c, — -1 . /o • Ate ^ *< °^ \ / *^ns <* *■ NOAA Technical Report NESS 78/^" % Geostationary Operational Environing Satellite/Data Collection System Washington, D.C. July 1979 U.S. DEPARTMENT OF COMMERCE National Oceanic and Atmospheric Administration National Environmental Satellite Service NOAA TECHNICAL REPORTS National Environmental Satellite Service Series j.tional Environmental Satellite Service (NESS) is responsible for the establishment and operation of the environmental satellite systems of NOAA. Publication of a report in NOAA Technical Report NESS series will not preclude later publication in an expanded or modified form in scientific journals. NESS series of NOAA Technical Reports is a continua- tion of, and retains the consecutive numbering sequence of, the former series, ESSA Technical Report onal Environmental Satellite Center (NESC), and of the earlier series, Weather Bureau Meteorological Satellite Laboratory (MSL) Report. Reports 1 through 39 are listed in publication NESC 56 of this ser- ies . Reports in the series are available from the National Technical Information Service (NTIS), U.S. Department of Commerce, Sills Bldg., 5285 Port Royal Road, Springfield, VA 22161, in paper copy or mi- crofiche form. Order by accession number, when given, in parentheses. Beginning with 64, printed copies of the reports, if available, can be ordered through the Superintendent of Documents, U.S. Gov- ernment Printing Office, Washington, DC 20402. Prices given on request from the Superintendent of Docu- ments or NTIS. ESSA Technical Reports N"ESC 43 Atlas of World Maps of Long-Wave Radiation and Albedo — for Seasons and Months Based on Measure- ments From TIROS IV and TIROS VII. J. S. Winston and V. Ray Taylor, September 1967, 32 pp. (PB-176-569) NESC 44 Processing and Display Experiments Using Digitized ATS-1 Spin Scan Camera Data. M. B. Whitney, R. C. Doolittle, and B. Goddard, April 1968, 60 pp. (PB-178-424) NESC 45 The Nature of Intermediate-Scale Cloud Spirals. Linwood F. Whitney, Jr., and Leroy D. Herman, May 1968, 69 pp. plus appendixes A and B. (AD-673-681) NESC 46 Monthly and Seasonal Mean Global Charts of Brightness From ESSA 3 and ESSA 5 Digitized Pic- tures, February 1967-February 1968. V. Ray Taylor and Jay S. Winston, November 1968, 9 pp. plus 17 charts. (PB-180-717) NESC 47 A Polynomial Representation of Carbon Dioxide and Water Vapor Transmission. William L. Smith, February 1969 (reprinted April 1971), 20 pp. (PB- 183-296) NESC 48 Statistical Estimation of the Atmosphere's Geopotential Height Distribution From Satellite Radiation Measurements. William L. Smith, February 1969, 29 pp. (PB-183-297) NESC 49 Synoptic/Dynamic Diagnosis of a Developing Low-Level Cyclone and Its Satellite-Viewed Cloud Patterns. Harold J. Brodrick and E. Paul McClain, May 1969, 26 pp. (PB-184-612) NESC 50 Estimating Maximum Wind Speed of Tropical Storms From High Resolution Infrared Data. L. F. Hubert, A. Timchalk, and S. Fritz, May 1969, 33 pp. (PB-184-611) NESC 51 Application of Meteorological Satellite Data in Analysis and Forecasting. Ralph K. Anderson, Jerome P. Ashman, Fred Bittner, Golden R. Farr, Edward W. Ferguson, Vincent J. Oliver, Arthur H. Smith, James F. W. Purdom, and Ranee W. Skidmore, March 1974 (reprint and revision of NESC 51, September 1969, and inclusion of Supplement, November 1971, and Supplement 2, March 1973), pp. 1 — 6C-18 plus references. NESC 52 Data Reduction Processes for Spinning Flat-Plate Satellite-Borne Radiometers. Torrence H. MacDonald, July 1970, 37 pp. (C0M-7 1-00 132) NESC 53 Archiving and Climatological Applications of Meteorological Satellite Data. John A. Leese, Arthur L. Booth, and Frederick A. Godshall, July 1970, pp. 1-1 — 5-8 plus references and appen- dixes A through D. (C0M-7 1-00076) NESC 54 Estimating Cloud Amount and Height From Satellite Infrared Radiation Data. P. Krishna Rao, July 1970, 11 pp. (PB-194-685) NESC 56 Time-Longitude Sections of Tropical Cloudiness (December 1966-November 1967). J. M. Wallace, July 1970, 37 pp. (COM-71-00131) (Continued on inside back cover) For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 Stock No. 003-019-00049-7 'WENT Of *i NOAA Technical Report NESS 78 Geostationary Operational Environmental Satellite/Data Collection System Office of System Engineering Washington, D.C. July 1979 U.S. DEPARTMENT OF COMMERCE £i Juanita M. Kreps, Secretary National Oceanic and Atmospheric Administration |S» Richard A. Frank, Administrator National Environmental Satellite Service David S. Johnson, Director DISCLAIMER. "Mention of a commercial company or product does not constitute an endorsement by NOAA National Environmental Satellite Ser- vice. Use for publicity or advertising pur- poses of information from this publication concerning proprietary products or the tests of such products is not authorized." 11 CONTENTS Glossary vi Abstract viii 1 Introduction 1 1.1 GOES system description 1 1.1.1 What is GOES? 1 1.1.2 How is the GOES system implemented? 1 1.1.3 What does GOES do? 1 1.1.3.1 Visible and Infrared Spin-Scan 1 Radiometer (VISSR) 1.1.3.2 Retransmission of VISSR and WEFAX data 1 1.1.3.3 Space Environment Monitoring (SEM) 2 1.1.3.4 Command and telemetry 2 1.1.3.5 Data collection 2 1.1.4 Is there a standby satellite? 2 1.1.5 Will service interruptions occur? 2 1.2 Description of the Data Collection Subsystem 2 1.2.1 How can an organization become a GOES/DCS 2 user? 1.2.2 What does it cost? 3 1.2.3 What geographical coverage is provided by 3 the DCS? 1.2.4 What is the DCS message capacity? 3 1.2.5 What type of user sensor platform can be used? 3 1.2.6 How are transmitter frequency assignments 6 obtained? 1.2.7 How do the sensor data reach the user? 6 1.2.8 How is the DCS operation controlled? 7 1.2.9 What data error probability can the DCS 8 user expect? 1.2.10 Can a user achieve direct data readout from 8 the GOES satellite? 2 A CLOSER LOOK AT THE DCS 2.1 DCS general operation 10 2.2 Sensor/platform interface 10 2.2.1 Sensor data format 10 in CONTENTS (Con. ) 2.2.2 Sensor platform transmitter response format 14 2.2.3 Command/ interrogate signal format 14 2.2.4 Platform radio set characteristics 17 2.2.5 Environmental considerations 17 2.3 Spacecraft function in the DCS 17 2.3.1 Communication Transponder 17 2.3.2 Conditions affecting access and performance 18 2.4 Command and data acquisition 18 2.4.1 Response data and command/interrogate signals 18 2.4.2 Time data and command/interrogate signal format 19 2.4.3 Response data demodulation and transmission to 19 CDF 2.5 Data Recovery-Central Distribution Facility 20 (CDF) 2.5.1 Recovered data characteristics 20 2.5.2 Error checks and abnormal responses 20 2.5.3 User access to recovered data 21 2.5.3.1 User dissemination circuits 21 2.5.3.2 Dedicated circuit 21 2.5.3.3 1200-baud DDD circuit 24 2.5.3.4 110-baud DDD circuit 27 REFERENCES Appendix A - Users Request Questionnaire Appendix B - Memorandum of Agreement Between NOAA and the User Appendix C - DCPRS Certification Standards (Interrogated & Self-Timed) Appendix D - Some Considerations in the Design and Installation of a Receiving System To Receive DCS Data Directly From the SMS/ GOES Family of Satellites Appendix E - Certification Specifications for International DCPRS IV LIST OF FIGURES 1. — Geometry of the DCS coverage 4 2. — SMS/GOES coverage 5 3. — GOES data collection system 9 4. — ASCII character assignments 11 5. — Pseudo-ASCII binary data format 12 6. — Modulation definition 13 7. — Platform transmitter response timing 13 8. — Interrogation message and time code formats 15 LIST OF TABLES TI 742 command characters 24 GLOSSARY ASCII BER BPS, bps CDA CPS CRC dB dBm DCPRS DCS DDD CDF e.g. : EIRP E/N GOES Hz IBM ID LRC LSB max American Standard Code for Information Interchange Bit Error Rate Bits per second Command and Data Acquisition Characters per second Cyclic Redundancy Check Decibels Decibels referred to 1 milliwatt Data Collection Platform Radio Set Data Collection System Direct Distance Dial Central Distribution Facility for example Effective Isotropic Radiated Power Bit Energy to noise spectral power density ratio Geostationary Operational Environmental Satellite Hertz International Business Machines Corporation Identification Longitudinal Redundancy Check Least Significant Bit Maximum VI GLOSSARY ( Con . ) min MLS NASA NBS NESS NOAA op. cit PSK Ptl RCP S/C s SEM SFSS SMS sync UHF UTC VISSR WEFAX Minimum Maximal Linear Sequence National Aeronautics and Space Administration National Bureau of Standards National Environmental Satellite Service National Oceanic and Atmospheric Administration In the work cited Phase shift keyed, -ing Probability that the transmission path loss will be less than the value used Right-hand Circularly Polarized Spacecraft Seconds Space Environment Monitoring Satellite Field Service Stations Synchronous Meteorological Satellite Synchronizat ion Ultra High Frequency Coordinated Universal Time Visible and Infrared Spin-Scan Radiometer Weather Facsimile vn GEOSTATIONARY OPERATIONAL ENVIRONMENTAL SATELLITE/ DATA COLLECTION SYSTEM ABSTRACT . The Data Collection System portion of the NOAA Geostationary Operational Environ- mental Satellite program has the potential and capacity for many and varied uses. The purpose of this report is to describe to potential us- ers the carrier system and its data processing capabilities. User qualifications and require- ments for participation in the Data Collection System are also defined. vm SECTION 1 INTRODUCTION 1.1 GOES System Description 1.1.1 What Is GOES? The United States of America Geostationary Operational Environmental Satellite (GOES) service is an integrated system of Earth and space environmental sensors which provide nearly continuous observational information to ground-based user stations. The service is operated and controlled by the National Oceanic and Atmospheric Administration (NOAA) of the U.S. Department of Commerce, and was developed at the National Environmental Satellite Service (NESS) in conjunction with the Synchronous Meteorological Satellite (SMS) program of the U.S. National Aeronautics and Space Administration (NASA). 1.1.2 How Is the GOES System Implemented? The GOES service currently operates two satellites located in Earth- synchronous orbits approximately 35,500 kilometers above the Equator at longitude 75°W and longitude 135°W. Both satellites are controlled from the NOAA Command and Data Acquisition (CDA) Station at Wallops Station, Virginia. 1.1.3 What Does GOES Do? The GOES service performs five missions. Each of the missions is carried out by a dedicated subsystem, as described below. 1.1.3.1 Visible and Infrared Spin-Scan Radiometer (VISSR) Each satellite has a visible/infrared radiometer that provides nearly continuous imaging of the viewed portion of the Earth's surface and cloud cover. The visible images are produced during daytime, whereas the infrared images provide day and night coverage. 1.1.3.2 Retransmission of VISSR and WEFAX Data The CDA is uniquely equipped to receive the high-speed VISSR data from the spacecraft. These data are reformatted at the CDA and retransmitted through the spacecraft transponder to Satellite Field Service Stations (SFSS) within view of the spacecraft. SFSS activities located in San Francisco, California; Kansas City, Missouri; Suitland, Maryland; Miami, Florida; Honolulu, Hawaii; and Anchorage, Alaska, currently receive and process, the retransmitted VISSR data for distribution to weather forecast stations throughout the United States. In a similar fashion, Weather Facsimile (WEFAX) pictures are transmitted through the spacecraft transponder to such users as ships at sea, aircraft, and weather forecast stations located within view of the space- craft . - 1 - 1.1.3.3 Space Environment Monitoring (SEM) SEM equipment onboard the spacecraft measures the energy and trajectory of incident particles. These data are transmitted to the Space Disturbance Forecast Center at Boulder, Colorado, for distribution and analysis. 1.1.3.4 Command and Telemetry The "housekeeping" functions necessary to maintenance of correct attitude and position, and monitoring spacecraft status, etc., are provided by the Command and Telemetry subsystem under control of the CDA Station. 1.1.3.5 Data Collection The Data Collection capability enables atmospheric and Earth-based sensors within view of the spacecraft to transmit synoptic or interrogated data to the CDA through the spacecraft transponder. These data are relayed to the Central Distribution Facility (CDF) at Suitland, Maryland, for dissemination to users. 1.1.4 Is There a Standby Satellite? To serve as a backup in event of failure of either operational satellite there is a third geostationary satellite in orbit at longitude 105°W (located midway between the other two). The backup satellite can readily be moved to either the east or west satellite position should such a need arise. 1.1.5 Will Service Interruptions Occur? There may be interruptions during the periods of solar eclipses. The GOES satellites undergo spring and autumn eclipses during a 46-day interval at the vernal and autumnal equinoxes. The eclipses vary from approximately 10 minutes at the beginning and end of eclipse periods to a maximum of approximately 72 minutes at the equinox. The eclipses begin 23 days prior to equinox and end 23 days after equinox; i.e., March 1 to April 15 and September 1 to October 15. The outages occur during local midnight for the satellites' mean meridian. There will also be shutdowns for periodic maintenance at the Wallops CDA station. These scheduled outages will be reduced in the future as redundancy is added to the Wallops Station facility. 1.2 Description of the Data Collection Subsystem In consonance with the primary purpose of this brochure, the following paragraphs discuss in some detail those aspects of the Data Collection System that will be of particular interest to users. 1.2.1 How Can an Organization Become a GOES/DCS User? An organization having a requirement for data collection, or which plans to collect data using the GOES DCS capability, must formally request permission to participate. Requests are processed by: National Oceanic and Atmospheric Administration, National Environmental Satellite Service, Washington, D.C. 20233. - 2 - The prospective user must describe the proposed use of the DCS for examination by the NESS DCS Review Committee, which recommends appropriate action to be taken by the Director of NESS on the request. A questionnaire (see Appendix A) is provided to facilitate presentation of information needed to properly consider the user's request . Upon approval of the user's request for participation in the DCS, a Memorandum of Agreement will be prepared to detail the rights and responsibilities of both NESS and the participating user. This agreement is attached as Appendix B. 1.2.2 What Does It Cost? Collection of data from user sensor platforms and processing the data for dissemination using the GOES DCS facilities is without charge to the user. The user will be responsible for his own costs of sensor platforms, such as procurement, maintenance, and installation, and such tests as are required to establish conformity to the DCS performance specifications. The user will also be responsible for the costs of communications lines, modem equipment, and data terminals necessary to the dissemination capability of the GOES/DCS. Unless an exception is justified, data collected for individual users will be made available by NESS to other users or interested parties upon request 1.2.3 What Geographical Coverage Is Provided by the DCS? The DCS was based on the practicality of an Earth-sited transmitter operating with a suitable antenna to provide an Effective Isotropic Radiated Power (EIRP) of 50 dBm (max.) at a minimum antenna elevation angle of less than 5°. The minimum antenna elevation angle defines the portions of the Earth's surface having geocentric angles up to approximately 77°, measured from the subsatellite point. Figure 1 described the geometry of the DCS. Figure 2 presents the actual Earth surface coverage for the DCS of both GOES satellites. 1.2.4 What Is the DCS Message Capacity? Access to the spacecraft transponder is shared among all platform site transmitters within its field of view. (See Figure 2.) The DCS has the capacity for handling at least 10,000 transmissions (30-s average message duration) from platform sites via the spacecraft transponder in each 1-hour period. Frequency division multiple access with time- shared channel occupancy is the technique used to meet the data traffic requirements. 200 channels are available for domestic user assignment with an additional 33 channels provided for cooperative working with international users. The channel spacing accommodates the 100-bits-per- second DCS message data rates. 1.2.5 What Type of User Sensor Platform Can Be Used? The simplest user platform configuration requires: (1) An UHF transmitter and antenna combination capable of producing +50 dBm EIRP. - 3 - GOES East North Area of Coverage 75 W Longitude South a Geocentric Angle of Satellite Coverage FIGURE 1 GEOMETRY OF THE DCS COVERAGE - 4 UJ UJ «s UJ CM > CD UJ <_> zz> co ~ V t» J 1 •J o M. fir tt a: C t/i 1 VI J | 1 3 1 1 * ' E CQ >. CO c o •H D P O a. +-> ~0 ^0 ] 0 0 1 0 1 0 0 1 1 1 0 0 1 0 1 1 1 0 1 1 1 0 0 0 0 _[_NUL_ ! SOU dleJ_ SPACE 0 9 p ^ p 0 0 0 1 DC1 ! 1 A Q .) C| 0 0 10 | STX DC 2 " 2 B n 1) 0 0 11 I ETX DC3 If- 3 C s c 0 10 0 I EOT, . DC'l S 4 D T d 0 10 1 ENO Inak 0/ 5 E U (; u 0 110 ACK ISYN & G F V 1 V 0 111 , j| " I MM. , i(l|BS«l!ll!> !etb | / 7 c; W 'J w 10 0 0 CAN i ( 8 II X h X 10 0 1 m ! 1 i i EM ! ) 9 I Y i V 10 10 iljiKil Isub 1 * J Z i z 10 11 VT 1 ESC 1 + * K L k \ 110 0 FF 1 \f's\ ' < L \ 1 1 110 1 life--: ill ■ Igs, ; - = M ] 111 \ 1110 SO III! ! rcs x N A II - 1111 si, US / 7 0 - ° 1 jDELl KM PRINTABLE CHARACTER PRINTER CONTROL CHARACTER ESSZ3 AUXILIARY DEVICE CONTROL CHARACTER CODES GENERATED 13".' KEYBOARD BUT NO ACTION TAKLN USASCII CONTROL CHARACTERS (From USA Standards Institute Publication X3.4-190B) ACK acknowledyu ETX end of to XI BEL hell II" form feed BS hack pare FS file separator CAN en '-t:l (IS ijroup separator CR carriage return MT I10ri20nt.1l t.ihul.ition DC1 - X-ON device control 1 IF line feed DC2 = TAPE device control 2 •VAK neq.iiive acknowledge UC2 - X-OFF device control 3 NUL null DC4 = TAPE device control 4 (stop) ns record separator 'DEL = RUB OUT delete SI shift in DLE (liiln link escape so shift out EM end of medium son Start of he.idin.i ENO = WHU '•iHHir.y SIX St. lit (if t'Xt TOT end of transmits Mil SUB substitute ESC escape SYN synchronous idle ETB end of transmission block US unit sep.ir.itoi VT vertical tabulation • ot strictly a control character Figure 4. — ASCII Character Assignments, - 11 - Bit # of Character: 8 7 6 5 4 3 2 1 (see note) ( ( 1 i 1 I 1 — -"Var"-- ^S Function : >> s 4-> •p .-1 «u cd •H s 0 XJ u ed en (0 >» ft 10 •H cd TJ £ oa a •d H •H o < SO CQ Figure 5. — Pseudo-ASCCII binary data format NOTE: Bit # is order of transmission — ■ #1 is first - 12 - SENSOR DATA 100 HZ CLOCK MANCHESTER CODED DATA +60° CARRIER PHASE -60° 5 MS I i l 1 i i 1 i i ! i i i i 1 . «— ONE — * ] «-ZER0-* Figure 6. — Modulation Definition TRANSMIT CARRIER ' UNMODULATED AT ' 0° PHASE TRANSMIT ±60° PSK ' ALTERNATE 1,0 '. FATTERN I TRANSMIT 46-BIT • PREAMBLE '" J TRANSMIT SENSOR DATA ASCII 4.9 S (MIN) 2.4 (MIN) 0.46 S 0.08 x N S TRANSMIT 3 ASCII , EOT CHARACTERS • I 0.24 S ■11.0 S (MAX) BOTE: N - NUMBER OF 8-BIT ASCII CHARACTERS OF SENSOR DATA Figure 7. — Platform Transmitter Response Timing - 13 - character. If necessary binary data may be transmitted when formatted into pseudo-ASCII characters, as shown in Figure 5. These special requirements are needed to ensure that binary data are not mis- interpreted as control characters, affecting communications link operation . The sensor data must not contain certain ASCII characters that have special control functions in the DCS dissemination system. These prohibited characters are: DLE,NAK, SYN, ETB, CAN, GS , RS , SOH , STX, ETX, ENQ, and ACK. EOT characters must appear only at the end of transmission and will be deleted from the data prior to dissemina- tion. Data characters containing parity errors will be replaced with NUL or $, depending on the specified dissemination link. 2.2.2 Sensor Platform Transmitter Response Format The platform response format satisfies the CDA requirements for response signal acquisition, clock recovery, data bit synchroni- zation, message synchronism, and optimum response channel utilization. The data are Manchester encoded to provide a self- clocking signal which is used to modulate the transmitted carrier by phase shift keying (PSK). Figure 6 defines the coding and modulation characteristics of the platform transmitter response signal . A platform response transmission (Figure 7) begins with unmodulated carrier (0° phase shift) for 5 seconds to allow the CDA data demodulator to acquire the signal and establish a phase reference. Next, the response signal is PSK'd with 2^ seconds of alternate "one," "zero" data (Manchester coded) so the data demodulator may obtain the 100-Hz bit rate clock and data bit synchronization. Then the 46-bit preamble is sent, consisting of the 15-bit MLS message sync word (100010011010111) followed by the 31-bit sensor platform address identifier word with the most significant bit (MSB) of those sequences transmitted first. The sensor data in serial ASCII character (odd parity) format are sent immediately following the preamble with the least-significant bit of each character transmitted first. A postamble consisting of 3 8-bit EOT characters marks the end of the response message. The CDF considers all the data in the platform response message framed by the preamble and postamble to be sensor data characters in ASCII. At the CDA, the received platform address identifier is compared with the expected address identifier obtained from the CDF computer, and the sensor data characters are tested for errors by examining received character parity. Detected error conditions will cause the appropriate error status to be transmitted along with the sensor data to the CDF computer. If no response is received at the CDA when one is expected, a "no message received" notice is sent to the CDF computer and on to the user. 2.2.3 Command/Interrogate Signal Format The signal transmitted from the CDA to the deployed sensor platform field via the spacecraft transponder contains two information items: - 14 - !:> } CD .— r - K - > >"n _l U1 \ s"" - } or n oo I *-> ac ,— J- o: o Iu a s oT o o ^l1 coT w a «* a C\J J_ =3 °T ~ J-1- lis lis ~1^ of — O I O Jill- I- . Ill- .lip o — -r sll" _ III- in- iir im-J =n !t S . o o =r I a »-T si*1 co T : ~1= -- o f z = 8 2 Us cc r W O ?H 0) CO u ■H ^Y~ - 15 - (1) Platform address identifier (or command word) which initiates the appropriate response activity from the platform to which it corresponds. (2) Coded time data, updated at J-minute intervals, referred to an NBS source. The CDA equipment which generates the Interrogate signal transmits a different frequency for each spacecraft; a sensor platform receiver must be equipped to receive the specific signal frequency trans- mitted via the spacecraft serving its site. Currently, both space- craft transmit identical address identifiers. The Interrogate signal modulation characteristics are those shown previously in Figure 6. The Interrogate message is formatted into blocks of 50 bits, as follows : (1) 4 bits of time-code data. (2) 15-bit MLS for message sync; same as used in platform reply (paragraph 2.2.2). (3) 31-bit address identifier. Message timing is derived from the CDA station atomic clocks which are maintained by NBS to within a few microseconds of the master NBS clock in Boulder, Colorado. As an approximate accommodation of the satellite transmission path time delay, the time signals as provided to the CDA Interrogate signal generating equipment are advanced 260,000 microseconds. The leading edge of the first bit of each message block coincides with the CDA station standard's ^-second mark. A complete time-of-year data record is transmitted piecemeal in the time code bits every 30 seconds, beginning on the minute-and l-minute marks. Figure 8 shows the format of the Interrogate message block and the time code record. The serial bit transmission sequence is shown in real-time reading from left to right. In all message segments the LSB is sent first. The format of the time-of-year data record is as follows: (1) Sync word, 40 bits in length, consisting of ten 4-bit time code characters which designate whether the trans- mission began on the minute-or J-minute mark. (1010 character for 1-minute records, 0101 for J-minute records; LSB sent first) . (2) Tirne-of-year word as eight 4-bit time code characters with the hexadecimal value of each character representing a decimal digit of the sequence; seconds (tens); minutes (units), (tens); hours (units), (tens); days (units), (tens), (hundreds), of Coordinated Universal Time (UTC). - 16 - (3) Correction to universal time as 2 time code characters; sign of correction (1111 = +, 0000 = -), and tenths of seconds corrected. (4) Satellite ephemeris in geocentric measure as thirteen 4- bit time code characters; longitude in degrees (hundreds), (tens), (units), (1/10's), (1/100's); latitude in degrees (sign), (units), (1/10's), (1/100's), orbital radius correction in microseconds (sign), (hundreds), (tens), (units) . NBS Technical Note 681, "A Satellite-Controlled Digital Clock," describes use of the time code data transmitted via the DCS Interrogate signal to obtain continuously updated time-of-year data at a sensor platform site to an accuracy of about 100 micro- seconds with +20-microsecond precision. 2.2.4 Platform Radio Set Characteristics A sensor platform does not need to be equipped to receive the Command/Interrogate signal for operation in the DCS. The additional flexibility provided by the Command/Interrogate capability may be desirable, for example, when the data reporting schedule requires modification or when the sensor activities must be directed remotely . The requirements for a sensor platform radio set are described in NOAA/NESS Specification No. 200.004, "Data Collection Platform Radio Set Specification." The design characteristics of platform radio sets essential to operation in the DCS are presented in Appendix C, "Platform Radio Set Certification Requirements." 2.2.5 Environmental Considerations Efficient utilization of the DCS capability for timely data collection from unattended sensor sites requires that platform radio sets be able to operate within specifications for all environmental conditions to which they are exposed. Environmentally induced performance changes may result in loss of important data from the affected sensor platform, as well as from other platforms which experience interference produced by the malfunctioning equipment. 2.3 Spacecraft Function in the DCS 2.3.1 Communication Transponder The spacecraft (GOES) operates as a cross-strapped transponder within the DCS communications chain. Command/ Interrogate signals from the CDA station are received by the spacecraft at S-band, then translated to UHF and retransmitted through an Earth-coverage antenna to the field of deployed sensor platforms. Response signals from the sensor platform sites are received by the spacecraft at UHF, translated to S-band, and sent to the CDA Station. The spacecraft transponder is fully redundant to guard against DCS outages, because of premature equipment failure. - 17 - 2.3.2 Conditions Affecting Access and Performance Periodic preventive maintenance is scheduled for the DCS well in advance, since no interrogated data collection can be carried out during these periods. Twice each year, the Earth's position will be such that the space- craft orbit carries it through the Earth's shadow. Because the spacecraft's prime power source is a solar array, safety considerations may require the transponder to be operated in a low-power mode if at all to conserve secondary power. This results in a satellite transmitter power drop of from 1 to 3 dB. DCP up-link response signals, occasional TIROS-N data relay signals, and satellite receiver noise share the down-link power output of the GOES. Increasing the number of active DCP channels thus decreases the power per channel, and also simultaneously reduces the down-link noise. Until the noise contributed by the CDA or other ground terminal receiving system becomes the limiting factor, the net signal- to-noise ratio is nearly independent of channel loading, and bit error probabilities of less than 10-5 may be anticipated, as described in paragraph 1.2.9 on page 8. 2.4 Command and Data Acquisition 2.4.1 Response Data and Command/ Interrogate Signals A UHF Pilot Response signal centered in the platform response frequency band and derived from the CDA station frequency standard is transmitted to the spacecraft transponder along with replies from the deployed platforms. The corresponding signal received at the CDA Station is recovered from the S-band receiver. The recovered signal is applied to the response link receiver. The recovered signal is applied to the response link Automatic Phase Control and Multicoupler Chassis which compensates for spacecraft oscillator variations, and divides the received CDA response signals in order to drive the response channel data demodulators. Forty data demodulators are presently implemented with the capacity to expand at a future date. 18 - Command/Interrogate data are applied to the Interrogate Channel Modulator which Manchester encodes the data and applies them to a Phase Shift Keying (PSK) modulator. The modulator output provides the input to the Interrogate Channel Frequency Control, which supplies the CDA S-band transmitter with the Interrogate Signal for transmission to the spacecraft transponder. The UHF Command/ Interrogate signal is received at the CDA and applies to the Interrogate Channel Data Demodulator which produces an error signal for automatic frequency control and demodulates the Interrogate signal for bit error rate performance testing. (See Figure 3, page 19. ) 2.4.2 Time Data and Command/Interrogate Signal Format Details of the modulation and data contained within the Command/ Interrogate signal are presented in paragraph 2.2.3. Since a Command/Interrogate signal is transmitted continuously, a dummy address is utilized in those periods when the CDF is not providing platform addresses for interrogation. In the absence of any address identifiers from the CDF for transmission to the sensor platform field, an idle pattern of 31 binary bits is used (00110100100001011101100011111). These bits are Manchester encoded and transmitted MSB first. 2.4.3 Response Data Demodulation and Transmission to CDF After recovery of the response signals as described in paragraph 2.4.1, the response data are demodulated in the appropriate CDA Channel Data Demodulator and held in buffer storage until the entire response is received at the CDA. The response address identifier is compared at the CDA with the expected platform address, and the message characters are checked for parity errors The platform sensor response and any applicable error conditions are then forwarded to the CDF for logging and dissemination, or for special attention based on error conditions. 19 Engineering tests of SMS-2 (op. cit . ) showed that the average time delay of platform site interrogation (from transmission of address identifier to reception of response preamble) was 12.6 seconds. 2.5 Data Recovery-Central Distribution Facility (CDF) 2.5.1 Recovered Data Characteristics Recovered data and other messages sent from the CDF to the users are in data record form, consisting of a heading and sensor platform data. Each record represents one of the following: (1) Platform sensor response. (2) Abnormal platform response message. (3) Operator message. (An "abnormal platform response message" is a "canned" message from the DPS describing an unexpected event in a platform response, and is preceded by two carets (AA) for rapid identification.) Each normal platform response data record contains: (1) The platform sensor address identifier (in hexadecimal representation ) . (2) A question mark (?) if the address had bit errors or a space if the address was received without error. (3) The time platform data were received, as day-of-year (3 characters), hour (2 characters), minute (2 characters) and second (2 characters) of Coordinated Universal Time. (4) Received data (as ASCII characters). Characters received with parity errors will be replaced in the data with either a dollar sign ($) or NUL character, depending on the means of dissemination as explained in 2.5.3 below. The EOT characters required to terminate the platform transmission are not included with data in the record. (For further details regarding this general subject, refer to the user interface manual listed in the references.) 2.5.2 Error Checks and Abnormal Responses The CDF identifies as abnormal all responses with error conditions. All scheduled response messages will be routed to users even if the address is received in error. (Only unscheduled unrecognizable messages will not be disseminated.) - 20 The following error conditions are recognized in the CDF: (1) A scheduled or interrogated message is not received when expected. (2) A scheduled message is received slightly off schedule. (3) An unscheduled response message is received (4) (4) An interrogation was not performed as scheduled for some reason. (5) A response message exceeded the expected channel occupancy time for the sensor platform. (6) Data are received with parity errors. (For interrogated responses, the user may specify whether all responses are required, or only those without error. The maximum number of reinterrogations to recover bad data (up to 15) may also be specified.). (7) Address identifiers received with 2 or fewer bit errors will be corrected within the data record by the CDF. 2.5.3 User Access to Recovered Data 2.5.3.1 User Dissemination Circuits Three classes of communication circuits are utilized by the CDF for dissemination of sensor platform data. These classes differ mainly in the user terminal capabilities the CDF presumes to exist, and their respective information transfer rate. The three classes of circuits are: (1) Full-time, dedicated. (2) 1200 baud dial-up. (3) 110 baud dial-up. 2.5.3.2 Dedicated Circuit For the first class of circuit, the CDF uses synchronous data transmission at modulation rates (determined by the modem equipment) up to 9600 baud. The "handshaking" protocol between the CDF and user terminal follows the IBM Binary Synchronous Communication procedures ("General Information, Binary Synchronous Communications," IBM No. 6A2730042). Communication over the dedicated, full-time circuits of the CDF has the following characteristics: - 21 - Channel Type Modulation Rate Coding Error Control Block Length Block Check Character full duplex; half-duplex protocol 0-9600 baud; 2400 baud nominal ASCII, odd parity, nontransparent block transmission; alternating ack 0, ack 1 190 data characters maximum and 3 (or 4) framing characters user option of Longitudinal Redundancy Check (LRC) or Cyclic Redundancy Check (CRC-16) Because the full-time circuits are permanent connections, the CDF knows which user is associated with each, and there is no need for identification procedures. If any platforms have emergency transmission capability, the CDF may identify such transmissions with a special header (optional) and data description in the disseminated data bulletin.2 Operator intervention is not required for routine or emergency data dissemination over a full-time dedicated circuit. The bulletin format used contains the following information sequence 35 CD 0) £ Function Control character (SOH); start of heading 3-digit message sequence number Catalog number; identification of user Control character (STX); start of text 6-digit data description; user constant Space character 2-digits each; day of month, hour, minute of dissemination DUP; Space, indicates possibly duplicate bulletin End-of-line; carriage return, linefeed Number of Characters 1 3 5 1 6 1 4 (Optional) 2 2Reference: User Interface Manual - 22 - 8 8 3 | CO 8 Number of Function Characters Control character (RS); record separator identifies start of platform replies 1 Sensor platform address identifier 8 ? or space; shows whether address was correct or not 1 Date-time of reception; # day of year, hour, minute, and second 9 Platform data; without final EOT characters Control characters (RS); separates platform replies within bulletin 1 Next reply follows, as above, from "Sensor Platform address identifier." Bulletins are segmented into uniform length blocks for transmission. Bulletins are generally less than 10 full blocks; the only exception being when a report beginning in the ninth block spans one or more blocks. Each block is formatted as follows: Control character; start of transmission block (SOH or STX) 1 Message characters 190 Control character; end of transmission block (ETB or ETX) 1 Block error control character; LRC (or CRC-16) 1 (or 2) NOTES : (1) Start of transmission block character is SOH for initial block, STX for subsequent blocks. (2) End of transmission block character is ETB for initial blocks, ETX for final block. (3) Final block may have 1 to 190 message characters (4) Block error control character: if longitudinal redundancy check (LRC), it is composed to the MOD-2 sum of corresponding character bit positions of all characters following the start of transmission block character, except that its parity bit is odd parity; if cyclic redundancy check (CRC-16), it is the 16-bit remainder after division of the message characters (following the start of transmission block character) by the polynominal x16 + x15 + x2 + 1. - 23 - 2.5.3.3 1200 Baud DDD Circuit The second class of circuit (1200 baud dialup) has the following characteristics : Channel Type Modulation Rate Coding Error Control Block Length Block Check Character DDD subscriber lines; half-duplex, AT&T type 202C modem or equivalent 1200 baud (120 characters/s ) asynchronous ASCII (10 bits) even parity block transmission: ACK/NAK 320 (max) data characters, and 6 framing characters with 6 device control characters Longitudinal Redundancy Check (LRC) NOTE: CDF assumes user terminal on 1200 baud circuits to have the characteristics of a Texas Instruments Model 742, with options: (1) "Auto-answer" line discipline. (2) "Answer-back memory" preprogramed with user ID. (3) Bell 202C data set compatible modem. The TI Model 742 can transmit and receive at 120 characters per second (CPS), but its printer operates only up to 30 CPS. The CDF uses control character sequences to remotely command the terminal recorder and printer. Table 3 lists the possible commands. The platform sensor data are disseminated at 120 characters per second rate with the terminal's printer OFF. An end-of-line (carriage return, line feed) is inserted following every 80 data characters by the CDF. TABLE 1 TI 742 COMMAND CHARACTERS Command Record ON Playback OFF Printer ON Printer OFF Auto Device Control ON (ADC ON) Auto Device Control OFF (ADC OFF) (*) Utilized by DPS/DCS. Code *DC2 *DC3 *DLE 9 *DLE 0 *DLE : *DLE ; - 24 - The following procedures are employed for user-CDF communications over 1200 baud dialup circuits: Procedures on 1200-bps Line (1) Utilizing a Texas Instruments Model 742 (or equivalent) terminal, the user dials the telephone number for the 1200-bps line. (2) The CDF answers the call, and when the answer tone sounds the user presses the DATA button or pushes down the exclusion key. (3) The CDF waits 8 seconds from occurrence of the ring to permit the terminal to identify itself by auto- matically triggering its answer-back device. (If the terminal is equipped with an answer-back device, it must be programed to transmit either STX, user ID, ETX, LRC, or STX, user ID, "DIS," ETX, LRC. The correct LRC must be hand computed and preprogramed for for the answer-back, since normally it is not generated automatically in this situation.) (4) If the CDF receives no identification within 8 seconds, it transmits the message: DCS/CDF. ENTER ID The user will sign on by responding with the framing character STX followed by the user ID (a maximum of six characters), and the framing character ETX. Optionally, he may enter STX, user ID, a comma, and one of the following: MSG, DIS, RLT. If DIS or RLT is specified the GMT (in the format DDDHHMMSS) may be included. All entries must be terminated by ETX and LRC. (5) For either answerback or manual sign-on messages, CDF determines whether the ID is valid; and if only the ID was entered, determines whether any new message data have been acquired since the previous user inquiry (6). (6) If the ID is invalid, the request is repeated up to five times, after which the CDF goes on-hook after sending the following message: TOO MANY ERRORS (7) If the ID is correct, and if only the ID was entered, CDF informs the user of the number of messages received since the previous inquiry and the time of the last dissemination. The CDF then transmits: ENTER MSG, DIS, RLT, OR STOP. - 25 - (7a) MSG Directive - the user may transmit a text message to the DCS operator (to be displayed on the console) by entering STX MSG ETX after the CDF informs him/her of the number of messages received since the previous inquiry (or by entering MSG following the user ID in the sign-on, separated by a comma). The CDF responds with STX ENTER MESSAGE ETX, after which the user may enter the message. (7b) PIS Directive - the user may request transmission of the sensor platform data by entering STX DIS ETX after the CDF informs him/her of the number of messages received since the previous inquiry (or by entering DIS following the user ID in the sign-on, separated by a comma). Transmission of sensor platform data, if requested, begins with the oldest undisseminated message and continues to the newest message on the user's queue. Abnormal platform response messages are transmitted in lieu of data when appropriate, and are identified by two carets (AA) preceding the text of the message. Alternatively, the user may specify a new time (other than "time of last inquiry") from which dissemination of data in the queue may begin, by entering STX DIS DDDHHMMSS ETX in GMT ( or UTC ) . (7c) RLT Directive - the user may request transmission of sensor platform data acquired after a specified time for information only, by entering STX RLT, DDDHHMMSS ETX after the CDF informs him/her of the number of messages received since the previous inquiry. As in (7b) above, a new time interval may be specified to begin playback. Data disseminated by this method are not recorded by the CDF. (8) Once transmission of the most recent sensor platform data (DIS only) has been completed, the CDF sends the following message: NNN MESSAGES DISSEMINATED. VERIFY: OK, NO, OR STOP. If the user enters OK, after DIS (dissemination) the UTC of the last message disseminated will be inserted into the user Queue Table as the "last time of dissemination" (for use the next time he/she calls.) The "number of messages" will be reset to zero, and the message pointers updated. If the user enters NO, then the user Queue Table will not be updated. After either OK or NO, the CDF solicits a new request with the following message: ENTER MSG, RLT, DIS, OR STOP. - 26 - If at any time the user enters STOP, the CDF sends an EOT and goes on-hook. If the user hangs up without answering STOP, the CDF will assume that the previously transmitted data were successfully disseminated . (9) If the user disconnects the circuit prior to transmission of all the messages, then the CDF considers that no messages have been disseminated. (10) Data transmitted on these circuits have all erroneous characters (containing parity errors) replaced with a dollar sign ($). (11) If the CDF receives STX CAN ETX LRC from the terminal, signifying a playback error (cassette I/O error), it will send the following message to the user: PLAYBACK ERROR. TRY AGAIN. The user will be able to reenter his request. 2.5.3.4 110-Baud DDD Circuit The third class of circuit (110 baud dial-up) has the following characteristics : Modem : AT&T Model 103A3 Channel Type : DDD subscriber lines; half-duplex Modulation Rate: 110 baud (10 characters/s) Coding : Asynchronous ASCII (11 bits) even parity Error Control : None The CDF assumes that the terminal on a 110-baud dialup line is similar to a Teletype Model ASR-33. User communications over these low-speed lines require the following procedures: Procedures on 110-baud Line (1) Utilizing a terminal, the user dials the telephone number for the 110-baud line. (2) The CDF automatically answers, and transmits the message : DCS. ENTER ID followed by the protocol character WRU (ENQ) - 27 - (3) The answer-back capability available on many terminals will respond to the WRU. At terminals not thus equipped, the operator will respond with the user ID (a maximum of six characters). Optionally, he/she may enter his/her user ID, a comma, and one of the following: MSG, DIS, RLT, or the time (in the form DDDHHMMSS of GMT). (4) The CDF determines whether the ID is valid, and, if only the ID was entered, determines whether any new message data have been acquired since the previous user inquiry. (5) If the ID is invalid, the request is repeated up to five times, after which the CDF goes on-hook after sending the following message: TOO MANY ERRORS. (6) If the ID is correct, and if only the ID was entered, CDF informs the user of the number of messages received since the previous inquiry and the time of the last dissemination. The CDF then transmits: ENTER MSG, DIS, RLT, OR STOP. (6a) MSG Directive - the user may transmit a text message to the DCS operator (to be printed on the console) by entering MSG after the CDF informs him/her of the number of messages received since the previous inquiry (or by entering MSG with the ID entry, separated by a comma). The CDF responds with ENTER MESSAGE, after which the user may enter the message. (6b) DIS Directive - the user may request transmission of the sensor platform data by entering DIS after the CDF informs him/her of the number of messages received since the previous inquiry (or by entering DIS after the ID entry, separated by a comma). Transmission of sensor platform data, if requested, begins with the oldest undisseminated message and continues to the newest message on the user's queue. Abnormal platform response messages are transmitted in lieu of data when appropriate, and are identified by two carets (AA) preceding the test of the message. Alternatively, the user may specify a new time (other than "time of last inquiry") from which dissemination of data in the queue may begin, by entering DDDHHMMSS (in GMT) . - 28 (6c) RLT Directive - the user may request transmission of sensor platform data acquired after the connection is made until the user hangs up, by entering RLT after the CDF informs him of the number of messages received since the previous inquiry (or by entering RLT after the ID entry, separated by a comma). Optionally, RLT, DDDHHMMSS may be specified to receive data disseminated earlier. As in paragraph (7b), page 26, a new time interval may be specified to begin playback. Data disseminated by this method are not recorded by the CDF. (7) Once transmission of the most recent sensor platform data has been completed, (DIS only) then CDF sends the following message: NNN MESSAGES DISSEMINATED. VERIFY: OK, NO, OR STOP. If the user enters OK, the GMT of the last message disseminated will be inserted into the user queue table as the "last time of dissemination." The "number of messages" will be reset to zero, and the message pointers updated. If the user enters NO, then the user queue table will not be updated. After either OK or NO, the CDF solicits a new request with the following message: ENTER MSG, RLT, DIS, OR STOP. If at any time the user enters STOP, the CDF dis- connects. If the user hangs up without answering STOP, the CDF will assume that the previously transmitted data were not successfully disseminated . (8) If the user disconnects the circuit at his/her end prior to transmission of all his/her messages, then the CDF considers that no message has been disseminated. (9) Data transmitted on these circuits have all erroneous characters (containing parity errors replaced with a dollar sign ($). - 29 - REFERENCES Cateora, J.V., Davis, D.D., and Hanson, D.W. , 1976. A Satellite Controlled Digital Clock. NBS Technical Note 681, Time and Frequency Division, Institute for Basic Standards, National Bureau of Standards, U.S. Department of Commerce, Boulder, Colorado, 46 pp. GOES Data Collection System, SMS2 Test Report, Test Report TER-383-002 for Office of System Engineering, NOAA/NESS, by Telcom, Inc., Vienna, Virginia, 24 pp. McManamon, P., 1973. GOES Data Collection System Performance Estimates. Telecommunications Technical Memorandum OTM- 73-125, Institute for Telecommunication Sciences, Office of Telecommunications, U.S. Department of Commerce, Boulder, Colorado, 66 pp. User Interface Manual, NESS/NOAA/Department of Commerce, by GTE Information Systems, Systems Division, Silver Spring, Maryland, May 1978, 50 pp. 30 APPENDIX A USERS REQUEST QUESTIONNAIRE 1. Describe fully your application Operational/Experimental If experimental, please complete the following: Name and address of the funding agency Administrator. Name and address of the party responsible for imple- menting your DCS program, i.e., the principal investigator . Give the starting and ending dates of the period during which you plan to collect data via satellite. Purpose of Data Data Perishability Final User of Data 2. Type of System Interrogated Self-time Hybrid 3. Number of Platforms Number of each Type Number of Platforms with Emergency Alarm Provision Time Scale for Deployment of each Type 4. Location of Platforms by Types State, ocean Nearest city if located in State Fixed station - Latitude/Longitude Mobile station operating area - Latitude/Longitude of Bounding Area 5. Data Format of Data Bits per Sensor Message 6. Reporting Times Interrogation Schedule Self-Timed Schedule 7. Data Delivery Data Form (Magnetic Tape, Paper Tape, Computer Printout , etc.) Address for Delivery How often required? (Delivery once per hour, per six hours, per day, etc. ) 8. Explain why commercial services cannot meet your program needs 9. Agency to install and maintain platform equipment. 31 APPENDIX B MEMORANDUM OF AGREEMENT BETWEEN NOAA AND THE USER INTRODUCTION The National Environmental Satellite Service (NESS) , of the National Oceanic and Atmospheric Administration (NOAA) hereinafter referred to as the operator (of the Synchronous Meteorological Satellite (SMS) and the Geostationary Operational Environmental Satellite (GOES) and the Command and Data Acquisition (CDA) (Station)) and the (user) (agency), hereinafter referred to as the user (the provider of Data Collection Platforms and the user of the data collected) agree on the "Joint Understanding" below and agree to fulfill the undertakings specified. I. Name of Program. The program to which this Memorandum applies shall be known as the "National Environmental Satellite Service - (user) GOES Data Collection System Program". II. Joint Understanding. A. To qualify for collection by the GOES, the data from the user's Data Collection Platforms must fall within the definition of environmental data. Environmental data are defined as obser- vations and measurements of physical, chemical, or biological pro- perties of the oceans, rivers, lakes, solid Earth and atmosphere (including space) . B. Authority for the GOES to utilize the radio frequency band 401.7 to 402.1 MHz as an uplink and the radio frequency band 468.750 to 468.950 MHz as a down link is contained in the Fre- quency Assignment Subcommittee/Interdepartment Radio Advisory Committee docket numbers 7422556 and 7422589, respectively. Docket number 7422556 grants the operator the authority to make frequency channels available to the user. However, it is understood that the user must obtain authority from appropriate national agencies to transmit on frequency channels, designated by the operator, within the uplink band. The operator will also provide address codes. C. The operator will not assign a channel to one user for full time use; however, time periods within a channel will be assigned and on a priority basis. D. The operator reserves the right to terminate or suspend the user's participation in this program in the event of space- craft or ground equipment limitations requiring curtailment or elimination of services. 33 E. Unless an exception is specified elsewhere in this memo- randum, data collected for users shall be made available from NESS to other interested parties as appropriate. F. Data Collection Platforms which the users plans to implement as part of the GOES Data Collection System are subject to certification by the operator before deployment. G. In consultation with the user, the operator will establish the collection times and data lengths for the user's Data Collec- tion Platforms and the schedules and methods for data dissemina- tion. H. All transmissions from the Data Collection Platforms to the GOES spacecraft will be coordinated with the operator prior to such transmissions. III. Undertaking by the user. The user shall : A. Provide the operator a list of the user's Data Collection Platforms showing the type (self -timed, interrogate) ; where each is to be located; and which platforms are equipped with emergency alarm provisions. B. Provide the operator notification prior to Data Collec- tion Platform relocation. C. Provide the operator with the data type and message load planned for each Data Collection Platform. D. Provide the personnel, funds, and equipment necessary to carry out the portion of the program at the Data Collection Plat- form location. E. Operate and maintain the Data Collection platforms in conformance with equipment performance standards as specified by the operator in: National Environmental Satellite Service Speci- fication for Data Collection Platform Radio Set (DCPRS) . Speci- fication No. 200.004, January 27, 1975. F. Provide the personnel, funds and equipment necessary to operate and maintain facilities for receipt of collected data. These responsibilities include the cost of the communication interface at the NESS facility and the means to forward the data to the terminal point designated by the user. The communication interface is specified by the operator in: NESS GOES DCS User Terminal Specifications, 1 January 1975. G. Provide periodic reports, upon request from the operator, on the present application of the user's DCS data. 34 IV. Specific Undertakings on the Part of the operator. The operator shall: A. Provide and operate the GOES spacecraft and the NESS ground facilities for receiving data collected from the satellite B. Provide telemetry reduction sufficient to monitor the user's Data Collection Platforms for meeting system performance standards. C. Notify the user by the most expeditious means available whenever NESS system monitoring indicates the user's Data Collec- tion Platform is performing outside system specifications or is inoperative. D. Assign priorities for participation in the GOES DCS, scheduling purposes, channel assignments and for special DCS data requests according to the following categories in order of priority: 1. Disaster Warning 2. Operational 3. Experimental E. Notify the user of modifications to the established operational schedule for collecting data from the user's Data Collection Platforms. Notification will be prior to activation of such schedule changes unless the operator must enact schedule modifications to provide services for emergency warnings. Sudden adverse spacecraft conditions may also preclude the operator from providing the user notification prior to schedule changes. In any event, notification will be made as soon as possible. This agreement shall enter into force and effect for one year after signature by both parties and if otherwise consistent with applicable authorization and apprpriation Acts of Congress, this agreement shall remain in force and effect unless and until terminated at the election of either the user or the operator provided notification of such termination is in writing and forwarded by one party to the other, no less than 90 days in advance of termination. Director, NESS Date User Date 35 APPENDIX C DCPRS Certification Standards (Interrogated & Self-Timed) 1. RF Power Output . The Effective Isotropic Radiated Power (EIRP) of a DCPRS and antenna shall not exceed 50dBm under any combination of service conditions. 2. Frequency Characteristics. DCPRS received radio frequency (RF) shall be 468.825 MHz1. The transmitter RF shall be in the 401.85 MHz to 402 MHz band. (See Table 1.) 3. Stability. A. Temperature. The transmitter carrier frequency shall change by less than 0.5 parts per million over the temperature range of -20°C to +50°C. B. Long-Term. The long-term stability (including temperature variations) shall be better than one part per million per year. C. Short-Term. The phase jitter on the transmit carrier shall be less than 3 degrees RMS. 4. Electromagnetic Interference (EMI). All transmitter spurious emissions, when measured with modulation and with antenna and diplexer connected, shall be down from the unmodulated carrier level by 50dB. 5. Transmission Format. After a minimum of 4.9 seconds of unmodulated carrier, the carrier shall be modulated with the bit and message synchronization patterns which are at least 2.4 seconds of alternate 1, 0 data bits, and the 46- bit preamble consisting of the 15-bit MLS sync word followed by the 31-bit BCH command word. Transmission of the address shall be complete within 11 seconds after receipt of an interrogation. The binary data shall be Manchester- encoded 8-bit ASCII, odd parity and shall modulate the carrier in the following manner: a data "0" shall consist of +60° (+5°) carrier phase shift for 5 milliseconds followed by -60° (+5°) carrier phase shift for 5 milliseconds, and a data "1" shall consist of -60° (+5°) carrier phase shift for 5 milliseconds followed by +60° (+5°) carrier phase shift for 5 milliseconds. Data rate shall be 100 BPS +.1 BPS. (See Figure 1 . ) 6- End of Transmission. Immediately after sending the sensor data, the DCPRS shall transmit three 8-bit ASCII, odd parity, End of Transmission (EOT) characters contiguously with the ASCII sensor data characters (no break) and return to the standby condition. :For the West Satellite. DCPRS assigned to the East Satellite will be required to receive on 468.8375 MHz. 37 7. Fail-Safe Design. The DCPRS shall incorporate a "fail-safe" design feature such that malfunctioning of the equipment shall in no way cause continuous transmission. Further, provision shall be made to retrigger the fail-safe via the interrogation link in 90-second intervals without interruption of data transmission by addressing the radio set. 8. Receive Signal. The DCPRS shall continuously receive and demodulate the standard GOES Data Collection System interrogation signal over an input signal level range of -100 dBm maximum to -130 dBm minimum centered at 468.825 MHz2 and modulated +60°PSK with 100-bit /second Manchester-coded data. The DCPRS shall be capable of simultaneous reception and transmission and meet all performance requirements of the DCPRS in this mode. The DCPRS shall be capable of auto- matically locking to the interrogation signal at an input signal level as low as -135 dBm (total signal power with the following data present: 15-bit Maximal Linear Sequence (MLS) sync word (100010011010111) followed by the 31-bit Bose-Chaudhuri-Hocquenqhem (BCH) command word (0011010010000101011101100011111)). 9. Acquisition Time. The receiver shall acquire lock on the interroga- tion signal in 2 minutes or less from standby condition, when the carrier is within _100 Hz of 468.825 MHz1. 10. Spurious Emissions. Reradiated local oscillator and mixing frequency signals shall be less than 50 microvolts at the antenna or primary power in-out terminal. 11. Antenna Polarization. Polarization shall be right-hand circular, according to IEEE Standard 65.34.159. 12 . Data Formatting Restrictions. The following ASCII control characters must not appear in the DCPRS message: DLE , NAK , SYN, ETB, CAN, GS , RS , SOH, STX, ETX, ENQ, and ACK . EOT characters may appear only at the end of transmission. 2For the West satellite. DCPRS assigned to the East satellite will be required to receive on 468.8375 MHz. 38 TABLE 1 INTERROGATED DCPRS TRANSMIT FREQUENCIES CHANNEL FREQUENCY 100 401.849569 101 401.851069 102 401.852569 103 401.854069 104 401.855569 105 401.857069 106 401.858569 107 401.860070 108 401.861570 109 401.863070 110 401.864570 111 401.866070 112 401.867570 113 401.869070 114 401.870570 115 401.872070 116 401.873570 117 401.875070 118 401.876570 119 401.878070 120 401.879571 121 401.881071 122 401.882571 123 401.884071 124 401.885571 125 401.887071 126 401.888571 127 401.890071 128 401.891571 129 401.893071 130 401.894571 131 401.896071 132 401.897571 133 401.899072 134 401.900572 135 401.902072 136 401.903572 137 401.905072 138 401.906572 139 401.908072 140 401.309572 141 401.911072 142 401-912572 143 401.914072 144 401.915572 145 401.917072 146 401.918573 147 401.920073 148 401.921573 149 401.923073 CHANNEL FREQUENCY 150 401.924573 151 401.926073 152 401.927573 153 401.929073 154 401.930573 155 401.932073 156 401.933573 157 401.935073 158 401.936573 159 401.938074 160 401.939574 161 401.941074 162 401.942574 163 401.944074 164 401.945574 165 401.947074 166 401.948574 167 401.950074 168 401.951574 169 401.953074 170 401.954574 171 401.956074 172 401.957575 173 401.959075 174 401.960575 175 401.962075 176 401.963575 177 401.965075 178 401.966575 179 401.968075 180 401.969575 181 401.971075 182 401.972575 183 401.974075 184 401.975575 185 401.977076 186 401.978576 187 401.980076 188 401.981576 189 401.983076 190 401.984576 191 401.986076 192 401.987576 193 401.989076 194 401.990576 195 401.992076 196 401.993576 197 401.995076 198 401.996577 199 401.998077 39 SELF-TIMED DCPRS DESIGN REQUIREMENTS 1. RF Power Output. The Effective Isotropic Radiated Power (EIRP) of a DCPRS and antenna shall not exceed 50 dBm under any combination of service conditions. 2. Frequency Characteristics. The DCPRS transmitted RF shall be in the 401.7-MHz to 401.85-MHz band. (See Table 1.) 3. Stability. A. Temperature. The transmitter carrier frequency shall change by less than 0.5 parts per million over the temperature range of -20°C to +50°C. B. Long-Term. The long-term stability (including temperature variations) shall be better than one part per million per year. C. Short-Term. The phase jitter on the transmit carrier shall be less than 3° RMS. 4. Electromagnetic Interference (EMI). All transmitter spurious emissions, when measured with modulation and with antenna and diplexer connected, shall be down from the unmodulated carrier level by 50 dB. 5. Transmission Format. After a minimum of 4.9 seconds of unmod- ulated carrier, the carrier shall be modulated with the bit and message synchronization patterns which are at least 2.4 seconds of alternate 1, 0 data bits, and the 46-bit preamble consisting of the 15-bit MLS sync word followed by the 31-bit BCH command word. Maximum duration of this preamble shall be 9.0 seconds. The binary data shall be Manchester-encoded 8-bit ASCII, odd parity, and shall modulate the carrier in the following manner: a data "0" shall consist of +60° (+5°) carrier phase shift for 5 milliseconds followed by -60° (+5 ) carrier phase shift for 5 milliseconds, and a data "1" shall consist of -60° carrier phase shift for 5 milliseconds followed by +60° carrier phase shift for 5 milliseconds. Data rate shall be 100 BPS +0.1 BPS. (See Figure 1.) 6. End of Transmission. Immediately after sending the sensor data, the DCPRS shall transmit three 8-bit ASCII, odd parity, End of Transmission (EOT) characters contiguously with the ASCII sensor data characters (no break) and return to the standby condition . 7. Fail Safe Design. The DCPRS shall incorporate a "fail-safe" design feature such that malfunctioning of the equipment shall in no way cause continuous transmission. 8. Antenna Polarization. Polarization shall be right-hand circu- lar, according to IEEE Standard 65.34.159. 41 9. Data Formatting Restrictions. The following ASCII control characters must not appear in the DCPRS message: DLE , NAK, SYN , ETB , CAN, GS , RS , SOH, STX, ETX, ENQ, and ACK. EOT characters may appear only at the end of transmission. 10. The DCPRS reporting time shall always be within 30 seconds of its assigned reporting time. 42 TABLE 1 SELF-TIMED DCPRS TRANSMIT FREQUENCIES Channel Frequency 1 401. 700996 2 401. 702495 3 401. 703994 4 401. 705493 5 401. 706992 6 401. 708491 7 401 709990 8 401. 711489 9 401. 712989 10 401 714488 11 401. 715987 12 401. 717486 13 401. 718985 14 401. 720484 15 401. 721983 16 401 723482 17 401. 724981 18 401. 726480 19 401 727979 20 401 729478 21 401 730977 22 401 732476 23 401 733976 24 401 735475 25 401 736974 26 401 738473 27 401 739972 28 401 741471 29 401 742970 30 401 744469 31 401 745968 32 401 747467 33 401 748966 34 401 750465 35 401 751964 36 401 753463 37 401 754962 38 401 756462 39 401 757961 40 401 759460 41 401 760959 42 401 .7^2458 43 401 . , 1^957 44 401 . V65456 45 401 766955 46 401 768454 47 401 .769953 48 401 771452 49 401 772951 Channel Frequency 50 401. 774450 51 401. 775949 52 401. 777449 53 401. 778948 54 401. 780447 55 401. 781946 56 401. 783445 57 401. 784944 58 401. 786443 59 401. 787942 60 401. 789441 61 401. 790940 62 401. 792439 63 401. 793938 64 401. 795437 65 401. 796936 66 401. 798435 61 401. 799935 68 401. 801434 69 401. 802933 70 401 804432 71 401 805931 72 401 807430 73 401 808929 74 401 810428 75 401 811927 76 401 813426 77 401 814925 78 401 816424 79 401 817923 80 401 819422 81 401 820922 82 401 822421 83 401 823920 84 401 825419 85 401 826918 86 401 828417 87 401 829916 88 401 831415 89 401 832914 90 401 834413 91 401 835912 92 401 837411 93 401 838910 94 401 840409 95 401 841908 96 401 843408 97 401 844907 98 401 846406 99 401 847905 43 DATA CLOCK MANCHESTER- CODED DATA +6CP CARRIER PHASE -60° 1 1 ' 1 1 l 1 1 1 1 1 ! 1 i- 1 1 1 1 1 ) | 1 i 1 i 1 i i 1 i • i i 1 ONE ZE RO 1 . ♦ 1 Figure 1. — Modulation Definition 44 APPENDIX D SOME CONSIDERATIONS IN THE DESIGN AND INSTALLATION OF A RECEIVING SYSTEM TO RECEIVE DCS DATA DIRECTLY FROM THE SMS/ GOES FAMILY OF SATELLITES December 1978 Prepared hy : JOHN J. NAGLE Office of System Engineering National Environmental Satellite Service National Oceanic and Atmospheric Administration Washington, D.C. 20233 45 CONTENTS I . Introduction 47 II. Equipment needed for a typical DCS direct readout station 48 III . DCS signal power 51 IV. Antenna/receiver considerations 54 V. Demodulator /decoder 57 VI . Frame synchronizer , 63 VII . Data processing equipment 63 VIII. Obtaining a direct readout receiving station 63 IX. Costs 63 APPENDIX A. Per-channel signal energy as a function of the number of active channels APPENDIX B. Calculation of the phase modulation introduced by the switched array antenna 46 ABSTRACT. The equipment needed to establish a readout receiving station for the Data Collection System (DCS) is discussed in general terms. The parameters and characteristics of the DCS are described as they affect ground station equipment design. I . INTRODUCTION The SMS/GOES1 family of meteorological satellites provides, along with other capabilities, a DCS to relay in situ environmental data obtained from a variety of sensors located in remote areas. Typical sensors measure hydrologic, oceanographic , meteorological, rnd other types of environmental parameters. The National Environmental Satellite Service (NESS) provides ground facilities to receive the data transmitted to the satellite from the user's ground-based sensors and makes these available to users at the World Weather Building, Camp Springs, Maryland. DCS users must supply the ground communication link between their own facilities and Camp Springs. Under some circumstances, users may wish to provide their own satellite ground receiving station facilities. i The equipment needed to receive DCS data directly from the satellite is both sophisticated and expensive; ordinarily this expense cannot be justified except where the communications links between the user and Camp Springs, Maryland, are long and expensive or where the time required to establish these links can compromise the usefulness of the data. Another consideration is that some emergency situations, e.g., floods, that the platforms are designed to monitor may destroy land line communication circuits from NESS to the user's processing facility. Further complicating the picture is the fact that a number of design tradeoffs are possible, so that there is no single optimum choice of equipment. This report lists some of the parameters of the DCS system and describes the manner in which they affect the components of a satellite receiving system. It is assumed that the reader has a basic knowledge of physics and some experience with satellite receiving systems. For the purpose of this discussion, the description of the DCS will will be divided into six parts. The first of these, Part II, is a brief description of a typical station for receiving satellite data. Part III discusses the signal level available on the Earth's XSMS stands for Synchronous Meteorological Satellite; GOES stands for Geostationary Operational Environmental Satellite. The SMS satellites are prototypes for the GOES satellites. 47 surface. Although this quantity is beyond the control of the user, if does determine the antenna and receiver noise temperature requirements . The remaining four items— antenna/receiver , demodulator/decoder, frame synchronizer, and data processing equipment--are under the user's direct control and are discussed in Parts IV, V, VI, and VII, respectively. A breakdown of the necessary equipment is shown in Figure 1 . II. EQUIPMENT NEEDED FOR A TYPICAL DCS DIRECT READOUT STATION The equipment that must be provided by the user in order to receive DC replies directly is listed below: ANTENNA The antenna is usually paraboloidal and must be mounted on a supporting structure or building; it should have an' unobstructed view of the satellite. For normal operations, the ground station antenna is pointed at a single satellite-~GOES East or GOES West. As these satellites are at fixed locations, the antenna can be adjusted to the proper look angles and locked in position. However, the mounting structure should be designed so that minor adjustments (say + 10 degrees) in both azimuth and elevation can be made conveniently to compensate for installation inaccuracies. Because the inclination of the satellite v/ill be held to about +0.1 degree, it should not ordinarily be necessary to reposition the antenna during normal operations . However, operation of the DCS may be changed from one satellite to another during eclipse periods, so that direct-readout users must reposition their antennas to another satellite to maintain continuity of operations. While the satellite is in eclipse, operational restraints are placed on the satellite, because of limited power available. These eclipse periods occur for intervals up to 72 minutes a day each day for 6 weeks, twice a year. If the antenna will be used with more than one satellite or family of satellites, it is recommended that repositioning of the antenna be done by remote control. The S-band transmissions from the SMS/GOES satellites are linearly polarized. If the full capability of the receiving antenna is to be realized, the antenna must also be linearly polarized so that it is parallel to the polarization of the received signal. Because of different aspect angles to different satellites, the polariza- tion of the signal cannot be predicted at the Earth's surface. For this reason, the polarization of the antenna should be adjustable, Where the antenna will be used with only one satellite, the 48 CO o Q < h. O — T CO o < Q. to H z: o UJ z ^* o > Q_ ixf 2 o O UJ ucc q: JjJoco-o^" L_CO_l£0 — 49 polarization angle can be adjusted for maximum received signal and locked in that position. If the antenna will be used with more than one satellite, it will be convenient to make the polarization as well as the antenna position remotely controlled. PREAMPLIFIER Considerable thought should be given to the preamplifier as it will play an important role in determining the error rate of the system and the size of the antenna. Many types of amplifiers are discussed in detail in the material that follows. For purposes of this discussion, the important parameter is noise temperature, with a low noise temperature being more desirable. To maximize performance of the system, it is standard practice to mount the preamplifier on the antenna, making the connection between the preamplifier and antenna as short as possible. For a similar reason, the downconverter is usually located as close to the preamplifier as practical. Several manufacturers' supply a pre- amplifier and downconverter in one package for direct mounting at the antenna's terminals. DOWNCONVERTER The purpose of the downconverter is to translate the satellite signal from the vicinity of 1694.5 MHz down to a more convenient frequency. If the user is independently assembling a station using a surplus VHF WEFAX or APT receiver2, the downconverter output frequency is usually in the vicinity of 137 MHz. Where the user is buying a complete package receiver, this frequency may be in the neighborhood of 70 MHz. Other conversion frequencies are also used. RECEIVING SYSTEM Although all the components discussed above can be considered part of the receiving system, the term "receiver" is generally reserved for that part of the receiving system that contains the power supplies and control circuits and provides most of the amplification. Special units may be acquired or available equipment adapted for this application. In addition to the usual functions provided by the receiving system, it is desirable that the receiver lock onto and track a pilot carrier, This carrier is transmitted from the Wallops CDA station and is returned on a nominal frequency of 1694.450 MHz. The receiver should track the deviation from this pilot carrier frequency (+ 20 kHz) to compensate for drift in the satellite translation frequency and control internal receiver drift as well. l_l Nagle, John J., "A Method of Converting the SMS/GOES WEFAX Frequency (1691 MHz) to the Existing APT/WEFAX Frequency (137 MHz)." Technical Memorandum NESS 54, NOAA/NESS Office of System Engineering; , April, 1974. 50 The input to the receiver must be compatible with the output of the downconverter , and the receiver output must be compatible with the input requirements of the demodulator. DEMODULATOR/DECODER The output of the receiver is fed to a separate demodulator/decoder external to the receiver at a convenient intermediate frequency (IF), The demodulator portion converts this IF signal to a series of d.c. pulses. The decoder portion separates the clock and the data stream to recover the information. If the user purchases a special receiver for DCS reception, the demodulator/decoder may be built into the receiver. In either case, the demodulator/decoder will recover the data from the phase shift signal. In principle, the output of the decoder should be the same as the sensor data input to the radio set transmitter at the remote platform. FRAME SYNCHRONIZER Following the decoder, a frame synchronizer must be provided to recognize the 15-bit Maximal Length Sequence (MLS) synchronizing data pattern and ensure that the data bits are outputted in the proper sequence. DATA PROCESSING EQUIPMENT Where a large amount of data is to be received, or where the data must be processed, automatic processing equipment is necessary. The design of this equipment depends on the requirements of the user and cannot be discussed in general . III. DCS SIGNAL POWER The energy available in the received signal plays an important role in determining the necessary quality and, hence, cost of the receiving system. Unfortunately, the received signal level can vary over a wide range; a lOdB variation may be typical (a numerical variation of 10). There are various causes for this variation, all of which are beyond the control of the user. The most probable cause is satellite loading; i.e., the number of other platforms transmitting at the same time. During emergency situations many more platforms may be programed to respond in their emergency mode than would normally be transmitting. Unfortunately, this tends to lower signal levels at a time when the data are most needed. The power radiated by the satellite in the DCS band under normal conditions is 2.5 watts. This power is known as the equivalent isotropic radiated power (EIRP). This 2.5 watts is available to be shared equally by all channels. Thus, if everything were ideal and 51 only one channel transmitting, the entire 2.5 watts would be available to this one channel. If two channels were transmitting simultaneously, the 2.5 watts would be shared equally between both channels. The power in each channel will be reduced by 3dB. (Note: A reduction of 3dB is equivalent to a numerical factor of one-half). Although there are 183 possible channels, statistically it is very highly unlikely that all 183 will ever be transmitting simultaneously. A more reasonable number of channels transmitting simultaneously might be 100, which will give a 20dB per channel reduction in signal level from 2.5 watts. (Note: A 20dB reduction corresponds to a numerical reduction of 100). The situation is not ideal, however. The satellite uplink receiver puts out noise which is rebroadcast by the transmitter along with the desired signals. This rebroadcast noise requires transmitter energy that would otherwise be available for useful signals. The amount of energy converted into noise is equivalent to approximately 10 simultaneous signal channels. This amounts to a reduction of lOdB in the useful, available power when a small number of channels are active. (Note: A reduction of lOdB is equivalent to a numerical reduction of one-tenth). When a large number of channels are active, say 100 or more, this noise component tends to be suppressed. The available energy is then equally distributed among the active channels. The power reduction for 100 channels is only 20dB instead of 30dB. (Note: 20dB is equivalent to a numerical value of 100 while 30dB is equivalent to 1000). This is discussed in more detail in Appendix A. From the above, it can be seen that available power per channel can vary from about 0.25 watts when only one channel is active to about 0.25 watts when 100 channels are transmitting simultaneously. This is a lOdB variation in the available received energy and can occur at random. Another factor to be considered is that, beginning with the GOES-2 satellite, a low-power mode is used operationally. In this mode the total satellite transponder output power is 0.4 watts or a reduction of 8dB from the 2.5-watt level. (Note: 8dB is a numerical reduction by one-sixth). However, the DCS signal will actually be reduced by only about 2dB (40 percent); this is because VISSR and S-VISSR are not transmitted in the low-power mode so, except for telemetry which is small, the entire transponder power is then available to the DCS. It is expected that the low-power mode will be used during the predictable eclipse periods. The free-space loss between isotropic antennas from synchronous altitude to the subsatellite point is 188dB; the free-space loss to Earth's edge is increased to 189. 3dB. The satellite trans- mitting antenna gain is 2dB less at Earth's edge than at the sub- satellite point. In addition, the radiation pattern of the satellites favors the Northern Hemisphere, so that there is an additional less of about 2dB at the Earth's edge in the Southern Hemisphere. The signal level that can be expected at Earth's edge in the Southern Hemisphere is, therefore, approximately 52 24dBm-(189.3 +2 +2) dB = -169.3dBm and about 2dB more in the Northern Hemisphere. (Note: In making signal level calculations it is convenient to express power levels in decibel form. Since a decibel represents a ratio of two power levels, it is necessary to specify a reference level when absolute power is specified in decibel form. By convention, one of two values is used as a reference. When a power level is specified in dBm, the reference is understood to be 1 milliwatt, 10 3 watts. When the power is expressed in dBw, the reference is understood to be 1 watt. Thus, +34dBm is a power level 34dB above 10 3 watts or 2.5 watts. Similarly, 4dBw is a power 4dB above 1 watt or 2.5 watts. A power level given in dBw may be converted to dBm, or vice versa, by adding or subtracting 30dB since the difference between 1 watt and 10 3 watts is 30dB). The natural noise against which the signal energy must compete is composed of two components. The first is cosmic noise with an effective noise temperature of about 75 degrees Kelvin ( K) for an elevation angle of 5 degrees or more. This noise will be nearly constant with increases occurring only when the antenna beam sweeps past radio stars or the Sun. The user has no control over this source of noise. The second major source of noise is the receiver. The noise contribution of the receiver is measured by its noise temperature; another term often used is noise figure. As the noise temperature of a receiver is difficult to measure directly, it is standard practice to measure the receiver noise figure and convert this to noise temperature. The noise temperature and noise figure are related by equation (1). NF = 10 log /Tree + l\ (1) ^Tref ) where vTp = receiver noise figure in dB Tree = receiver noise temperature (degrees Kelvin) Tref = reference temperature (degrees Kelvin see discussion below) log = is the logarithm to the base 10 The reference temperature, Tref, is usually taken to be in the vicinity of ambient room temperature, approximately 293°K. In the material that follows, a reference temperature of 289.855 K will be used. The reason for this choice is that when 289.855 K° is multiplied by Boltzmann's constant (1.38 x 10'23 joules per °K) 53 -21 the product is a round number (4 x 10 ) that is convenient to9 manipulate in both algebraic and logarithmic form (10 log 4 x 10~ = -204dBw per Hertz bandwidth). It is important to determine the reference temperature when comparing specifications from different manufacturers. IV. ANTENNA/ RECEIVER CONSIDERATIONS The noise temperature of a receiver is largely determined by the input stage of the receiver. Typical noise figures and noise temperatures of various commonly used input amplifiers in the 1.7 GHz frequency range are given below: Typical bipolar transistor Premium grade bipolar trans- istor Gallium-Arsenide (GAS) FET Noncooled (ambient temp.) parametric amplifier Cryogenic parametric amplifier NF dB No ise Temp. °K Approx. Cos 4 438 $ 350.00 3 289 $ 600.00 1.5 120 $ 1,500.00 0.69 50 $ 25,000.00 0.29 20 $100,000.00 (Note: These numbers are approximate as they neglect the noise contribution of succeeding stages. This should be small if the gain of the input amplifier is large, say lOdB or more. As an example, if a premium grade biopolar transistor amplifier with a noise temperature of 289°K is used, to which must be added the antenna noise temperature of 75°K, the result is a total system noise temperature of approximately 364°K. The noise power in a 1-Hz bandwidth is given by: -23 Noise power = 1.38 x 10 (joules per °K) x 365 °K - 5.02 x 10~21 watts/Hz = -203dBw/Hz = -173dBm/Hz As the DCS information is 100 bits/s, the postdetection band- width must be 100 Hz. The noise power in a 100-Hz bandwidth will therefore be 100 times (or 20dB) greater than the noise power in a 1-Hz bandwidth. The total noise power at the receiver output will be -153dBm/100 Hz. This is the natural noise against which the signal must compete. 54 In addition to natural noise described above, there is' a source of noise peculiar to the SMS/GOES family of satellites, called spin modulation or "spin mod." The SMS/GOES satellites are spin stabilized, which means they must spin continuously to maintain correct attitude. The spin rate is nominally 100 revolutions per minute (rpm). The antenna elements for the communications system are located around the periphery of the satellite. As the satellite spins, it is necessary to continuously switch the output of the trans- mitter to those elements that are facing the Earth. This is called an "electronically despun antenna." As an antenna element on one side of the satellite is passing out of view of the Earth and is being switched off, an antenna element on the opposite side of the satellite is coming into the Earth's view and is being switched on. It can, therefore, be visualized that the electrical center of the antenna rotates around the satellite in a direction opposite to the rotation of the satellite. Therefore, the antenna pattern is always pointing toward the Earth. The spin mod noise is peculiar to the electronically despun antennas that are used on the SMS-1 and 2, and GOES-1, 2, and 3 satellites. If the phase center of the antenna were located on the spin axis of the satellite, the problem would not exist. A more detailed descriptin of this source of noise is given in Appendix B. At the time this report is being written, it is expected that the next generation of GOES satellites (GOES-D, E, and F) will use a mechanically despun antenna. With this type Of antenna, the antenna mechanically spins in a direction opposite to the spin direction of the satellite, so that the antenna appears stationary with respect to the Earth. As there will be no switching of antenna elements, the phase center of the antenna will remain constant and there will be no spin mod. This will improve the operation of the system by reducing the noise against which the desired signal i.iust compete. The spin-mod has been measured as 40° peak-to-peak for Earth stations located on the same longitude as the satellite and 70° peak-to-peak for Earth stations located on the horizon at the Equator. Unfortunately, the desired signal information is also phase modulated on the carrier so that spin mod caused by the antenna switching appears as noise along with the desired information. This noise is in addition to the natural noise described previously, It is therefore necessary to have a higher carrier-to-noise ratio (CNR) than would otherwise be required. The CNR actually needed depends on the highest bit-error rate (BER) that is considered acceptable for the user. Considering 55 only natural noise (excluding spin modulation noise), the BER for the DCS system is given by r -0 794 — BER = 1/2 e u" 'y* N (2) where C/N is the CNR as a numeric (not in dB) and e = 2.71828. This equation is derived from statistical communication theory. Because of the spin modulation noise, the required CNR will be somewhat higher than given by equation (2). Direct measurements on GOES A, B, and C show that a CNR of 14dB on the DCS over natural noise is necessary to give a BER of 1 in 10 5. This CNR is greater than predicted by equation (2) because it takes into account the spin modulation noise which equation (2) does not consider. As mentioned earlier, it is expected that the GOES-D, E, and F generation of satellites will not have spin modulation noise. Lower CNR's, which are more in agreement with equation (2), should be usable with these satellites. The user should bear in mind that a CNR of 14dB is necessary for a BER of 1 in 105. If the user requires more accurate data (higher BER), then a higher CNR is necessary. This in turn will require a larger and more expensive antenna and/or lower noise preamplifier, Conversely, if a lower error rate is acceptable, a lower CNR is permissible, resulting in a more economical receiving system. Users should, therefore, carefully analyze their BER requirements to ensure that they obtain adequate accuracy at the lowest cost. As stated previously, the noise power at the receiver output will be -153dBm/100Hz and that a CNR of 14dB is necessary for a BER of 1 in 10s. Hence, a carrier level of -153dBm +14dB = -139dBm is required. As a signal level of only -169.3dBm can be expected the difference, -139dBm -(-169.3dBm) = 30.3dB, must be made up by using an antenna with 30.3dB. This requires a paraboloidal antenna with a diameter of 2.5 meters and assumes an aperture efficiency of 55 percent, which is typical for antennas of this type. If the DCS system is heavily loaded (100 channels transmitting), the received signal per channel will be lOdB lower and an antenna diameter of 3.16 times 2.5 meters = 7.9 meters will be necessary. (Note: The gain of a paraboloidal antenna is directly proportional to its area. Therefore, the antenna gain is proportional to the square of the diameter. Increasing the antenna diameter by /TO = 3.16 will increase its area and hence its gain by a numerical factor of 10. This is equivalent to a lOdB increase. ) If an uncooled parametric amplifier is used instead of a premium grade transistor amplifier, the receiver and antenna noise temperatures are now 50 K +75 K respectively = 125 K. The noise power in a 100-Hz bandwidth is now -157,6dBm instead of -153dBrn. This represents an improvement of 4.6dJ3 and a reduction in antenna 56 size by the same amount (4.6dB and a reduction in antenna area by the same amount (4.6dB = a reduction of 1.7 in antenna diameter). Hence, for the single channel case, the required diameter becomes 2.5 meters/1.7 = 1.47 meters and for the 100-channel loading case, 4.6 meters. These values can be used as points on two curves, one curve for a single channel loaded DCS, and the other for a fully loaded system. Points for the other types of preamplifiers can be plotted, and a smooth curve drawn between them. Figure 2 shows a family of curves giving the required antenna diameter as a function of system noise temperature for various numbers of simultaneous active channels. In using these curves the following conditions should be noted. 1. The abscissa (noise temperature) is the overall receiving system noise temperature and is the sum of the preamplifier noise temperature and the 75 K cosmic noise. The ordinate gives the required antenna gain. 2. A 3dB margin, as well as miscellaneous losses of 3dB, are included. 3. All calculations are based on the receiver being located at the subsatellite point. For convenience, Tables 1 and 2 give the margin obtained with some typical preamplifiers and antenna sizes of 3 and 10 meters. A wide variation in the available margin is evident. All users must bear in mind that the National Environmental Satellite Service (NESS) cannot guarantee that usage of the DCS will be limited to any maximum number of platforms, and therefore cannot guarantee a minimum signal level at the Earth's surface. Users must, therefore, carefully consider the consequences of lost or inaccurate data against the costs of building a better ground station . V. DEMODULATOR/DECODER To recover the data, the output of the receiver must be demodulated and decoded. A separate demodulator/decoder should be used for each DCS channel in use, although a single antenna-receiver combination can be used to drive a number of them. It is recommended that the demodulator/decoder be purchased from a manufacturer experienced with the GOES DCS. Four problem areas must be considered; these are: 1. The Spin/Phase Noise: As described earlier, the spacecraft rotation causes spin modulation which appears as noise at 57 CRYOGENIC PARAMP AMBIENT TEMP PARAMP co -< CO H m GAS-FET- oo m PREMIUM TRANSISTOR 4* o o TYPICAL TRANSISTOR Figure 2 Ol o 3 ANTENNA DIAMETER- METERS ro a o> oo o 1 I OJ CD 4* en en CL o. w CD OJ CO o. CD 4* 1 4* O ro cn • 4* o. CL CD CD ANTENNA GAIN AT 1690 MHZ -Antenna diameter as a function of system noise temperature for various numbers of channels. 58 Table 1.— Typical Gain Marg ins Usi ng a 2-Meter Antenna Number of Simultaneous Active Channels Premium Grade Transistor Preamp T +T , =364°K (=25.6dB-°K) rcvr sky Gallium- Arsenide FET T +T , =195°K (=22.9dB-°K) rcvr sky 1 10 50 100 1 10 50 100 EIRP dBm +34 +34 +34 +34 34 34 34 . 34 Share Loss + Noise Power dB -10.4 -13 -17.8 -20.4 -10.4 -13 -}7.8 -20.4 Free Space Loss dB -188 -188 -188 -188 -188 -188 -188 -188 '■ Receiver Antenna Gain dB (55%) +32 +32 +32 +32 32 32 32 32 (Antenna Diameter-Meters) (3) (3) (3) (3) (3) (3) (3) (3) Rx Input Power Level dBm -132.4 -135 -139.8 -142.4 -132.4 -135 -139.8 -142.4 System Noise Temp. dB-°K .25.6 25.6 25.6 25.6 22.9 22.9 22.9 22.9 Boltemans Constant (dBm/Hz - °K) -198.6 -198.6 -198.6 -198.6 -198.6 -198.6 -198.6 -198.6 Rx Input Noise No dBm/Hz -173 -173 -173 -173 -175.8 -175. -175. -175. Rx Input C/N0 dB/Hz 40.6 38.0 33.2 30.6 43.4 40.8 35.9 33.3 Required C/^ (dQ_Ife) 34 ; 34 I 34 34 34 34 34 34 Miscellaneous Losses dB 3 3 3 3 3 3 3 3 Margin (dB) 3.6 1.0 -3.8 -6.4 6.4 3.8 -1.1 -3.7 * Field Effect Transistor. 59 Table 2. — Typical Gain Marg ins Using a 10-Meter Antenna Number of Simultaneous Active Channels Premium Grade Transistor Preamp. Trcvr+Tsky=3640K (=25.6dB-°K) * Gallium-Arsenide FET Preamp. Trecr+'Isky=1950K <=22-9dB-°K> 1 10 50 100 1 10 50 100 EIRP dBn +34 +34 +34 +34 34 34 34 34 Share Loss + Noise Power dB -10.4 -13 -17.8 -20.4 -10.4 -13 -17.8 -20.4 Free Space Loss dB -188 -188 -188 -188 -188 -188 -188 -188 Receiver totenna Gain db (55%) 42.5 42.5 42.5 42.5 42.5 42.5 42.5 42.5 (Antenna Diameter-Meters) (10) (10) (10) (10) (10) (10) (10) (10) Rx Input Power Level dBn -121.9 -124.5 -129.3 -131.9 -121.9 -124.5 -129.3 -132.0 System Noise Temp. dB-°K 25.6 25.6 25.6 25.6 22.9 22.9 22.9 22.9 ! Boltzmans Constant (dBn/Hz - OK) -198.6 -198.6 -198.6 -198.6 -198.6 -198.6 -198.6 . -198.6 Rx Input Noise No dEm/IIz -173 -173 -173 -173 -175.7 -175.7 -175. 7 -175.7 Rx Input C/^ dB/Hz 51.1 48.5 43.7 41.1 53.8 51.2 46.4 43.7 Required C/M 34 34 34 34 34 34 34 34 Miscellaneous Losses dB 3 3 3 3 3 3 3 3 Margin (dB) 14.1 11.5 6.7 4.1 16.8 14.2 9.8 6.7 * Field Effect Transistor. 60 the demodulator output. Unfortunately, the switching rate of the electronically despun satellite antenna is fairly close to the data rate; therefore, the decoder must be able to differentiate between valid data and spin noise. 2. Manchester Coding: The DCS uses Manchester coding to transmit data through the system. This type of coding has the advantage that the clock rate is imbedded in the data stream and can be recovered from the data by the decoder. It is, therefore, not necessary to transmit the clock information separately or to impose extreme stability requirements on the oscillator at the remote platform. Manchester coding may be described in terms of digital logic as the Exclusive OR function between the clock and data. The Truth Table for the Exclusive OR function is given below: TRUTH TABLE EXCLUSIVE OR DATA CLOCK 0 1 0 0 1 1 1 0 As can be seen, the output is zero if the clock and data are both one or zero, but is one if the clock and data are opposite. The advantage of this may be seen in the timing diagram (Figure 3). Regardless of the input data, the Manchester-encoded data will change state once and only once during each input data bit. These changes may be used to synchronize the decoder clock. In this manner the remote platform clock information will be carried along with the data. 3. Pattern Sensitivity: As stated above, the data collection platforms use a Manchester Code to transmit data. The decoder must be capable of extracting the clock rate from the data in the presence of the spin noise described above. Furthermore, it must be able to do this with any possible data bit pattern. The GOES DCS has standardized on the use of ASCII code with odd parity. The decoder, therefore, must be able to accommodate data having as few as two "bit transitions" per 8-bit character. Otherwise, loss of synchronization will permit errors to be received until the decoder resynchronizes. Unfortunately, consecutive zeros can represent valid data from some types of platforms. Some of the electronic techniques which can be used to minimize the spin modulation noise complicate the clock slip problem. 61 Lio.oi-t2*| CLOCK 100 HZ DATA 0 0 0 APPLY THE EXCLUSIVE OR TRUTH TABLE AND OBTAIN AS MANCHESTER- ENCODED DATA NOTICE THAT THE ENCODED DATA CHANGES STATE ONCE DURING EACH INPUT DATA BIT FIGURE 3 62 4. Interface : In addition to the above, the demodulator/decoder must properly interface with the receiver which immediately precedes it and with the frame synchronizer equipment which it feeds. VI. FRAME SYNCHRONIZER After the signal has been decoded and the 100-Hz clock in the decoder "locked up" by the 2.5-second alternating 1-0 pattern, the decoder is ready to present data to the outside world. The next segment of the reply format is a 15-bit Maximal Length Sequence (MLS) pattern. The 15-bit MLS is recognized by a device called a "frame synchronizer" which must wait for decoder lock before looking for the 15-bit MLS. At the conclusion of the MLS, the frame synchronizer must be ready to present the received data in 8-bit segments to the data processing equipment . VII. DATA PROCESSING EQUIPMENT It is not possible to recommend a specific system for processing the received data, as this depends on the requirements of the user and on how the remote sensor outputs its data to the radio set. The cost of data processing system can be minimized by considering the processing, if any, that is done at the platform and user's data processing equipment. Obviously, the form of the data provided by the sensor and the computing system must be designed together if a cost effective system is to be developed. These depend on the user's specific requirements and can only be defined by the user. Generally, the data processing equipment must, as a minimum, include the following: A. A proper interface to the frame synchronizer. B. Interpretation of the 31-bit DCP address. C. Parity error checking. D. Storage of the data. E. A means of retrieving the data from storage. VIII. OBTAINING A DIRECT READOUT RECEIVING STATION The above material has briefly described system parameters; readers should bear in mind that NOAA/NESS cannot provide designs for particular users. Anyone considering establishing his own direct readout facility should contact vendors experienced in this area. IX. COSTS The following are rough estimates on costs of the various components: 63 1 . Antenna and Feed (Excluding Mounting Structure ) Diameter (meters) Cost (U.S. dollars) 1.2 (= 4 feet) 500 1.8 (= 6 feet) 800 2.4 (= 8 feet) 1,200 3.0 (= 10 feet) 1,750 3.6 (= 12 feet) 3,600 4.6 (= 15 feet) 8,950 9.1 (= 30 feet) 25,000 Preamplifier Typical bipolar transistor: 350 Premium grade bipolar transistor: 600 Gallium-arsenide FET: 1,500 Ambient temperature parametric amplifier: 25,000 Cryogenic parametric amplifier: 100,000 3. Receiver 3,000 4. Demodulator: (Includes 4,000 channel filter) 5. Data processing equip- (Depends on application) ment : 64 Note: This report describes the equipment and operation requirements for the GOES/DCS in a general way. Additional information about the DCS system and its use can be ob- tained by writing to: David S. Johnson, Director (S) National Environmental Satellite Service FOB #4, Room 2069 Washington, D.C. 20233 Readers should bear in mind that the U.S. Government cannot comment on the relative merits of a particular design of a particular vendor. 65 APPENDIX A PER-CHANNEL SIGNAL ENERGY AS A FUNCTION OF THE NUMBER OF ACTIVE CHANNELS The Equivalent Isotropic Radiated Power (EIRP) on a per-channel basis is given by W EIRP/,-,. -, = watts (in watts per channel) (1) X + N where W = the total EIRP available over the 400-kHz DCS channel in watts (not in dBm). This is normally 2.5 watts, X = the number of equal power users transmitting simultaneously. The non- equal power users case will be discussed later, and N = the noise-to-signal ratio at the input to the spacecraft receiver. This ratio must be converted into a numerical form and not expressed in dB. The noise power is calculated over the entire 400-kHz DCS bandwidth and not over just the 100-Hz DCS data bandwidth. Typical numbers are: noise power over a 400-kHz bandwidth at the input to the spacecraft receiver = -117dBm and the signal energy from a fixed platform is approximately -127dBm. Hence, the noise-to- signal ratio is nominally lOdB which also happens to be a numerical ratio of 10. The EIRP as given by equation (1) will be given in watts which must be converted into an equivalent dBm number for most link calculations. Applying the above to equation (1) gives, for typical cases, 2.5 E IRP = 'in watts/channel. (2) As can be seen from equation (2), when the number of channels transmitting simultaneously is small, say less than about five, the EIRP per channel is approximately constant at about 0.2 watts (= 23dBm) per channel. When the number of channels transmitting simultaneously is large, greater than say about 20, the EIRP will decrease linearly in watts as the number of channels transmitting increases. 67 Equations (1) and (2) assume that all platforms are received at the same level by the spacecraft. Where the signal levels of the platforms as received at the spacecraft are not the same, as may be the case when fixed and buoy platforms are received simultaneously, the equation is slightly different. In this case equation (1) becomes W ftrp/ watts (watts per (3) 'Channel Z (X + N ) channel) where W = EIRP in watts, as before a number of channels received simultaneously at a given level, and N = noise-to-signal ratio (expressed as numeric) of those channels. The summation is taken over all the different power levels and their respective noise-to-signal levels. Equation (2) or (3) can also be used when the satellite is trans- mitting in low-power mode., as it will be during eclipse periods beginning with GOES-2 satellite. In this case, the numerator is 0.4 watts (+26dBm) instead of 2.5 watts. 68 APPENDIX B CALCULATION OF THE PHASE MODULATION INTRODUCED BY THE SWITCHED ARRAY ANTENNA The UHF antenna used for SMS is switched array. As the satellite spins, the transmitter and receiver are disconnected from one set of antenna elements and connected to another set (one element for transmit and one for receive). Since the radiating elements are in separate locations about the satellite periphery, the propaga- tion path length from antenna to Earth terminal suddenly changes when the elements are switched. This effect causes a jump in RF carrier phase. Also, because the phase center of the UHF satellite antenna is located off the axis of spin rotation, the propagation path length to the Earth terminal varies as the satellite rotates. This path variation introduces a phase (frequency) modulation on the trans- mitted and received carriers which may degrade link performance. Figure B-l , shows the basic geometry, and Figure B-2 shows the phase shift as a function of time. The step in phase occuvs when a new antenna is switched. The fundamental frequency of this phase modulation is eight times the spin rate (13.3Hz at 100 rpm). The worst case occurs on the DCP interrogation and reporting links with the DCP on Earth edge at the equator (in the plane of spin motion). From Figure B-l, AL = R - R Cos (0 - a) = R (1 - Cos (G - a) = 2R sin2 6 - d where R is the antenna radius, a is the angle from the vertical to the Earth terminal (in the equatorial plane); and 6 is the position of the UHF antenna element relative to the vertical. Also, A* = AL (360°), ♦Obtained from the Ford Aerospace and Communications Corp. 69 for a = -9.2 (Earth edge +0.5 tolerance), 9 - 22.5°, and R = 26 in. Then, 3 = B - a= 31.7° and the phase shift is 47.7° at 401.9MHz and 55.6° at 468.8MHz. A new element is switched in at this point, changing G to -22.5 and 3 to -13.3°. The phase shift jumps to 8.6° at 401.9MHz and 10.0° at 468.8MHz. The step change in phase due to switching is 39.1° at 401.9MHz and 45.6° at 468.8MHz. This phase step occurs once every 75 ys for a spin rate of 100 rpm. Figure B-3 shows how the peak phase and phase jump varies with a. 70 ANTENNA PHASE CENTER AL R - R cos(0 -*u) 0 a - 2Rsin" ( T~ > ANTEMNA. GROUND TERMINAL VERTICAL WHEN ANTENNA PHASE CENTER ROTATES TO B. THE ARRAY IS SWITCHED AND THE PHASE CENTER JUMPS TO A. THIS CAUSES A JUMP IN PHASE OF THE RF CARRIER. Figure Rl.— Geometry of Propagation Path Length to ground Variation Resulting from Satellite Spin Motion 71 FREQUENCY: 468.8 MHz (401.9 MHz) EARTH TERMINAL LOCATED ON EQUATOR AT EARTH EDGE (a = 9.18) ANTENNA RADIUS: 26 INCHES (AL = 0) ■1/8 SPIN PERIOD- Figurc 32. — Phase Shift Introduced by Satellite Spin Motion and Element Switching in UHF Antenna. 72 60 r- 50 - 40 - CO LU LU o LU 30 - 20 10 - • ANTENNA SWITCHED ± 22.5J OFF VERTICAL • RADIUS = 26 INCHES PEAK PHASE DEVIATION 4 6 a - DEGREES PHASE JUMP EARTH EDGE + 0.5° 10 Figure S3.— phase Modulation Caused by UHF Satellite Antenna Due to Spin Motion . 73 APPENDIX E Certification Specifications for International DCPRS 1. RF Power Output The Effective Isotropic Radiated Power (EIRP) of a DCPRS and antenna shall not exceed 52 dBm under any combination of service conditions. 2. Frequency Characteristics The DCPRS transmitted RF shall be in the 402. 002 57 7- MHz to 402.098582-MHz band. (See Table 1.) 3. Stability A. Temperature and Long Term The transmitted carrier frequency stability shall be better than 1.5 parts per million against temperature variations and aging altogether. This specification applies typically over the temperature range of -20°C to +50°C and over 1 year, unless specified differently by the DCP operator. B. Short Term The phase jitter on the transmit carrier shall be less than 3° RMS when measured through a phase lock loop two-sided noise bandwidth (2BL) of 20 Hz and within ± 2 KHz. (See Fig. 1. ) 4. Electromagnetic Interference Any transmitter spurious emissions, when measured with modulation and with antenna and diplexer connected, shall be down from the unmodulated carrier level by 60 dB, (referred to a measurement bandwidth of 500 Hz). International ship- board EMI specification should also be included. 5. Transmit Data After 5 seconds of unmodulated carrier, the carrier shall be modulated with the bit and message synchronization data patterns which are 2.5 seconds of alternate 1,0 data bits, and the 46-bit preamble consisting of the 15-bit MLS synch word (100010011010111) followed by the 31-bit BCH address word (different for each platform), and a message proper consisting of 8-bit data words. The binary data shall be Manchester encoded and shall modulate the carrier in the following manner: A data "0" shall consist of +60° carrier phase shift for 5 milliseconds followed by -60° carrier phase shift for 5 milliseconds, and a data "1" shall consist of -60° carrier phase shift for 5 milliseconds followed by +60° carrier phase shift for 5 milliseconds. (See Fig. 2.) The phase of the 5 seconds unmodulated carrier shall correspond to the phase of the modulated carrier. 75 End of Transmission Immediately after sending the sensor data, the DCPRS shall transmit 31-bit End-of-Transmission (EOT) ,code (bit pattern 0010000010111010100111100011) continuously with the sensor data (no break) and return to the standby mode. The first 8 bits of the 31-bit EOT shall be the International Alphabet No. 5, 8 bits EOT (ODD parity Po): First transmitted bit - 0010000010111011010100111100011 last transmitted I | bit IA. No. 5 EOT 31 bits PRN sequence 7. Fail-Safe Design The DCPRS shall incorporate a fail-safe design feature such that malfunctioning of the equipment shall in no way cause discontinuous transmission. Furthermore, provision shall be made to automatically terminate the transmission at a time not to exceed the platform's allocated transmission plus 30 seconds. 8 . Antenna Polarization Polarization shall be right-hand circular according to CCIR Report No. 321 (XHIth Plenary Assembly, 1974 - Vol. XII). 9. Timing Accuracy The timer that determines the DCPRS reporting time shall be of sufficient accuracy to ensure that the DCPRS reporting time is maintained to within 30 seconds of its assigned reporting time. The timer shall provide for a reporting interval of 1 to 12 hours in 1-hour steps. Furthermore, the timer shall be capable of being set in steps of 60 seconds. 10. Clock Output The DCPRS shall provide a 100 Hz clock frequency. This frequency shall be used to clock in the reply data. The 100-Hz clock frequency shall have a long-term temperature stability better than 50 parts per million. 11 . Data Input The DCPRS shall accept, from an interface unit with environmental sensors or manual data input device, a serial bit flow NRZ-L, 100 bits/s coded in International Alphabet No. 5. (See Fig. 3.) 76 12. Start Signal The DCPRS shall provide a start signal at the required time of transmission. This start signal generated from the timer will initiate the read-out of data from the interface unit. 77 TABLE 1 FREQUENCY ALLOCATION FOR INTERNATIONAL DCP RESPONSE CHANNEL No. of Channel 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 Frequency MHz 402.002577 402.005577 402.008577 402.011577 402.014577 402.017578 402.020578 402.023578 402.026578 402.029578 402.032578 402.035579 402.038579 402.041579 402.044579 402.047579 402.050579 402.053579 402.056580 402.059580 402.062580 402.065580 402.068580 402.071580 402.074581 402.077581 402.080581 402.083581 Remarks 78 No. of Frequency Channel MHz Remarks 29 402.086581 30 402.089581 31 402.092581 32 402.095582 33 402.098582 79 DATA CLOCK MANCHESTER- CODED DATA +6CP CARRIER PHASE -60° 1 I 1 1 1 1 i i l i i ! 1 1 1 I I i 1 i i 1 1 1 i t i • i i - ONE ; ze RO i i i Figure 1. — Modulation Definition ■tiUS. SOVBWMDfT PRIffTINS OFFICE : 1979 0-281-C*7 (2X0) 80 (Continued from Inside front cover) NOAA Technical Reports NESS 55 The Use of Satellite-Observed Cloud Patterns in Northern Hemisphere 500-mb Numerical Analysis. Roland E. Nagle and Christopher M. Hayden, April 1971, 25 pp. plus appendixes A, B, and C. (COM-73-50262) NESS 57 Table of Scattering Function of Infrared Radiation for Water Clouds. Gilchl Yamamoto, Masayuki Tanaka, and Shoji Asano, April 1971, 8 pp. plus tables. (COM-71-50312) NESS 58 The Airborne ITPR Brassboard Experiment. W. L. Smith, D. T. Hilleary, E. C. Baldwin, W. Jacob, H. Jacobowitz, G. Nelson, S. Soules, and D. Q. Wark, March 1972, 74 pp. (COM-72-10557 ) NESS 59 Temperature Sounding From Satellites. S. Fritz, D. Q. Wark, H. E. Fleming, W. L. Smith, H. Jacobowitz, D. T. Hilleary, and J. C. Alishouse, July 1972, 49 pp. (COM-72-50963) NESS 60 Satellite Measurements of Aerosol Backscattered Radiation From the Nimbus F Earth Radiation Budget Experiment. H. Jacobowitz, W. L. Smith, and A. J. Drummond, August 1972, 9 pp. (COM-72- 51031) NESS 61 The Measurement of Atmospheric Transmittance From Sun and Sky With an Infrared Vertical Sounder. W. L. Smith and H. B. Howell, September 1972, 16 pp. (COM-73-50020) NESS 62 Proposed Calibration Target for the Visible Channel of a Satellite Radiometer. K. L. Coulson and H. Jacobowitz, October 1972, 27 pp. (COM-73-10143) NESS 63 Verification of Operational SIRS B Temperature Retrievals. Harold J. Brodrick and Christopher M. Hayden, December 1972, 26 pp. (COM-73-50279) NESS 64 Radiometric Techniques for Observing the Atmosphere From Aircraft. William L. Smith and Warren J. Jacob, January 1973, 12 pp. (COM-73-50376) NESS 65 Satellite Infrared Soundings From NOAA Spacecraft. L. M. McMillin, D. Q. Wark, J.M. Siomkajlo, P. G. Abel, A. Werbowetzki, L. A. Lauritson, J. A. Pritchard, D. S. Crosby, H. M. Woolf, R. C. Luebbe, M. P. Weinreb, H. E. Fleming, F. E. Bittner, and C. M. Hayden, September 1973, 112 pp. (COM-73-50936/6AS) NESS 66 Effects of Aerosols on the Determination of the Temperature of the Earth's Surface From Radi- ance Measurements at 11.2 m. H. Jacobowitz and K. L. Coulson, September 1973, 18 pp. (C0M-74- 50013) NESS 67 Vertical Resolution of Temperature Profiles for High Resolution Infrared Radiation Sounder (HIRS). Y. M. Chen, H. M. Woolf, and W. L. Smith, January 1974, 14 pp. (COM-74-50230) NESS 68 Dependence of Antenna Temperature on the Polarization of Emitted Radiation for a Scanning Mi- crowave Radiometer. Norman C. Grody, January 1974, 11 pp. (C0M-74-50431/AS) NESS 69 An Evaluation of May 1971 Satellite-Derived Sea Surface Temperatures for the Southern Hemisphere. P. Krishna Rao, April 1974, 13 pp. (COM-74-50643/AS) NESS 70 Compatibility of Low— Cloud Vectors and Rawins for Synoptic Scale Analysis. L. F. Hubert and L. F. Whitney, Jr., October 1974, 26 pp. (COM-75-50065/AS) NESS 71 An Intercomparison of Meteorological Parameters Derived From Radiosonde and Satellite Vertical Temperature Cross Sections. W. L. Smith and H. M. Woolf, November 1974, 13 pp. (COM-75-10432) NESS 72 An Intercomparison of Radiosonde and Satellite-Derived Cross Sections During the AMTEX. W. C. Shen, W. L. Smith, and H. M. Woolf, February 1975, 18 pp. (COM-75-10439/AS) NESS 73 Evaluation of a Balanced 300-mb Height Analysis as a Reference Level for Satellite-Derived soundings. Albert Thomasell, Jr., December 1975, 25 pp. (PB-253-058) NESS 74 On the Estimation of Areal Windspeed Distribution in Tropical Cyclones With the Use of Satel- lite Data. Andrew Timchalk, August 1976, 41 pp. (PB-261-971) NESS 75 Guide for Designing RF Ground Receiving Stations for TIROS-N. John R. Schneider, December 1976, 126 pp. (PB-262-931) NESS 76 Determination of the Earth-Atmosphere Radiation Budget from NOAA Satellite Data. Arnold Gruber, November 1977, 31 pp. (PB-279-633) NESS 77 Wind Analysis by Conditional Relaxation. Albert Thomasell, Jr., January 1979. PENN STATE UNIVERSITY LIBRARIES AD00D7aDl""a The National Oceanic and Atmospheric Administration was established as part of the Department of Commerce on October 3. 1970. The mission responsibilities of NOAA are to assess the socioeconomic impact of natural and technological changes in the environment and to monitor and predict the state of the solid Earth, the oceans and their living resources, the atmosphere, and the space environment of the Earth. The major components of NOAA regularly produce various types of scientific and technical informa- tion in the following kinds of publications:, . % PROFESSIONAL PAPERS — Important' -.definitive research results, major techniques, and special "iiYves-- tigations. CONTRACT AND GRANT REPORTS — Reports prepared by contractors or grantees under NOAA sponsorship. 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