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Full text of "Investigation of site coverage and associated problems at the O'Hare Airport, Chicago, Illinois, enroute radar beacon test site"

Report No. FAA-RD- 73-49 

INVESTIGATION OF SITE COVERAGE AND ASSOCIATED 

PROBLEMS AT THE O'HARE AIRPORT, CHICAGO, ILLINOIS, 

ENROUTE RADAR BEACON TEST SITE 



U56 in 



George F. Spingler 










APRIL 1973 V^ 



INTERIM REPORT 



Document is available to the public through 
the National Technical Information Service, 
Springfield, Virginia 22151 



Prepared for 

IRAN DEPARTMENT OF TRANSPORTATION 

FEDERAL AVIATION ADMINISTRATION 

TL 
7X5. iT7 Systems Research & Development Service 

Washington D. C, 20591 



iransporwioh 

CENTER 
LIBRARY 






NOTICE 

This document is disseminated under the sponsorship 
of the Department of Transportation in the interest of 
information exchange. The United States Government 
assumes no liability for its contents or use thereof. 



\ 



1 . Report No. 

FAA-RD-73-49 



2. Government Accession No. 



3. Re 




ge 



3 5556 031 147283 



4. Title and Subtitle 

•^INVESTIGATION OF SITE COVERAGE AND ASSOCIATED 

PROBLEMS AT THE O'HARE AIRPORT, CHICAGO, ILLINOIS, 
ENROUTE RADAR BEACON TEST SITE^) 



5. Report Date 



April 1973 



6. Performing Organization Code 



7. Author's) 

George F. Spingler 



8. Performing Organization Report No. 

FAA-NA-73-28 



9. Performing Organization Name and Address 

^-'Federal Aviation Administration 

i-National Aviation Facilities Experimental Center 
Atlantic City, New Jersey 08405 



10. Work Unit No. (TRAIS) 



1 1 . Contract or Grant Nc 



Project No. 031-241-020 



12. Sponsoring Agency Nome and Address 

Department of Transportation 
Federal Aviation Administration 
Systems Research and Development Service 
Washington D.C. 20591 



13. Type of Report and Period Covered 

Interim 
September 1972-September 1972 



14. Sponsoring Agency Code 



15. Supplementary Notes 



16. Abstract 

A temporary beacon test site was installed adjacent to the Chicago, Illinois, 
O'Hare Airport and operational tests were conducted to determine its suita- 
bility for possible use as a future enroute radar beacon site. Photographic 
data were collected using "targets-of -opportunity" flying within the coverage 
area of the test site. The data were analyzed at NAFEC to determine the 
extent of the radar beacon coverage and further scrutinized to uncover any 
anomolies which might derogate the operation of an enroute radar beacon site 
installed at the test location. As a result of the initial data analysis, 
flight tests were conducted in the vicinity of the O'Hare Enroute Radar Beacon 
Test Site using a NAFEC jet aircraft. The NAFEC flight tests confirmed the 
originally-suspected problem areas and provided additional justification for 
linking the anomolies to the vertical radiation pattern of the standard radar 
beacon directional antenna. The total test effort showed that: (1) the 
procedure of using a temporary beacon test facility to determine coverage and 
problem area of future radar beacon sites was sound, and (2) that this pro- 
cedure should be utilized whenever there is some question about the adequacy 
of the coverage that a future site might provide. 



17. Key Words 

Portable beacon test site 

ATCRBS 

Site coverage 

Temporary beacon test facility 



18. Distribution Stotement 



Document is available to the public 
through the National Technical Information 
Service, Springfield, Virginia 22151 



19. Security Classif. (of this report) 

Unclassified 



20. Security Classif. (of this page) 

Unclassified 



21. No. of Pages 

61 



22. Price 

$3.00 PC 

$ .95 MF 



Form DOT F 1700.7 (8-72) 



Reproduction of completed page authorized 



PREFACE 



I wish to express my appreciation to my associates from the Great Lakes 
Region who participated in the testing of the O'Hare Airport Enroute Radar 
Beacon Test Site. 

I also wish to thank Air Traffic Controllers Robert J. Lucas, (NAFEC) , 
Duane L. Johnson (NAFEC), and Joseph Chaloka (Great Lakes Region); pilots 
Kenneth B. Johnson, Jesse S. Terry, and Fredrick G. Auer ; photographers 
John J. Bradley and James P. McGrail; engineer Barry J. Saltzman; and 
technicians Stanley L. Scull and Joseph H. Reed for their much needed 
assistance. 



in 



Digitized by the Internet Archive 

in 2012 with funding from 

CARLI: Consortium of Academic and Research Libraries in Illinois 



http://www.archive.org/details/investigationofsOOspin 



TABLE OF CONTENTS 



INTRODUCTION 



Page 



Purpose 1 

Background 1 

Description of Site Equipment 1 

DISCUSSION 4 

General 4 

Description of Flight Tests 7 

Targets-of-Opportunity 7 

Flight Inspection Aircraft Flight Tests 7 

High-Altitude Flight Tests 7 

Low-Altitude Flight Tests 9 

NAFEC Aircraft Flight Tests 9 

Results of the Flight Tests 14 

Results of the Targets-of-Opportunity Tests 14 

Site Coverage 14 

Reflected Beacon Replies 20 

Vertical Lobing 23 

Results of FIDO Aircraft Flight Tests 25 

High-Altitude Flight Tests 25 

Low-Altitude Flight Tests 25 

Results of NAFEC Aircraft Flight Tests 28 

CAUSES OF SITE PROBLEMS AND POSSIBLE SOLUTIONS 40 

Reflected Beacon Reply Problems 40 

Vertical Lobing Problems 44 

Future Expansion of the Beacon Test Site Area 45 

CONCLUSIONS 47 

APPENDIX 



v 



LIST OF ILLUSTRATIONS 



Figure Page 

1 Aerial Photograph Showing Relationship of the 2 

Beacon Test Site to the O'Hare Airport 

2 Layout of Beacon Test Site Showing Temporary 3 

Tower and Equipment 

3 Equipment Layout as Installed Inside the 5 

Equipment Van 

4 Flight Path of FIDO High-Altitude Flight Test 8 

5 Flight Path of FIDO Low-Altitude Flight Test 10 

Conducted 21 September in the Morning 

6 Flight Path of FIDO Low-Altitude Flight Test 11 

Conducted 21 September in the Afternoon 

7 Flight Path of FIDO Low-Altitude Flight Test 12 

Conducted 22 September in the Morning 

8 , NAFEC Aircraft Convair 880M Used for Flight Tests 13 

9 Flight Path of NAFEC Aircraft Coverage Flight Tests 15 

10 Two Examples of Reflected Beacon Replies Recorded 21 

South of the Beacon Test Site During Targets-of- 
Opportunity Tests 

11 Two Examples of Reflected Beacon Replies Recorded 22 

North of the Beacon Test Site During Targets-of- 
Opportunity 

12 Loss of Beacon Replies Due to Vertical Lobing on 24 

the 13° Radial of the Beacon Test Site 

13 Loss of Beacon Replies Due to Vertical Lobing on 26 

the 84° Radial of the Beacon Test Site 

14 Loss of Beacon Replies Due to .Vertical Lobing on 27 

the 172° Radial of the Beacon Test Site 

15 Airborne System Equipment Configuration Installed 29 

in NAFEC Aircraft N 112 



VI 



LIST OF ILLUSTRATIONS (continued) 

Figure Page 

16 Synchronization System Block Diagram 30 

17 Modified Standard Gain Horn Antenna That was Used 31 

at the Beacon Test Site to Transmit the "Synchronizing" 
Interrogation 

18 Data From the Beacon Test Site That was Recorded, in 33 

the NAFEC Aircraft, Using the Synchronizing System 

19 Reflected Beacon Replies Recorded South of the Beacon 35 

Test Site During the NAFEC Aircraft Flight Tests 

20 Reflected Beacon Replies Recorded North of the Beacon 36 

Test Site During the NAFEC Aircraft Flight Tests 

21 Loss of Beacon Replies Due to Vertical Lobing on 13° 38 

Radial of the Beacon Test Site During NAFEC Aircraft 
Flight Tests 

22 Loss of Beacon Replies Due to Vertical Lobing on 84° 39 

Radial of the Beacon Test Site During NAFEC Aircraft 
Flight Tests 

23 Reduction of Beacon Replies Due to Vertical Lobing on 41 

170° Radial of the Beacon Test Site During NAFEC 
Aircraft Flight Tests 

24 Panoramic View of Terrain Surrounding the Beacon Test 42 

Site From a Height of 75 Feet 

25 Future Development Planned for Beacon Test Site Area 46 



vix 



INTRODUCTION 



PURPOSE . 

The purpose of this effort was to conduct tests in the vicinity of the Chicago 
O'Hare Airport to determine the suitability of a test site location for possible 
use as a future enroute radar beacon site. The coverage (within 150 miles) of 
the O'Hare Airport Radar Beacon Test Site was investigated as well as any 
problems which might derogate the operation of an enroute radar beacon site 
installed at the test location in the future. 

BACKGROUND . 

In August 1971, the Central Region of the Federal Aviation Administration (FAA) 
requested that the National Aviation Facilities Experimental Center (NAFEC) 
participate in an investigation of reflected radar beacon replies at the 
McCook Enroute Radar Site. Flight tests were conducted within the coverage 
area of the McCook Site using a NAFEC aircraft. The result of the flight 
tests indicated that the occurrence of reflected replies from nearby buildings 
was so extensive that the reflected reply problem could not be resolved unless 
the site was relocated. 

The Central Region had been looking for a new site for the McCook Enroute 
Radar Site prior to the NAFEC investigation and this effort was continued 
when the Central Region, in early 1972, turned over the responsibility for 
the McCook Enroute Radar Site to the Great Lakes Region. 

The beacon test site property, owned by the city of Chicago (see Figure 1) , 
was located adjacent to the runway complex of the O'Hare Airport. The property 
is presently being used as an agricultural nursery. 

Operational tests were conducted at the O'Hare Airport Radar Beacon Test 

Site (beacon test site) using targets-of-opportunity , flight inspection aircraft, 

and a NAFEC aircraft. The result of these tests are documented herein. 

DESCRIPTION OF SITE EQUIPMENT . 

In order to simulate a radar beacon directional antenna installed on a con- 
ventional 50-foot enroute radar tower, a 70-foot tower was fabricated at 
the test site using typical construction scaffolding (see Figure 2) . Since 
the tests were conducted at radar beacon frequencies only, an enroute radar 
antenna was not required. The Air Traffic Control Beacon Interrogator (ATCBI) 
Directional Antenna, Type FA-8043, was mounted directly on top of the scaf- 
folding tower on an Antenna Pedestal, Type AB-294/FPS-8 . 

An omnidirectional antenna was also installed at the test site so that 
Improved 3-Pulse Side Lobe Suppression (SLS) could be implemented. The 
Ommnidirectional Antenna, Type FA-8044, was installed on the tower so that 
the bottom of the omnidirectional antenna was at the same vertical height as 




FIGURE 1. 



AERIAL PHOTOGRAPH SHOWING RELATIONSHIP OF THE 
BEACON TEST SITE OF THE O'HARE AIRPORT 



DIRECTIONAL 
ANTENNA 
TYPE FA-8043 



OMNI- DIRECTIONAL 
ANTENNA TYPE FA- 8044 




FIGURE 2. 



LAYOUT OF BEACON TEST SITE SHOWING TEMPORARY 
TOWER AND EQUIPMENT 



the top of the directional antenna. The omnidirectional antenna was also 
installed so that the centers of the omnidirectional and directional antennas 
were separated in the horizontal plane by approximately 15 feet. 

An equipment van (26' x 8') was used at the test site to house the radar beacon 
interrogator, defruiter, decoder and indicator equipment. Power for all of 
the test site equipment was supplied by a 25KVA 3-phase 110/208 volts Portable 
Generator (see Figure 2). The mobile van, with its equipment, the antenna ped- 
estal and the antennas were all part of a Mobile Enroute Radar Facility (MERF) 
loaned to the Great Lakes Region by the Aeronautical Center, Oklahoma City, 
Oklahoma, to implement the beacon test site. 

The radar beacon equipment installed in the mobile equipment van was used 
to provide a video display for observation by an air traffic controller and 
simultaneously photographed for future reduction and analysis of data (see 
Figure 3). The main and standby interrogators were Air Traffic Control Beacon 
Interrogators, Model ATCBI-4. The Model ATCBI-4 interrogators provided the 
normal, Mode 3/A only, beacon interrogator functions as well as improved 
3-pulse SLS operation. Removal of non-synchronous radar beacon replies result- 
ing from other interrogators was accomplished by utilizing an Interference 
Blanker, Type MX-8757/UPX, on both the main and standby channels. 

Decoding of the interrogator output was performed within the mobile equipment 
van by the main or standby Common (Decoder) Rack, Type FA-6193. Selection of 
the video that was displayed on the Console Cabinet, Type CA-4080A (ARSR-1 dis- 
play), was controlled by a Master Control Box (10-Channel) , Type FA-6191A. 



DISCUSSION 



GENERAL . 

Prior to conducting coverage tests at the beacon test site, the interrogation 
mode of the ATCBI-4 interrogators was set for Mode 3/A only and the pulse 
repetition frequency (PRF) of the interrogators was set for 350 interrogations 
per second. The pulse parameters of both interrogators were also adjusted 
so they met the U. S. National Aviation Standard for the Mark X (SIF) Air 
Traffic Control Radar Beacon System (ATCRBS) characteristics. 

The average pulse power for two pulses was measured on the directional antenna 
transmission line at the transmitter and at the directional antenna to deter- 
mine the attenuation afforded the radar beacon interrogations by the trans- 
mission line. The average power at the transmitter measured 8.4 milliwatts (mw) 
through 20dB of additional attenuation (1,500 watts peak power), while the 
average power at the directional antenna measured 1.4 mw through the same 20dB 
of additional attenuation (250 watts peak power) . This difference in power 
indicated that the RG-8U coaxial cable, RG-218U coaxial cable, and pedestal 
rotary joint attenuated the radar beacon directional antenna signal by 7.78dB. 




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Receiver tangential sensitivity measurements were made on both channels of 
the ATCBI-4 interrogator and the Sensitivity Time Control (STC) curve was 
set to 36dB attenuation on each receiver. 

The measurements were as follows: 

Channel 1. 

Receiver tangential sensitivity: -88.8dBm 

STC curve readings : 

Reduced Receiver 



Delay 


(ys) 


Attenuation(dB) 


Sensitivity (-dBm) 


15 




36.0* 


52.8 


27 




30.6 


58.2 


52 




25.0 


63.8 


102 




19.0 


69.8 


200 




12.8 


76.0 


398 




6.7 


82.1 


794 







88.8 



Channel 2. 

Receiver tangential sensitivity: -87.6dBm 
STC curve readings: 



Reduced Receiver 



Delay (ys) 


Attenuation (dB) 


Sensitivity (-dBm) 


15 


36.2* 


51.4 


27 


30.0 


57.6 


52 


24.0 


63.6 


102 


18.0 


69.6 


200 


11.5 


76.1 


398 


5.5 


82.1 


794 





87.6 



*The dB attenuation at 15 us delay designates the value of the STC curve. 

Measurements were made of the radar beacon system overall sensitivity by 
determining the minimum signal at the receiver input that would be displayed 
on the ARSR-1 Plan Position Indicator (PPI) display. In this manner, the 
effects of the receiver, interference blanker (defruiter) , decoder and display 
circuitry were all taken into account. The minimum receiver input signal 
that was displayed on the ARSR-1 dPI Display measured -83dBm on both the 
main and standby channels. 



DESCRIPTION OF FLIGHT TESTS . 

Three types of flight tests were performed during September and October 1972, 
to determine the radar beacon coverage of the beacon test site. They were: 

1. Targets-of-opportunity , 

2. Flight Inspection District Office (FIDO) high- and low-altitude 
aircraft flight tests, and 

3. NAFEC aircraft flight tests. 

All of the flight tests were observed by an air traffic controller from either 
the Great Lakes Region or NAFEC. The 35 mm camera (Figure 3) that was mounted 
on the ARSR-1 PPI display contained a semi-transparent, periscope-type mirror 
which allowed simultaneous viewing of the PPI display by the air traffic con- 
troller and recording of data by the camera. During all of the flight tests, 
the air traffic controllers recorded any unusual occurrences; e.g., loss 
of replies, reflected replies or change of altitude. 

TARGETS-OF-OPPORTUNITY . 

The major portion of the targets-of-opportunity were recorded on Friday, 
15 September, and Friday, 22 September, between the hours of 1500 and 2000 
hours. This is the time when the traffic reaches a maximum in the Chicago 
O'Hare Airport area. The high level of air traffic during these times provided 
a complete but random coverage of all portions of the airways within the beacon 
test site coverage area. 

FIDO AIRCRAFT FLIGHT TESTS . 

HIGH-ALTITUDE FLIGHT TESTS . High-altitude flight tests of the beacon test 
site were conducted using a FIDO aircraft on 20 September 1972, between 1400 
and 1800 hours. An Aero Commander, Type AC21, was flown to the area from 
Oklahoma City, Oklahoma, for this purpose. The route of the flight tests 
are shown in Figure 4 along with any pertinent flight test information. The 
test was broken into two segments to allow the aircraft to replenish the 
fuel that was consumed on the flight from Oklahoma City. The segmenting 
of the flight test allowed the aircraft to enter the test area at an altitude 
of 29,000 feet (FL 290), but also caused an interruption in the high-altitude 
testing in the area of Green Bay, Wisconsin. 

From Figure 4, it can be seen that the initial portion of the flight 
test commenced over the Capital VORTAC and ended in Green Bay, where the 
pilot landed the aircraft to refuel. The major portion of this leg of the 
flight test was flown at 26,000 feet (FL 260) until the pilot descended the 
aircraft to land at Green Bay. 

After refueling, the pilot climbed the aircraft to 27,000 feet (FL 270) 
where it remained until the end of the flight test. The pilot diverted the 
aircraft over the Wheatland Intersection from its intended course, the 175° 
radial of the Northbrook VORTAC, to the Woodland Intersection. The diversion 




NAUTICAL MILES 



FIGURE 4. FLIGHT PATH OF FIDO HIGH- ALTITUDE FLIGHT TEST 

8 



allowed for a more complete high-altitude flight test coverage while still 
allowing a sufficient reserve of fuel to land the aircraft at the W.K. 
Kellogg Airport in Battle Creek, Michigan, after the flight test was com- 
pleted. Prior to ending the tests, the aircraft was flown over the beacon 
test site so that the overhead coverage could be measured at 27,000 feet 
(FL 270). 

LOW-ALTITUDE FLIGHT TEST . Low-altitude flight tests of the beacon test site 
were conducted using a FIDO aircraft on 21 and 22 September 1972. A Douglas 
DC-3 aircraft was flown to the area from Battle Creek, Michigan, for this 
purpose. The low-altitude flight tests were so extensive that 2 days were 
required to complete the testing. A morning and afternoon flight test was 
conducted on 21 September, and a morning flight test was required on 
22 September. The routes of the flight tests are shown in Figures 5, 6, and 7 
along with the flight test altitude of each path. The figures are presented 
in a chronological order starting with the morning of 21 September. 

All of the low-altitude flight tests were made at altitudes between 2,500 
and 5,000 feet. One segment of the flight test, during the morning of 
21 September, extended to a maximum range of 62 nmi from the beacon test site. 
This occurred over the Vermillion Intersection where a change in altitude 
from 4,000 feet to 2,500 feet was also requested. The descent maneuver com- 
bined with the extreme range was made to determine the minimum coverage in 
this area. 

Before the FIDO aircraft completed the flight test on the morning of 
21 September, the aircraft was landed at the O'Hare Airport for fuel. After 
refueling, the flight test was continued until lunch time. A beacon test 
site overhead-flight was included as part of the afternoon low-altitude flight 
tests on 21 September. 

During the flight tests on the morning of 22 September, the improved 3-pulse 
SLS was turned off. This occurred when the aircraft was in the vicinity of 
the Crib Intersection, and was done to determine whether the improved 3-pulse 
SLS had caused any loss of replies or changes in the system operation. The 
FIDO flight test aircraft was also diverted during the morning flight test 
in the vicinity of the Chicago Heights VORTAC. The pilot was concerned about 
the operation of some of the aircraft's equipment and placed the aircraft 
in a holding pattern while he requested maintenance consultation. 

NAFEC AIRCRAFT FLIGHT TESTS . 

The flight tests that were conducted at the radar beacon test site, utilizing 
a NAFEC aircraft, were made during the week of 2 October 1972. A Convair 
880 M Jet Aircraft (FAA N-112) (see Figure 8) was used to verify problem 
areas uncovered during the analysis of the targets-of-opportunity data and 
the FIDO flight test data. The analysis of the targets-of-opportunity data 
and the FIDO flight test data took place during the week of 25 September. 




FIGURE 5. 



NAUTICAL MILES 

FLIGHT PATH OF FIDO LOW- ALTITUDE FLIGHT TEST 
CONDUCTED 21 SEPTEMBER IN THE MORNING 

10 




NAUTICAL MILES 

FIGURE 6. FLIGHT PATH OF FIDO LOW- ALTITUDE FLIGHT TEST 
CONDUCTED 21 SEPTEMBER IN THE AFTERNOON 

11 




NAUTICAL MILES 

FIGURE 7. FLIGHT PATH OF FIDO LOW- ALTITUDE FLIGHT TEST 
CONDUCTED 22 SEPTEMBER IN THE MORNING 

12 




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Two areas where reflected beacon replies occurred were Investigated using 
the NAFEC aircraft and three areas where vertical lobing occurred were also 
flight tested. Prior to the completion of the flight testing on 2 October, 
the aircraft was flown over the beacon test site at 41,000 feet to measure 
the overhead coverage of the site at this altitude. An orbital flight test 
also conducted on 3 October (see Figure 9) . The radius of this flight test 
was 150 nmi with the NAFEC aircraft flying at an altitude of 27,000 feet 
(FL 270) . 

RESULTS OF THE FLIGHT TESTS . 

Even though the air traffic controllers did an excellent task of observing 
the beacon test site PPI display, they could not study all portions of the 
PPI display with equal measure. This was particularly true when more than 
one aircraft was displayed at a time; e.g., targets-of-opportunity data. 
When the filmed data was reviewed at NAFEC, during the week of 25 September, 
it was rerun many times. Each time the film was run, a different portion 
of the filmed PPI display was studied. This resulted in the detection of 
more problem areas than was originally suspected. 

RESULTS OF THE TARGETS-OF-OPPORTUNITY TESTS . 

SITE COVERAGE . On 14 September 1972, the air traffic controllers at the beacon 
test site recorded target information while observing targets-of-opportunity. 
The aircraft type, position, and altitude of displayed replies were coordi- 
nated with the Chicago Air Route Traffic Control Center located at Aurora, 
Illinois. Observations were made simultaneously at the Chicago Center and 
the beacon test site. The Chicago Center observed the McCook ARSR-2 radar 
beacon display, while the beacon test site observed their radar beacon dis- 
play. The results of this radar beacon coverage coordination was as follows: 

RANGE SELECTED - 100 NMI 

Hours - 1000 to 1130 

Aircraft 

Type Position Altitude 

B707 5 Mi West of the TADPOLE Intersection 11,000 

BE99 TADPOLE Intersection , 8,000 

BE80 15 Mi North East of the MIU VOR 9,000 

AC21 10 Mi North East of the SBN VOR FL-230 

Cessna 310 Grand Beach Intersection 7,000 

G159 5 Mi North East of TADPOLE Intersection. 14,000 



14 




FIGURE 9. FLIGHT PATH OF NAFEC AIRCRAFT COVERAGE 
FLIGHT TESTS 



15 



Aircraft 
Type 

BE55 



PA28 

BE90 

Cessna 411 
Twin Otter 
Cessna 182 
Unknown 
DC 9 

Cessna 182 
PA28 
Cessna 172 

B727 
CV88 
BE55 
BE80 

Unknown 

Twin Otter 
BE90 
PA28 
C119 



Position Altitude 

12 Mi East of the Sunfish Intersection 8,000 

(NOTE: Ident. Feature Loud and Clear) 

10 Mi North East of the Taylor 9,000 
Intersection 

Grand Beach Intersection 5,000 

10 Mi North East of the RBS VOR 11,000 

10 Mi East of the RBS VOR 8,000 

5 Mi North West of the CGT VOR 3,000 

5 Mi North East of Musky Intersection 7,000 

2 Mi North East of ORD 2,500 

5 Mi West of the VPZ Airport 2,500 
10 Mi East of the 0X1 VOR 6,000 
12 Mi South East of the 0X1 VOR 4,000 

Hours - 1245 to 1415 

6 Mi North West of the JOT VOR 7,000 
12 Mi North West of the RFD VOR 17,000 
4 Mi North of the Lowell Intersection 5,000 
10 Mi North East of the BDF VOR 7,000 

(NOTE: Ident. Feature Loud and Clear) 

7 Mi South East of the Kentland 9,000 
Intersection 

15 Mi South West of the JOT VOR 6,000 

15 Mi North East of the BDF VOR 7,000 

10 Mi North of the BDF VOR 6,000 

3 Mi South of the Lakewood 4,000 
Intersection 



16 



Aircraft 
Type 

UHI Helicopter 



Fairchild 

C119 

BE90 

G159 

Cessna 172 

NA265 

Unknown 

Unknown 

BE55 

BE99 

AC68 

P2 
BE55 



Position 



Altitude 



3,000 



45 Mi West of ORD 
(Ident. Loud and Clear) 

10 Mi South East of the PLL VOR 9,000 
Malta Intersection 4,000 

15 Mi North East of the JVL VOR 3,500 
10 Mi North East of the 0X1 VOR 13,000 
10 Mi West of the 0X1 VOR 4,000 
8 Mi West of the GSH VOR 11,000 
15 Mi West of the SBN VOR 6,500 
7 Mi North East of Benton Harbor 7,500 

4,000 



2 Mi East of the Lowell 
Intersection 

5 Mi East of the Zoro 
Intersection 

3 Mi South of the Manteno 
Intersection 

Musky Intersection 

5 Mi West of the EON VOR 



5,000 

Climbing to 
17,000 

5,000 

4,000 



Radar beacon coverage recordings were also made by the Air Traffic Controllers, 
at the beacon test site, on 15 September 1972. The individual radar beacon 
replies were grouped by transponder code and not by exact altitude. Observations 
were made of the azimuth, direction of flight, and the maximum range that 
the reply could be tracked. The results of these coverage tests were as follows 



17 



RANGE SELECTED - 200 NMI 
HOURS: 1000 to 1145 
Code 1100 (Surface to flight level 230) 
Azimuth (Degrees) Maximum Range (nmi) 



360 
170 
280 
330 
040 
085 
220 
190 
110 
095 



150 
150 
160 
125 
110 
145 
155 
95 
120 
130 



Code 1500 (Surface to flight level 230) 



085 
100 
090 
110 
170 
190 
220 
260 
360 



70 

85 

110 

100 
90 
80 
70 
65 
85 



Code 1700 (Surface to flight level 240) 



350 
080 
070 
100 
110 
170 
240 
270 
280 



60 
70 
80 
90 

100 
85 

100 
90 

110 



Direction 

North 
South 
West 

North West 
North East 
East 

South West 
South 
East 
South East 



East 

East 

East 

East 

South 

South 

South West 

West 

North 



North 

East 

East 

East 

East 

South 

South West 

West 

West 



18 



Codes 2100 and 2300 (Above flight level 240) 

Azimuth (Degrees) Maximum Range (nmi) Direction 

160 160 South 

130 150 South East 

300 160 North West 

170 170 South 

080 160 East 

275 165 West 

220 160 South West 

290 165 North West 

130 160 South East 

Code 1200 (Surface to 9,500 Feet) 

210 80 South West 

190 50 South 

160 90 South 

320 110 North West 

070 90 North East 

130 120 South East 

Code 1400 (Between 10,500 and 17,500 Feet) 

100 145 East 

260 110 West 

280 100 West 

350 150 North 



19 



REFLECTED BEACON REPLIES . 

During the analysis of the targets-of -opportunity data at NAFEC, it was noted 
that aircraft replies suddenly appeared on the display and then just as suddenly 
disappeared. Some of these replies were approximately 60 to 80 nmi south 
of the beacon test site. From past experience it was assumed that these were 
reflected beacon replies. The criteria for determining whether a certain 
beacon reply is, in reality, a reflected beacon reply are as follows: 

- The range of the reflected reply is always greater than the range 
of the normal beacon reply. Sometimes the added range is so 
insignificant that the difference is undetectable on the PPI dis- 
play. (The flight path of the aircraft, during the rotation time 
of the antenna, must not be ignored in calculating this range), and 

- The reply code of the normal beacon reply and the reply code of 
the reflected beacon reply must be identical; e.g., (a) both 
should display a single-slash or both should display a double-slash, 
(b) if the normal reply shows the "identification" feature, the 
reflected reply should also show the "identification" feature. 

When this criteria was applied to the intermittent aircraft replies that 
occurred at the ranges of 60 to 80 nmi south of the beacon test site, the 
existence of reflected beacon replies in this area was confirmed. These 
reflected beacon replies extended from 203° to 172°, as the aircraft flew 
from 354° to 026°. The reflected beacon replies did not occur every rotation 
of the antenna, as the normal aircraft beacon reply covered the above azimuth 
changes, but a large number of reflected replies did occur during such flights. 

Figure 10 shows two examples of reflected replies that were recorded on 
15 September in this area. These photographs were selected because both the 
normal and the reflected beacon replies show the "identification" feature 
which confirmed the reflected beacon reply. The normal beacon reply was 
normally double-slashed; then showed the "identification" feature for two 
antenna rotations; and finally returned to a double-slashed return. The 
reflected reply followed the same sequence. 

When the film data collected on 15 September was analyzed at NAFEC, a persist- 
ent "flashing" of replies was noted on the display 90 to 150 nmi north of 
the beacon test site. These intermittent reflected beacon replies were recorded 
between the azimuths of 10° to 15°. The reflected beacon replies in this area 
were traced to aircraft flying, at approximately the same range, in the 
vicinity of the 85° radial. The identification was made by comparing double- 
slashed codes and replies utilizing the "identification" feature. Figure 11 
shows two examples of reflected replies that appeared in this area. 

The reflected beacon replies that occurred between the ranges of 90 and 150 nmi 
and azimuths of 10° to 15° did not persist for more than four or five antenna 
rotations. Most of the reflected replies in this area lasted for only one 
or two antenna rotations. These reflected beacon replies were far less persistent 
than the reflected replies that occurred south of the beacon test site. 

20 





FIGURE 10. 



TWO EXAMPLES OF REFLECTED BEACON REPLIES 
RECORDED SOUTH OF THE BEACON TEST SITE 
DURING TARGETS-OF-OPPORTUNITY TESTS 

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VERTICAL LOBING . During the analysis of the targets-of-opportunity data 
at NAFEC, the beacon replies were seen to diminish in width as the aircraft 
flew through certain coverage areas. Sometimes this decrease in the width 
of the beacon reply was so severe that the beacon reply was not seen for a 
number of antenna rotations. If the aircraft was changing course, the loss 
of beacon replies could have been attributed to the shielding of the aircraft 
antenna due to the turn maneuver. But in certain areas, the beacon reply 
was lost consistently when aircraft flew through this area even though the 
aircraft was not turning. 

From past experience obtained from analyzing field site data, it was assumed 
that the narrowing and complete loss of beacon replies could, in this case, 
be attributed to propagation vertical lobing. Vertical lobing is a phenomenon 
that is produced by the reception of a radar beacon interrogation or reply 
over multiple propagation paths. If two propagation paths are available from 
the ground antenna to the aircraft antenna a cancellation of signal intensity 
will occur when the phasing of the two signals differ by approximately 180° . 
The intense signals that are reflected from large flat areas of terrain, due 
to the terrain being illuminated by the broad vertical pattern of the direc- 
tional antenna, can cause very serious vertical lobing problems. 

During the analysis of the targets-of-opportunity data at NAFEC, it was noted 
that aircraft replies were lost north of the beacon test site as aircraft 
flew along Jet Airway 38-106. The loss of replies occurred at 10°, 13° and 
20° to a large number of aircraft that flew through the area. The range where 
the beacon replies were lost on Jet Airway 38-106 was approximately 145 nmi 
from the beacon test site. Figure 12 shows an example of the vertical lobing 
that occurred in this area. A series of photographs are shown so that the 
loss of the beacon reply could be associated with the position of the aircraft 
prior to and after the time the beacon reply was lost. The aircraft reply 
that exhibited vertical lobing was circled on the figure for easier 
identification. 

The reason that all of the replies from aircraft that flew through the vertical 
lobing area were not lost is that the reduction of signal intensity, due to 
vertical lobing, is dependent upon the vertical angle of the aircraft relative 
to the beacon test site. The altitude of the aircraft that were seen on the 
PPI display during the analysis of the targets-of-opportunity data was not 
known. Therefore, the vertical angle of the aircraft relative to the beacon 
test site was also unknown. Two aircraft that appear at almost identical 
ranges and azimuths could still be separated by altitude. This would mean 
that the two aircraft were at different vertical angles relative to the beacon 
test site. 

Another factor that should not be overlooked in analyzing flight test data, 
is that all aircraft transponders do not have equal receiver sensitivity or 
output power. The receiver sensitivity and output power are significant 
factors in the determination of why vertical lobing can reduce the beacon 
reply of one aircraft and not another even though the two aircraft are flying 
at similar ranges and azimuths. 



23 






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FIGURE 12. 



LOSS OF BEACON REPLIES DUE TO VERTICAL LOBING 
ON THE 13° RADIAL OF THE BEACON TEST SITE 



24 



During the analysis of the targets-of-opportunity data, vertical lobing was 
also noted in the vicinity of the 71° to 89° radial, at ranges approximat- 
ing 100 to 138 nmi. Aircraft flying on Jet Airways 94-547 and 584 covered 
these radials during their outbound and inbound flights. The three azimuths 
where the vertical lobing was most intense were 71°, 84°, and 89°. Vertical 
lobing was noted at two ranges on the 84° and 89° radials. Only one area 
of vertical lobing was noted on the 71° radial, this occurred at 130 nmi. 
Figure 13 shows an example of vertical lobing that took place on the 84° 
radial at 100 nmi. 

Some loss of radar beacon replies was observed on the targets-of-opportunity 
data when aircraft flew in the vicinity of the 164° radial at ranges between 
140 to 157 nmi. and on the 172° radial at ranges approximating 142 nmi. The 
loss of beacon replies did not occur too often in this area and it was ques- 
tionable whether the loss of replies in this area was really very serious. 
Figure 14 shows an example of the loss of beacon replies that occurred in 
the vicinity of the 172° radial at a range of 142 nmi. 

RESULTS OF FIDO AIRCRAFT F LIGHT TESTS. 

HIGH-ALTITUDE FLIGHT TESTS . The results of the high-altitude flight tests, 
using the FIDO Aero Commander, Type AC21, reconfirmed the occurrence of ver- 
tical lobing in the beacon test site coverage, but no positive areas of 
reflected replies were recorded. The loss of the FIDO aircraft beacon return 
was discounted during turn maneuvers, but there were five instances when the 
beacon reply was lost while the aircraft was not turning. 

Loss of the FIDO aircraft beacon reply occurred at azimuths of 310°, 331°, 
10°, 13° and 19°. All of these losses of replies occurred at ranges between 
110 and 143 nmi. 

The loss of the FIDO aircraft beacon reply at 10° and 13° reconfirmed losses 

of radar beacon replies that had been previously recorded at these same azimuths 

during the targets-of-opportunity tests. 

At the very end of the high-altitude flight tests, using a FIDO aircraft, 
the aircraft was flown directly over the beacon test site at an altitude of 
27,000 feet. During this maneuver, the flight test aircraft reply was lost 
at a slant range of 5 nmi on one side of the site and reappeared at a slant 
range of 4 nmi on the other side of the site. 

LOW-ALTITUDE FLIGHT TESTS . During the low-altitude flight tests, using a FIDO 
aircraft, the aircraft remained within 62 nmi of the beacon test site. Because 
the range of the flight test aircraft did not extend beyond the effective 
range of the improved 3-pulse SLS system, there were no reflected beacon replies 
recorded. Some reflected replies could have occurred if there was a reflecting 
surface less than 1,000 feet from the beacon test site antenna, but there 
was no indication that any efficient reflecting surface was within this range. 
The major contribution of the low-altitude FIDO flight tests was to confirm 
the beacon test site coverage. 



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Some "selected code" breakthrough that resembled reflected beacon replies, 
was seen during the low-altitude FIDO flight tests, but further investi- 
gation showed that these returns were due to code garbling that occurred between 
two unrelated aircraft replies. The flight test aircraft was also lost a 
number of times during turn maneuvers when the aircraft antenna was shielded 
by the aircraft wings or fuselage. 

The low-altitude flight tests included a station overflight at 2,500 feet. 
During this flight maneuver, the aircraft beacon reply was lost at a slant 
range of 3/4 of a nmi on one side of the test site and reappeared at a 
slant range of 3/4 of a nmi on the other side of the test site. 

RESULTS OF NAFEC AIRCRAFT FLIGHT T ESTS . 

Two different airborne equipment configurations were used during the NAFEC 
aircraft flight tests. The first equipment configuration used the aircraft's 
calibrated transponder, a Bendix Corp. Type TRU-1 Serial No. 55, which was 
interrogated by, and responded to, the beacon test site normal Mode 3/A 
interrogations. The antenna for this transponder was installed on the 
bottom of the aircraft beneath the cockpit. 

The second equipment configuration that was used during the NAFEC aircraft 
flight tests, consisted of an airborne and ground-based synchronization system. 
This system was used ONLY during vertical lobing flight tests. A photograph 
of the airborne portion of this system equipment configuration is shown in 
Figure 15. A Radio Corporation of America (RCA) Transponder, Type 2.3NAlb/ 
RT-1 Serial No. 1014, was used for the airborne portion of the system. 
The location of the antenna (Type 237Z-1) , that was used with this transponder, 
is shown in Figure 8. 

A block diagram of the entire equipment configuration, that was used during 
the vertical lobing flight tests is shown in Figure 16. This airborne and 
ground-based system was used as a synchronizing system to ensure that only 
the interrogations generated by the beacon test site were recorded in the 
aircraft, on the digital recorder. 

The ground-base equipment for this system consisted of a transmitter, logic 
circuitry and an antenna. The transmitter was a Power Pulsed Signal Source, 
Model PG 5KA, fabricated by Applied Microwave Laboratory Inc., which was capable 
of producing 5 KW pulses at 1030 MHz. The transmitter was triggered by three 
pulses supplied by the logic circuitry. The first two pulses were separated 
by 25 Us to form a Mode D radar beacon interrogation. The third pulse (parity 
pulse) was transmitted 10 ys later, to prevent capture of the airborne equip- 
ment circuitry by false Mode D interrogation pulse spacings that might be 
occasionally produced by two nonsynchronous interrogators. The antenna that 
was used for the ground-based portion of the system was a Scientific Atlanta 
Standard Gain Horn, Model 11-0.9 (see Figure 17). The antenna was modified 
at NAFEC, by increasing the flare length. This provided increased directivity 
and gain. 

The ground-based portion of the system used the "Beacon Sync" of the ATCBI-4 

interrogator and the "North Mark" of the antenna system together with 

the logic circuitry, to generate 60 3-pulse "synchronizing" interrogations 

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for each antenna rotation. The logic circuitry provided the means for not 
only controlling the precise azimuth where the 60 "synchronizing" interroga- 
tions began, but also supplied a strobe for the PPI display. The timing of 
the 60 3-pulse "synchronizing" interrogations were made to coincide with the 
interrogation of the flight test aircraft by the main beam of the directional 
antenna. This was accomplished by shifting the position of the strobe on 
the PPI display. The strobe was placed on the counterclockwise edge of the 
flight test aircraft reply. This positioning of the strobe caused the next 
60 3-pulse "synchronizing" interrogations to be transmitted to the flight 
test aircraft, approximately 12 us, prior to the transmission of the 
normal Mode 3/A interrogation from the directional antenna. 

The airborne portion of the synchronizing system, (Figure 16), consisted 
of an antenna, transponder, logic circuitry, analog-to-digital converter and 
a digital recorder. The reception of the Mode D interrogation by the trans- 
ponder produced a trigger for the logic circuitry which was "ANDED" with the 
parity pulse. The pulse that was produced by the "AND" circuit was delayed 
to coincide with the Pi pulse of the normal Mode 3/A beacon site interrogation, 
so that the amplitude of the Pi interrogation pulse could be sampled. The 
analog-to-digital converter changed the amplitude of the Pi interrogation 
pulse into a digital code which was recorded on the digital recorder along 
with the time of the recording. 

In order to record the amplitude of 60 consecutive Pi interrogation pulses 
in the main beam of the beacon test site directional antenna, the digitized 
amplitude of each Pi interrogation pulse was stored in a 7 x 60 bit shift 
register. After a time interval equivalent to five interrogation periods had 
elapsed without receiving a Mode D interrogation, the digitized amplitude of 
the Pi interrogation pulses that were stored in the shift register were 
transferred to the digital recorder and printed. Figure 18 shows a 
typical recording produced in this manner. The time of recording is shown 
on the left in hours, minutes, seconds and tenths of seconds while the data 
is recorded on the right. Each of the two digits of data can assume a value 
between zero and *. An increase or decrease of one in the numerical value of 
the number in the far righthand column indicated a change in the pulse 
amplitude of 16 millivolts. A recorded one was equivalent to 16 millivolts, 
while a recorded one* was equivalent to 496 millivolts. The series being 0, 
1,2, 3,4,5,6, 7,8,9, + ,-,V, A, ft,*. Figure 18 shows data that was recorded during 
the vertical lobing flight testing. The recorded data show how the ampli- 
tude of the directional antenna main beam varied with azimuth. 

A system calibration was performed, at NAFEC, prior to departure for Chicago, 
to allow converting the transponder input, in -dBm, to the appropriate 
digits that were recorded on the digital recorder. The calibration tests 
showed that a -64 dBm level at the input to the transponder was recorded as 
a three on the digital recorder, while an input level of -15 dBm was recorded 
as a 69. The synchronization system was effective to ranges beyond 150 nmi 
but the amplitude of the normal Mode 3/A interrogation P^ pulse could not 
be recorded at levels below -65dBm. This limited the effective range of the 
entire system to approximately 120 nmi. This limitation was caused by a 
Honeywell 3C Schmitt Trigger Circuit which converted the Mode 3/A video pulse. 

32 



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DATA FROM THE BEACON TEST SITE THAT WAS 
RECORDED, IN THE NAFEC AIRCRAFT, USING 
THE SYNCHRONIZING SYSTEM 



33 



A more sensitive Schmitt Trigger is needed to extend the range of the system 
to at least 150 nmi . The system was still effective in verifying vertical 
lobing that took place in Chicago at ranges less than 120 nmi. 

The flight tests that were conducted, using NAFEC aircraft N-112 on 2 October, 
were flown to reconfirm reflected beacon replies that were obtained during 
the targets-of -opportunity tests. The NAFEC aircraft flight tests were made 
at ranges approximating 64-80 nmi in an attempt to reproduce reflected 
replies south of the beacon test site. The flight tests were made at 25,000, 
27,000, 29,000 and 31,000 feet. All of the flight tests produced some refelcted 
beacon replies at azimuths from 205° to 150° as the aircraft was flown between 
site azimuths of 345° to 24°. Figure 19 shows two examples of dual reflections 
that occurred during these flight tests. 

The NAFEC aircraft was flown directly over the beacon test site at 41,000 feet 
on 2 October 1972. During this maneuver, the aircraft replies were lost at 
344° at a slant range of 9.5 nmi and were received, after the station cross- 
ing, at 169° at a slant range of 9 nmi. 

Flight tests were conducted on 3 October in the vicinity of the beacon test 
site 82° to 90° radials. The purpose of these flight tests was to produce 
reflected beacon replies north of the beacon test site which would recon- 
firm the reflected beacon replies recorded during the targets-of-opportunity 
tests. During the NAFEC aircraft flight tests at altitudes between 24,000 and 
31,000 feet, reflected beacon replies were recorded between 6° and 15°. The 
reflected replies that occurred at an azimuth of 15° reconfirmed the reflected 
replies recorded during the targets-of-opportunity tests at the same radial. 
The reflected replies that occurred at an azimuth of 15° were produced as 
the aircraft was flown in the vicinity of the 83° radial. Figure 20 shows 
reflected beacon replies that occurred at 15° and 355° when the NAFEC air- 
craft was flown at 83° and 99°, respectively. The reflected replies that 
occurred at 355° were produced during the orbital flight test that was con- 
ducted at the end of the flight testing on 3 October. 

Vertical lobing was also recorded during the NAFEC aircraft flight tests that 
took place on 3 October. The most severe case of vertical lobing occurred 
as the aircraft was flown on the 77° and 84° radials. 

The orbital flight test that was flown at 27,000 feet on 3 October was con- 
ducted to investigate the beacon test site coverage and determine if obstruc- 
tions on the site horizon would interfere with the beacon test site coverage. 
The only extended loss of the NAFEC aircraft beacon reply that did occur during 
the orbital flight was not due to obstructions but due to a change in the 
transponder code that was requested by the Indianapolis, Indiana, Center. 
The code change was not coordinated with the beacon test site. This caused 
a loss of the NAFEC aircraft beacon reply for 10 antenna rotations between 
the site azimuths of 180° and 175°. During the orbital flight test, reflected 
beacon replies were recorded on the 355°, 1°, 5° and 10° radials as the NAFEC 
aircraft was flown through site azimuths of 99°, 87°, 87° and 267°, respectively, 



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Vertical lobing caused a loss of the beacon reply in the northern sector 
at 358° and 15° during the orbital flight test. A more detailed listing 
of the reflected beacon replies that occurred during the flight testing on 
2 and 3 October can be found in the Appendix. 

Flight tests, using the NAFEC aircraft, were conducted on 4 and 5 October, 
to investigate areas that had produced a loss or narrowing of beacon replies 
during the targets-of-opportunity tests. The first area that was investi- 
gated, in an attempt to record vertical lobing, was north of the beacon test 
site. The flight tests were made in the vicinity of the 10° to 18° radials 
at an altitude of 25,000 feet. The major portion of the beacon replies that 
were lost during these tests were between 98 and 153 nmi on the 10° and 16° 
radials. Some beacon replies were also lost between 98 and 133 nmi on the 
13° radial of the beacon test site. Figure 21 shows a loss of beacon replies 
that occurred on the 13° radial at 125 nmi due to vertical lobing. A series 
of three photographs are shown to illustrate where the aircraft reply 
appeared prior to and after the loss of replies occurred. 

During the morning of 4 October, the timing signal that was used for the digital 
recorder clock was supplied directly from the aircraft's 60-Hertz power source. 
This introduced an error due to the 60-Hertz power source frequency shift 
which occurred over a period of time. A Time-Mark Generator, Tektronix Type 
2901, used to provide a very accurate timing signal for the digital recorder 
clock, was not functioning during the morning flight on 4 October. The time- 
mark generator problem was resolved at noon on 4 October, and the generator 
functioned normally for the remainder of the flight tests. 

Flight tests were conducted, in the afternoon of 4 October, on the beacon 
test site radials between 60° and 75°. A large portion of the losses of beacon 
replies that occurred during the targets-of-opportunity tests were lost on 
the 71°, 84° and 89° radials. Since extensive flight testing was accomplished 
on the 84° radial during the flight tests that were performed on 3 October, 
flight testing on the 84° radial was not repeated. On 3 October, losses and 
narrowing of targets were detected on the 84° radial at 120 nmi (Figure 22) . 
Some loss of beacon replies was also recorded on 3 October on the 84° radial 
at 110, 155 and 170 nmi. Losses and narrowing of beacon replies were recorded 
on 3 October at ranges of 115, 123 and 133 nmi on the 77° radial and at 135 nmi 
on the 89° radial (see Appendix) . 

The flight tests that were conducted, during the afternoon of 4 October, showed 
definite loss and narrowing of beacon replies on the 71° radial at ranges 
of 65, 106, 115, 126 and 134 nmi. Some narrowing of beacon replies was 
recorded on the 60° radial at 71 nmi. 

During the flight testing that was conducted on 4 October, some reflected 

beacon replies were recorded. The reflected replies occurred between 201° 

and 177° when the aircraft was flown between 357° and 21° at ranges between 
51 and 76 nmi (see Appendix) . 



37 






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39 



The flight testing that was conducted on 5 October, using the NAFEC aircraft, 
commenced by investigating the 165° to 175° beacon test site radials at 27,000 
to 29,000 feet. The flight tests were made in an attempt to produce losses 
of beacon replies in this area, but no complete loss of beacon replies was 
noted except during turn maneuvers. Some narrowing of the beacon reply was 
recorded on the 162° radial at 72 nmi and on the 170° radial at 79 nmi . 
Figure 23 shows the narrowing of the beacon reply that occurred on the 170° 
radial at 79 nmi. 

At the completion of the flight testing in the vicinity of the 165° to 175° 
radials, the weather at the 0'Hare Airport was below the minimum safe landing 
requirements so the aircraft could not land. Flight testing was continued 
on the 83° radial of the beacon test site awaiting a weather change. After 
the flight test was completed, the aircraft was flown to Green Bay, Wisconsin, 
via the Peacock Intersection. By this time the weather had cleared and the 
aircraft was flown back to the O'Hare Airport where it was landed. During 
the maneuvering after the aircraft left the 83° radial, the pilot did not 
try to maintain the aircraft in straight and level flight as he had done 
prior to this time. This resulted in more serious losses of beacon replies 
during the later portion of the flight testing on 5 October 1972. 

CAUSES OF SITE PROBLEMS AND POSSIBLE SOLUTIONS . 

The problems that were observed during the flight testing and analysis of 
data recorded at the beacon test site can be divided into two groups. There 
were beacon replies due to reflections and loss or narrowing of the beacon 
replies. Both of these problem areas can be related to the broad vertical 
pattern of the directional antenna, particularly the loss or narrowing of 
the beacon replies. If the energy radiated from the directional antenna 
could have been confined to vertical angles above the horizon, the beacon 
test site would have been virtually free of problems. Considerable FAA research 
and development effort is presently being expended in the development of radar 
beacon antennas that radiate a controlled vertical pattern. This is in the 
hope of eliminating or greatly reducing future beacon problems due to reflected 
replies and the loss of beacon replies due to vertical lobing. 

REFLECTED BEACON REPLY PROBLEMS . The reflected beacon replies, that were 
recorded at the beacon test site, were limited mainly to two areas. These 
areas were both north and south of the beacon test site. The reflected replies 
that occurred in the south were confined to site azimuths between 150° and 
205° (see Appendix ). From the panoramic photographs of Figure 24 that were 
taken at the beacon test site at a height of 75 feet, it can be seen that 
the reflecting surface is either the fence along Irving Park Road or the 
buildings and boxcars in the freight yard across Irving Park Road. 

The replies that were caused by the reflecting surface in the south were limited 
to ranges between 53 and 80 nmi. These reflected replies should have been 
eliminated by the improved 3-Pulse SLS system radiation but the data shows 
that the system capability was limited to approximately 53 nmi. Normally, 
the improved 3-Pulse SLS system is effective to ranges approximating 80 nmi. 

40 







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41 




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42 



The reason for the restricted range at the beacon test site was thought 
to be caused by the type of transmission line used on the omnidirectional 
antenna. Prior to the erection of the beacon test site, it was agreed that 
RG-197 (low-loss) cable would be used on both the directional and 
omnidirectional antennas. A normal enroute radar site uses 7/8-inch low- 
loss cable, and in some instances 1-5/8 inch low-loss cable, to feed the 
directional and omnidirectional antennas. RG-218U coaxial cable was used 
at the beacon test site. The RG-218U coaxial cable attenuates radar beacon 
frequencies the same as RG-17U coaxial cable, which is 4.4 dB per 100 feet. 
The loss afforded the radar beacon transmission by the 7/8 inch low-loss cable 
is nominally 1 . 5 dB per 100 feet while the loss afforded by the 1-5/8 inch 
low-loss cable is nominally .78 dB per 100 feet. 

If the 1-5/8 inch low-loss cable were used to feed the omnidirectional antenna 
at the beacon test site, the range of the improved 3-pulse SLS system would 
have been extended by a factor of 1.52. This would have increased the minimum 
range of the improved 3-pulse SLS system from 53 nmi to 80 nmi and would 
have probably eliminated the reflected beacon replies that occurred south of 
the beacon test site. If some reflected replies remained after the trans- 
mission line cable was changed, then the power input to the omnidirectional 
antenna could be increased while the directional antenna input remained the 
same. Since both the directional and omnidirectional antenna radiation 
power would probably be increased by the changing of transmission lines 
to both antennas, the power input to the omnidirectional antenna will prob- 
ably have to be increased by 3 dB to ensure that the reflected beacon replies 
south of the beacon test site were eliminated. 

Some experimentation could also be performed on the fence along Irving Park 
Road. An additional fence could be installed on the beacon test site side of 
the original fence, and tilted as described in NAFEC Report No. NA-69-36 
entitled "Experimentation and Analysis of Siting Criteria," dated September 
1969, pages 85 through 89. Tilting the fence at approximately one-half the 
Brewster Angle would reflect the energy that strikes the fence into the 
ground where it would be absorbed. 

The reflected beacon replies, that were recorded to the north of the beacon 
test site were confined to site azimuths between 355° and 15° (see Appendix). 
The range at which these reflected beacon replies occurred varied between 
105 and 178 nmi which is far beyond the range of the improved 3-pulse SLS 
system. The reflecting surface that was thought to be responsible for pro- 
ducing these reflected replies was the airline hangars approximately 2 to 3 
nmi to the north of the beacon test site. 

Most of the reflected replies that occurred north of the beacon test site 
were caused by aircraft flying at site radials between 82° and 99°. One 
isolated case was recorded when an aircraft flying at 267° caused a reflected 
reply at 10°. Since most of these reflected replies are caused by aircraft 
flying in the vicinity of the 84° radial (see Appendix) , a system could be 



43 



installed at the beacon test to increase the range of the improved 3-pulse 
SLS system in just this one sector. A NAFEC Data Report entitled "Investiga- 
tion of Reflected Reply Problem at Trevose, Pennsylvania, Enroute Radar 
Beacon Site," dated January 1972, and associated with Project No. 031-241-01X, 
deals with this same problem that occurred at the Trevose, Pennsylvania, 
ARSR-2 Site. A horn antenna (similar to that shown in Figure 17) was used to 
provide increased gain for the omnidirectional antenna radiations in a certain 
sector where aircraft were flying when reflected replies were produced. 

VERTICAL LQBING PROBLEMS . The vertical lobing problems that were encountered 
during the investigation of the beacon test site can be grouped into three 
areas. Vertical lobing caused a loss or narrowing of replies at site azimuths 
in the vicinity of the 13°, 84° and 170° radials. The vertical lobing that 
occurred, in the vicinity of the 170° radial, was rather minor and no further 
discussion will be addressed to this area. 

The vertical lobing that caused a loss or narrowing of beacon replies in the 
northern sector and in the vicinity of the 84° radial was moderate in severity 
and was thought to be caused by reflections from the smooth terrain of the 
O'Hare Airport. The drainage lake on the O'Hare Airport was thought to play 
a major role in the loss of replies that occurred in the vicinity of the 84° 
radial . 

Almost all of the loss or narrowing of beacon replies occurred at ranges 
beyond 100 nmi . This was due to: (1) a decrease in the intensity of the bea- 
con signals with increased range, (2) an increase in the reflection coeffi- 
cient of the terrain at the lower angles of propagation, and (3) an increase 
in the "apparent" smoothness of the terrain at angles closer to the horizon. 
If the transmission line that feeds the directional antenna were made of a 
low-loss 1-5/8 inch coaxial cable, fewer beacon replies would be lost or 
narrowed. At first it might seem that increasing the radiated power also 
increases the terrain reflection and nothing is gained. But, suppose a 1-volt 
signal was originally received in the aircraft direct from the site and a 
. 4-volt signal was reflected from the terrain and then received in the aircraft 
The resultant signal would be .6 volts when the direct and reflected signals 
were 180° out-of -phase . Now, suppose we increase the power output of the 
site, in fact, quadruple it. The signal received in the aircraft direct 
from the site increases to 2 volts (6 dB) and naturally the reflected signal 
is also doubled to .8 volts. But, now the resultant signal would be 1.2 
volts when the direct and reflected signals were 180° out-of -phase . This 
means that increasing the power output of the directional antenna by 6 dB 
will result in doubling the range where the vertical lobing begins. 

By changing the directional antenna cable from the RG-218U coaxial cable 
which was used during the test to a 1-5-/8 inch low-loss coaxial cable, the 
effective power output of the directional antenna would be increased by more 
than 3 dB. This increase in effective power could extend the range of the 
beacon signals by a factor of 1.4. If nothing else, this should emphasize 
the necessity of using low-loss coaxial cable at enroute radar beacon sites 
and using this same type of cable for testing of prospective radar beacon 
sites . 

44 



Besides increasing the power output of the directional antenna, there is 
really very little that can be done to reduce the vertical lobing problem at 
any future O'Hare Airport enroute radar site. The terrain that is responsible 
for the vertical lobing problem is so extensive that only a directional 
antenna with a controlled vertical pattern would provide the total solution 
required. 

FUTURE EXPANSION OF THE BEACON TEST SITE AREA . 

At the present time, a storm water control and recreation preserve is planned 
for the area surrounding the beacon test site. Figure 25 shows a detailed 
plan of the lake, athletic field and picnic area that is expected to be devel- 
oped. When the lake area is completely flooded, vertical lobing problems 
could start at site azimuths in the vicinity of the 300° radial where none 
exist at this time. 

Figure 25 shows a U.S. Government Post Office Building approximately 1,000 feet 
southeast of the beacon test site. A two-floor structure has been proposed 
for this building. There should be no problem with this building if the 
structure is limited to just two floors. If a reflection problem were to 
develop in the future, due to the Post Office building, a screen of shrubs 
or short trees could be used to shield the building from the radar beacon 
site radiation. 

The area designated on Figure 25 as "Plane Watch Hill" may also form an 
obstruction at site azimuths in the vicinity of the 260° radial. The develop- 
ment plan shows contour lines which seem to indicate that a rather large hill 
may be formed in the future, along with an observation area on the top of this 
hill. A detailed study of the elevations planned for "Plane Watch Hill" 
should be undertaken prior to making any firm decisions on the acceptability 
of the area as the site for a future O'Hare Airport enroute radar beacon 
site. 



45 



^^ 



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CITV = f CUIC&60 CHAQt FIE-LO 



< VI v: ;# \vj;. \" 

; V/7te Ae " 



FUTURE RELOCATED 
IRVING PARK ROAD 




ENTRANCE 



O'HARE STORM WATER CONTROL - 
RECREATION PRESERVE 
DEVELOPMENT PLAN 



FIGURE 25. 



FUTURE DEVELOPMENT PLANNED FOR BEACON 
TEST SITE AREA 



46 



CONCLUSIONS 



Based on the test data that was collected within the coverage area of the 
O'Hare Airport enroute radar beacon test site, it is concluded that: 

1. The flat terrain and hangars of the adjacent airport make the O'Hare 
Airport enroute radar beacon test site something less than an ideal site from 
the standpoint of siting criteria. 

2. Any future construction in the vicinity of the radar beacon site could 
limit the site coverage and increase the occurrence of reflected beacon replies 

3. Detailed coverage information can be obtained, for the siting of future 
radar beacon sites, through the use of temporary test facility similar to 
that used at the O'Hare Airport enroute radar beacon test site. 

4. The existence and intensity of abnormalities, such as vertical lobing 
and reflected beacon replies, can be determined through the use of flight 
tests using the temporary test facility. 

5. The use of coaxial transmission cable, other than that normally 
installed at radar beacon sites, required that an intuitive interpretation 
be performed on the test data in lieu of a direct one-to-one correlation. 



47 



APPENDIX 



The following reflected beacon replies were noted during the analysis of the 
O'Hare Airport enroute radar beacon test site data using NAFEC Aircraft N-112 

2 October 1972 

Aircraft Azimuth Reflection Azimuth Aircraft Range Reflection 
Frame In Degrees In Degrees Nautical Miles Range NMI 



60 


000 


198 


66 


66 


62 


003 


197 


66 


66 


68 


009 


192 


70 


70 


73 


013 


186 


72 


72 


79 


018 


180 


76 


76 


80 


019 


179 


76 


76 


147 


024 


176 


80 


80 


153 


022 


179 


78 


78 


158 


017 


199 


75 


75 


161 


015 


194 


72 


72 


166 


012 


192 


68 


68 


167 


010 


164 


6 7 


6 7 


169 


009 


192 


65 


65 


170 


008 


193 


64 


64 


171 


007 


193 


64 


64 


197 


345 


165 


68 


68 


216 


000 


200 


70 


70 


217 


003 


199 


70 


70 


218 


002 


197 


70 


70 


224 


009 


190 


57 


67 


227 


013 


190 


66 


66 


231-1 


017 


184 


65 


65 


231-2 


017 


199 


6 5 


66 


232 


019 


155 


65 


65 


235-1 


021 


181 


66 


66 


235-2 


021 


150 


66 


75 


284 


025 


175 


8U 


80 


288 


022 


177 


78 


79 


295 


018 


198 


72 


72 


302 


011 


189 


71 


71 


307 


007 


193 


68 


68 


312 


003 


198 


68 


68 


314 


001 


199 


68 


68 


315 


000 


200 


69 


69 


316 


359 


200 


69 


69 


317 


359 


201 


70 


70 


318 


358 


202 


70 


70 


320 


357 


205 


70 


70 



A-l 



Aircraft flew over the site at 41,000 feet altitude to determine the overhead 
coverage of the site. The following azimuth and ranges were observed: 

Lost radar beacon reply — 344° azimuth at 9.5 nmi slant range. 
Regained radar beacon reply — 169° azimuth at 9.0 nmi slant range. 

3 October 1972 

Aircraft Azimuth Reflection Azimuth Aircraft Range Reflection 
Frame In Degrees In Degrees Nautical Miles Range nmi 



124 


085 


013 


178 


178 


188 


090 


006 


116 


116 


189 


090 


007 


115 


115 


190 


090 


007 


114 


114 


394 


083 


015 


105 


105 



Reflected replies obtained during the 150-nmi orbital flight test: 

010 
010 
355 
356 
355 
005 
001 
005 
010 



898 


267 


899 


268 


1258 


099 


1259 


097 


1260 


099 


1278 


087 


1280 


087 


1281 


086 


1291 


082 



145 


145 


145 


145 


145 


145 


146 


149 


146 


149 


146 


148 


145 


147 


145 


147 


145 


147 



4 October 1972 



70 


357 


73 


000 


74 


001 


75 


001 


77 


001 


80 


005 


81 


005 


85 


005 


242 


012 


247 


012 


252 


013 


262-1 


015 


262-2 


015 


265 


019 


267 


020 


268 


021 


272 


021 


273 


021 



201 61 62 

200 62 63 

199 62 63 
198 63 64 
197 64 65 
195 66 67 
194 66 67 
192 69 70 
186 72 72 
186 65 ; 66 
188 61 61 
180 74 76 

200 74 77 
180 76 77 

177 59 59 

178 60 60 
180 62 63 

179 63 64 



A- 2 



Aircraft Azimuth Reflection Azimuth Aircraft Range Reflection 
Frame In Degrees In Degrees Nautical Miles Range nmi 



274 


020 


179 


64 


65 


474 


016 


185 


5b 


57 


475 


015 


188 


54 


55 


476 


015 


187 


53 


54 


477 


014 


188 


54 


55 


478 


012 


188 


54 


55 


479 


009 


190 


55 


56 


480 


008 


192 


5 5 


56 


481 


006 


193 


56 


57 


482 


006 


195 


56 


57 


483 


005 


195 


5 7 


58 


500 


012 


187 


64 


65 



The following loss or narrowing of beacon replies were observed during the 
analysis of the O'Hare Airport Enroute Radar Beacon Test Site data using 
NAFEC Aircraft N-112: 

3 October 1972 



Frame Degrees Nautica l Miles Comments 

77 84 120 Normal Target 

78 84 121 Weak Target 

79 84 122 Very Weak Target 

80 84 123 Normal Target 

81 84 124 Weak Target 

82 84 125 Very Weak Target 

83 84 126 Normal Target 

84 84 127 Weak Target 

85 84 No Target Return 

86 84 129 Normal Target 

87 84 130 Very Weak Target 

88 84 131 Normal Target 

103 84 153 Normal Target 

104 84 154 Weak Target 

105 84 No Target Return 

106 84 156 Very Weak Target 

107 84 No Target Return 

108 84 158 Weak Target 

109 84 159 Normal Target 

114 84 165 Normal Target 

115 84 166 Weak Target 

116 84 No Target Return 

117 84 168 Very Weak Target 

118 84 169 Normal Target 



A- 3 



Frame Degrees Nautical Mile s Comments 

119 84 No Target Return 

120 84 171 Weak Target 

121 84 173 Weak Target 

122 84 No Target Return 

123 84 176 Weak Target 

124 84 177 Normal Target 

291 77 114 Normal Target 

292 77 No Target Return 

293 77 115 Normal Target 

299 77 122 Normal Target 

300 77 No Target Return 

301 77 125 Normal Target 

307 77 131 Normal Target 

308 77 132 Weak Target 

309 77 133 Weak Target 

310 77 134 Normal Target 

455 89 132 Normal Target 

456 89 133 Very Weak Target 

457 89 134 Normal Target 

458 89 135 Very Weak Target 

459 89 136 Normal Target 

632 17 150 Normal Target 

633 16 150 Weak Target 

634 16 No Target Return 

635 15 149 Normal Target 

669 001 148 Normal Target 

670 000 148 Weak Target 

671 359 148 Weak Target 

672 358 148 Normal Target 

675 357.5 148 Normal Target 

676 357 148 Very Weak Target 

677 356.5 148 Weak Target 

678 356 148 Very Weak Target 

679 355.5 148 Weak Target 

680 355 148 Normal Target 

713 341 148 Normal Target 

714 340.5 148 Weak Target 

715 340 148 Normal Target 



A-4 



Frame Degrees Nautical Miles Comments 

1103 170.5 146 Normal Target 

1104 170 146 Weak Target 

1105 169.5 146 Normal Target 

1106 169 146 Weak Target 

1107 168.5 146 Normal Target 

4 October 1972 (Morning) 

116 10 101 Normal Target 

117 10 102 Weak Target 

118 10 103 Weak Target 

119 10 104 Normal Target 

120 10 105 Normal Target 

121 10 106 Normal Target 

122 10 No Target Return 

123 10 No Target Return 

124 10 109 Normal Target 

125 10 110 Normal Target 

126 10 111 Weak Target 

127 10 112 Weak Target 

128 10 113 Normal Target 

129 10 114 Weak Target 

130 10 115 Normal Target 

137 10 123 Normal Target 

138 10 124 Weak Target 

139 10 No Target Return 

140 10 126 Very Weak Target 

141 10 127 Very Weak Target 

142 10 No Target Return 

143 10 129 Very Weak Target 

144 10 130 Weak Target 

145 10 131 Weak Target 

146 10 132 Normal Target 

153 10 140 Normal Target 

154 10 No Target Return 

155 10 No Target Return 

156 10 143 Weak Target 

175 13 133 Normal Target 

176 13 132 Weak Target 

177 13 131 Normal Target 

184 13 126 Normal Target 

185 13 No Target Return 

186 13 124 Normal Target 



A- 5 



Frame Degrees Nautical Miles Comments 

211 13 101 Normal Target 

212 13 100 Very Weak Target 

213 13 99 Very Weak Target 

214 13 98 Normal Target 

306 16 98 Normal Target 

307 16 No Target Return 

308 16 101 Weak Target 

309 16 102 Normal Target 

310 16 103 Very Weak Target 

311 16 104 Normal Target 

317 16 110 Normal Target 

318 16 No Target Return 

319 16 No Target Return 

320 16 113 Normal Target 

322 16 115 Normal Target 

323 16 116 Weak Target 

324 16 117 Weak Target 

325 16 118 Normal Target 

330 16 123 Normal Target 

331 16 124 Very Weak Target 

332 16 125 Weak Target 

333 16 126 Weak Target 

334 16 127 Very Weak Target 

335 16 128 Very Weak Target 

336 16 129 Weak Target 

337 16 No Target Return 

338 16 131 Normal Target 

347 16 142 Normal Target 

348 16 143 Weak Target 

349 16 No Target Return 

350 16 145 Weak Target 

351 16 146 Normal Target 

353 16 148 Normal Target 

354 16 149 Weak Target 

355 16 No Target Return 

356 16 No Target Return 

357 16 152 . Weak Target 

358 16 153 Normal Target 

417 16 112 Normal Target 

418 16 111 Very Weak Target 

419 16 110 Very Weak Target 

420 16 109 Normal Target 



A- 6 



Azimu 


th 


In 


Range In 


Deg 


rees 


Nautical Miles 


16 






104 


lb 






103 


lb 








16 






101 


16 






100 


16 








16 






98 



Frame Degrees Nautical Miles Comment s 

426 16 104 Normal Target 

427 16 103 Weak Target 



428 16 No Target Return 

429 16 101 Normal Target 

430 16 100 Normal Target 

431 16 No Target Return 

432 16 98 Normal Target 

4 October 1972 (Afternoon) 

256 71 113 Normal Target 

257 71 No Target Return 

258 71 115 Very Weak Target 

259 71 116 Weak Target 

260 71 No Target Return 

261 71 119 Normal Target 

267 71 125 Normal Target 

268 71 126 Very Weak Target 

269 71 127 Normal Target 

273 71 131 Normal Target 

274 71 132 Weak Target 

275 71 No Target Return 

276 71 No Target Return 

277 71 135 Weak Target 

278 71 136 Normal Target 

357 60 73 Normal Target 

358 60 72 Weak Target 

359 60 71 Weak Target 

360 60 70 Normal Target 

387 71 63 Normal Target 

388 71 64 Weak Target 

389 71 65 Very Weak Target 

390 71 66 Broken Target 

391 71 67 Normal Target 

426 71 105 Normal Target 

427 71 No Target Return 

428 71 No Target Return 

429 71 No Target Return 

430 71 No Target Return 

431 71 No Target Return 

432 71 110 Normal Target 



A- 7 



Frame 

434 
435 
436 
437 
438 
439 
440 

445 
446 
447 
448 

450 
451 
452 
453 
454 
455 
456 



Azimuth 


In 


Range In 


Degrees 


Nautical Miles 


71 




112 


71 




113 


71 




114.5 


71 




116 


71 




117.5 


71 




119 


71 




120 


71 




125 


71 




126 


71 






71 




128 


71 




130 


71 




131 


71 






71 






71 




135 


71 




136 


71 




137 



Comments 

Normal Target 
Weak Target 
Weak Target 
Normal Target 
Weak Target 
Weak Target 
Normal Target 

Normal Target 
Weak Target 
No Target Return 
Normal Target 

Normal Target 
Weak Target 
No Target Return 
No Target Return 
Weak Target 
Weak Target 
Normal Target 



5 October 1972 



203 
204 
205 
206 



162 
162 
162 
162 



70 
71 
72 
73 



Normal Target 
Weak Target 
Weak Target 
Normal Target 



526 
527 
528 
529 
530 
531 



170 
170 
170 
170 
170 
170 



81 
80 

79 
78 
77 
76 



Normal Target 
Weak Target 
Very Weak Target 
Normal Target 
Weak Target 
Normal Target 



1231 
1232 
1233 
1234 
1235 
1236 
1237 
1238 
1239 
1240 



25 
23 



143 
143 



20 



142 



Normal Target 
Weak Target 
No Target Return 
No Target Return 
No Target Return 
No Target Return 
No Target Return 
No Target Return 
No Target Return 
Normal Target 



A-8 



Azimuth In Range In 

Frame Degrees Nautical Miles Comments 

1247 17 142 Normal Target 

1248 No Target Return 

1249 No Target Return 

1250 No Target Return 

1251 15 142 Normal Target 

1266 10 142 Normal Target 

1267 9 142 Weak Target 

1268 8 142 Weak Target 

1269 No Target Return 

1270 No Target Return 

1271 6 142 Normal Target 



A- 9