NEL/REPORT 1350 PAS i | | iy a" | In Ng OSEL LHOdaH/13N ~ DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED SS re i. yee: 4 THE PROBLEM Analyze Omega phase differences recorded in port aboard the oceanographic survey ships HMS VIDAL and HR, MS, SNELLIUS. RESULTS Approximately 2500 hours of recorded phase differences from 24 ports of call and five different lines-of-position indicate that a 24-hour navigation capability of about 0.9 n.mi. (c.e.p.) may be expected in an implemented Omega system, assuming only present radiated powers and prediction ability. RECOMMENDATIONS Plan general refinements in the methods of synchronization and prediction, employing the data compiled for this report, to improve predictions. No further analysis specifically restricted to the Atlantic is envisioned, ADMINISTRATIVE INFORMATION Work was performed from October 1963 to December 1965 under SS 161 001, Task 6101 (NEL A10461). The report was approved for publication 10 January 1966. OCT O 0301 0040514 : The authors wish to express appreciation to the many per- sons who contributed to the success of the operation, but especially to the engineers at the transmitting stations and the officers and men of both ships. R. Gallenberger and R. Finkle, both of San Diego State College, assisted in the data analysis. Mrs. Rita Brown programmed the 1604 computer to predict skywave corrections. CONTENTS INTRODUCTION. . . page 5 INSTALLATION OF EQUIPMENT. . . 5 SUPPORDE is.) © RESUILUSHAG SEA. teed RIES Ui RSeINGe ORM.) seme el CONCLUSIONS. . . 26 REFERENCES... 27 TABLES 1 Average Phase Differences at Night. . - page 15-15 2 Typical Discrepancies Between Observed and Predicted Phase Differences. . . 24 ILLUSTRATIONS 1 Early Omega system accuracy (Atlantic): Forestport-Summit and Criggion-Summit. . . page 9 ; 2 Early Omega system accuracy (Atlantic): Criggion-Summit and Haiku-Summit. . . 10 3 Typical 10.2-kc/s Omega phase difference. . . 12 4 Phase perturbation over short paths at night. . . 17 5 Bathurst, Gambia: Forestport-Summit, 10.2-kc/s Omega phase difference. . . 20 6 Bridgetown, Barbados: Criggion-Summit, 10. 2-ke/s Omega phase difference. . . 21 7 Charleston, §.C.: Haiku-Forestport, 10.2-kc/s Omega phase difference. . . 22 8 Den Helder, Netherlands: Forestport-Summit, 10. 2-kc/s Omega phase difference. . . page £3 9 Las Palmas, Canary Islands: Criggion-Forestport, 10. 2-ke/s Omega phase difference. . . 24 REVERSE SIDE BLANK i i i A 5 5 at 7 7 3 a as é ae SOF 1G “Fi: ERTS tse jae Bee INTRODUCTION Although still developmental and not in continuous operation, the Omega navigation system is frequently in de- mand for special operations. Such an operation was the Project NAVADO oceanographic survey of the North Atlantic. Installation of an Omega receiver on HMS VIDAL was made in the fall of 1963. Subsequently the receiver was transferred to HR. MS, SNELLIUS and remained onboard until August 1965. 10.2-kc/s This report analyzes 10. 2-kc/s signals received by measurements in the survey ships in 24 ports of call. port analyzed INSTALLATION OF EQUIPMENT Installed on The installation of HMS VIDAL was made at the re- VIDAL, October quest of the Bureau of Ships (Code 362A). Routine installa- 1963 tion was accomplished at Portland Dockyard, Portland, England, in October 1963 with the assistance of a represent- ative of the U. S. Navy Electronics Laboratory. The field engineer also accompanied the ship for a period sufficient to provide necessary assistance in operation and training. Transferred Survey assignments within Project NAVADO were to SNELLIUS, changed in fall of 1964 with the VIDAL being reassigned to October 1964 resurvey the first four lines. The Dutch ship HR, MS, SNELLIUS continued the northern survey lines in the Atlantic. Because of the change in assignment and at the request of the Hydrographer, Royal Netherlands Navy, the Omega receiver was transferred from the VIDAL to the Removed August 1965 SUPPORT Close liaison Tables and skywave correc- tions provided Data received SNELLIUS in October 1964 at Portland dockyard. Again, the field engineer assisted in the installation and rode the ship to assist in operation and training. Because of the loss of signals from Criggion, due to other commitments, the equip- ment was removed at Halifax, Nova Scotia, in August 1965. Both installations were conventional and used the Omega receiver manufactured by ITT (Type I, Serial 2) with various whip and wire antennas. However, the internal Manson oscillator was not used on the VIDAL which had a precision frequency standard. An engineer from the U. 8S. Navy Electronics Laboratory spent a total of approximately five months aboard the ships during the two years of operation. When aboard, he assisted in navigation, receiver operation, and maintenance and helped train the officers and crew. Equally important, from the present viewpoint, he was responsible for monitoring when the ship was in port. NEL also provided support to the ships in the form of navigation tables and skywave corrections. Because of the large area involved (approximately 10 million square miles), this effort proved substantial although skywave corrections were computed more sparsely than would have been done for an operational system. All totalled, approxi- mately 20 hours of computer time were used for support and evaluation of the operation. Support was received as well as given. Both ships monitored Omega signals in port and forwarded the results to this Laboratory. Excellent cooperation was received and the data have been of substantial importance in refining the Omega system calibration. RESULTS AT SEA Equipment reliable Transmission schedule intermittent Correction problems While it is not the intent of this report to evaluate the actual operation at sea, it is germane to note that the equipment functioned reliably. Exclusive of recorders, only a few failures occurred during each year of operation, including routine tube replacements. Reliable functioning was also noted in informal letter reports from Rear Admiral G. S. Ritchie, R.N., presently the Royal Hydrographer but then Captain commanding the VIDAL, and from LT CDR F. Bradander, R. Neth. N., commanding the SNELLIUS. * It is regretted by all concerned that the developmental nature of the Omega system precluded con- tinuous transmissions from all stations. Most transmis- sions were scheduled either on a 16-hour day, seven days a week, or on a 24-hour day five days a week. Frequently, it was possible to provide only one line-of-position (LOP). It should be emphasized that intermittent operation is particu- larly inconvenient because of the necessity of reestablishing lane count. Indeed, the discontinuity of transmissions was such as to render operation nearly marginal at certain times. Particularly during the early part of the operation, skywave corrections were based on limited data. To im- prove calibration, the computed skywave corrections were frequently adjusted by local monitoring in port. This tech- nique, while unnecessary for a navigator in an implemented and well calibrated system, is believed to have substantially improved the actual navigation accuracy obtained at sea during the present operation. Also, it should have mitigated or eliminated the effects of an offset mistakenly included in the Criggion-Forestport computations. *See also reference 1 in list at end of report. Good signals Poor geometry In general, Forestport, Summit, and Criggion were easily received throughout the entire North Atlantic while infrequent attempts to measure Haiku were routinely success- ful in the Western Atlantic. In the Eastern Atlantic, Haiku was marginal in the Canary Islands (about 7000 n.mi.) and usually could not be used when the propagation path passed over Greenland (although measurements were made in the Netherlands). It is the consensus that a system such as Omega is particularly useful for oceanographic survey work in mid- ocean. The system was also of value in standard navigation problems such as making a land-fall after prolonged opera- tion without star sites. In addition to skywave correction problems, one of the most significant accuracy limitations in the Atlantic area is the purely geometric one resulting from the loca- tions of the existing developmental stations. Figures 1 and 2 illustrate the expected Omega fix accuracy for various lines-of-position in the North Atlantic. These are based on system geometry existing during the operation and a theoretical model which assumes 4-centilane timing errors believed indicative of daytime conditions. The accuracy at night and during transitions would be less than that indi- cated by the figures. ((a484yMAarea p'Q 40 UO!{D/ 91105 pud | a> p JO UO! {D/Aap paDpuDys pauinssD upd UO pasbg sinoyuo5 ADDANIID [D314a410 3y | ) ~y)Wwing-u0! 66145 ‘41 WUING-140d4sas0 4 :(214uD] 47) ADD4ND5DD watsks pBawg AjinFZ i ainbi 4 oS l o0E oS 009 oGL ie Vas ail Sa NVagglavo yassa \ Ooly oluwand vano g0€ yvanwaaa 341W % YONV1avSv2 YOA MAN ATW % NIVdS SAYNOZV : VILODS VAON FW 1 Sy (oy SaTiWw AL CNV TGNNOAMAN “HD HSI ISNA SATIW “AL vas CNV 13a! yoavagva - — 7.7. = - a 7 a i ((a48YMAIBAS f'Q JO UO!4D/a1403 pUD Ja> p 4$O UO!{D/ Aap P4Dpudis pauinssp UD UO pasng s4inojUod ADD4ND5D | DI!,a10aY /) “4/WLING-NyIDpy ‘4! wuing-UolBBi4> :(514UD]/ 47) ADD41NDDD Watsdks DBaw¢G AjsDg 7 aunbiy re!) oSl o0E oSP 909 OSL o0E of 10 RESULTS IN PORT Whenever the survey ships were in port, available Omega signals were monitored. These data are especially valuable since the positions of the ships were carefully determined.* Most of the in-port data were measured by the respective navigators and forwarded to NEL for analysis. The data have proved to be virtually free from incidental errors and nearly always significant. Slaved Phase differences were measured in 24 ports of call transmissions from 10.2-kc/s transmissions in the "'slaved'' mode. As measured in 24 changes in the transmission schedule caused unusual lines- ports of call of-position to be available at various times and since some of the ports were revisited, more data were obtained than might be expected from single visits using existing equip- ment. Also, manual and automatic time sharing were used 60 site-LOP’s for afew measurements. The net result was that 60 site- LOP's were obtained using five different lines-of-position. The average monitoring duration was three days. Half-hourly readings from a typical phase differ- ence recording are shown in figure 3. The scatter on the graph is that actually displayed on the original recordings since no time averaging was used in the reduction. The scatter is indicative of the receiver time constant of about 30 seconds. For the particular measurement shown in figure 3, all component propagation paths are dark between *Each position determination was referenced to local datum. Accurate conversion to international coordinates could not be made for all locations. The overall error resulting from inadequate geodetic information is believed small although probably appreciable. Errors at individual sites, however, may be important. PHASE DIFFERENCE (CEC) 0 0000 0400 - 0800 1200 1600 2000 2400 TIME (GMT) Figure 3. Typical 10.2-kc/s Omega phase difference. Half-hourly measurements on Criggion-Summit in-port at Chaguaramas, Trinidad, 21-24 Dec 63. 2300 and 0800Z while all are sunlit between 1130 and 1600Z. The figure shows the usual features of the Omega phase difference, namely, constant (or ''flat'') at night with a slow change (or ''curvature'') during the day. Because phase differences tend to be constant at night, a single average nighttime phase difference may be meaningfully computed for each of the site-LOP's. The average values observed, combined with appropriate lane assignments, may then be compared with theoretical compu- tations. Such a comparison is shown in table 1 where the theoretical computations were based on the determination of phase coefficients made early in 1965.° The last column is TABLE 1. AVERAGE PHASE DIFFERENCES AT NIGHT 4 Lop oe No. We a iy discrepancy Nights verage with computation LOP? Site (lanes) (cec)* EE SU Al Yadida, Jul 64 2 64.54 1.5 Morocco EP-SU Azores Jun 64 2 ] 59.67 4.3 CR-SU Azores Jun 64 3 2 152.08 - 6 CR-SU Bathurst, Gambia Dec 63 2 ] 202.04 -10.1 EP-SU Bathurst, Gambia Dec 63 2 ] 116.06 2.6 CR-SU Bridgetown, Jan 64 5 2 432.53 - 8.0 Barbados FP-SU Bridgetown, Jan 64 ] ] 179.82 12 : Barbados CR-SU Cadiz, Spain Jul 64 2 ] 74.65 -29 Be SU Cadiz, Spain Jul 64 ] ] 56.65 1.6 CR-SU Casablanca, Jul 64 ] ] 90.63 - 5.8 Morocco CR-SU Casablanca, Apr 65 ] | 5S | -99 Morocco FR-SU Casablanca, Jul 64 2 ] 63.35 = 4 Morocco CR-SU Chaguaramas, Nov 63 10 3 452.50 - 79 Trinidad FP-SU Chaguaramas, Nov 63 5 2 194.26 - 6.5 Trinidad FP-SU Den Helder, Nov 64 a 3 24.42 = 5.3 Netherlands CR-SU Gibraltar Jun 64 2 1 73.09 25 FP-SU Gibraltar Jun 64 2 ] 57.07 ja LOR? CR-FP HK-FP FP-SU CR-FP CR-SU CR-FP FP-SU FP-SU FP-SU FP-SU FP-SU CR-SU FP-SU CEE HK-FP CR-SU FP-SU HK-SU 14 Site-LOP Site Hamilton, Bermuda Hamilton, Bermuda La Coruna, Spain Las Palmas, Canary Islands Lisbon, Portugal Lisbon, Portugal Lisbon, Portugal Matosinhos, Portugal Portland, U.K. Portland, U.K. Rotterdam, Netherlands San Juan, P.R. San Juan, P.R. Charleston, S.C. Charleston, S.C. Curacao Curacao Curacao Date Table 1. (Continued) Wt Night Average (lanes) Normal Site-LOP’s (Continued) Mar 65 Mar 65 Noy 64 Feb 65 May 64 May 65 May 64 Oct 63 Oct 63 Nov 64 Oct 64 Dec 64 Dec 64 Mar 65 Mar 65 Dec 64 Dec 64 Dec 64 WO WN OH ] ] mo Nw S| NY ‘‘Near’’ Site-LOP’s DDAaA WwW Ww Gon SC4 Gs Gs eG) 308.39 522.46 43.56 89.08 76.23 49.02 53.20 47.95 29.35 29.39 26.08 445.74 168.14 351.20 485.08 490.44 209.00 566.51 Residual discrepancy with computation (cec)© 10.7 6.6 1.0 3.2 2.0 Table 1. (Continued) re Lop . Ne : Night Residual discrepancy . ate Nicute Wt Average with computation LOP Site g (lanes) (cec)~ ““Near’’ Site-LOP’s (Continued) CR-FP Den Helder, Jul 65 2 0 1.84 22.6 Netherlands CR-SU La Coruna, Nov 64 ] 0 54.84 - 6.2 Spain CR-FP Norfolk, Va. Apr 65 13 0 349.88 112 HK-FP Norfolk, Va. Apr 65 0 S10 19 = 15 CR-SU Portland, U.K. Nov 64 0 7.49 - 1.6 CR-SU Rotterdam, Oct 64 ] 0 2 28.8 Netherlands Site-LOP’s With Arctic Path Component HK-SU Den Helder, Nov 64 6 0 381.98 23.5 Netherlands HK-FP Las Palmas, Feb 65 2 ) 527,95 - 18 Canary Islands HK-FP Tenerife, Feb 65 ] 0 528.26 5.0 Canary Islands Notes: a. LOP’s coded slave minus master as follows: HK, Haiku; FP, Forestport; SU, Summit; CR, Criggion. ‘‘Near’’ site-LOP’s have path components less than 700 n.mi.; site-LOP’s with arctic path components include propagation in arctic regions of poor conductivity. b. Weights approximately equal to square root of number of nights of observation (except ‘‘near’’ site-LOP’s and LOP’s with arctic path components). c. Residual positive if observation greater than computation, i.e., positive residual indicates additional net delay. Note that a delay on the path to the master station will yield a negative residual. 15 Discrepancy of night averages 5.8 cec Perturbations over short paths at night Theoretical perturbations not large the residual discrepancy between the observed average and the computed value in centicycles (cec). Since the observa- tional average from only a few nights of monitoring is not necessarily equal to a long-term mean at a particular site, the rms residual discrepancy of 5.8 cec for the 30 site-LOP's, where the prediction theory is applicable, is not completely due to prediction error. Presently, the prediction theory is invalid near transmitting stations where significant energy may be propagated by the second wave-guide mode or in cases where one of the component propagation paths passes over arctic regions of very poor conductivity.* Although practical navigation was not recommended using a station within 1000 n.mi., for tabulation purposes only measurements with component propagation paths shorter than 700 n.mi. (1300 km) have been identified as 'near."" A theoretical estimate of the effect of the second mode on the resultant phase over short propagation paths is shown in figure 4. The curve was produced from the work of Wait and Spies ° based on the isotropic model ionosphere. * The theo- retical work is important since it indicates peak deviations of on the order of one n.mi. or less. However, the work cannot yet be considered sufficiently reliable to warrant addi- tional corrections. In particular, if the quasi-wave length were only slightly off, corrections of the wrong sign might be *Specifically an ionospheric height of 90 km, gradient of 0.5 km’ and infinite surface conductivity were assumed. The curves of Wait and Spies were then used to obtain the relevant parameters for propagation at 10.2 ke/s, viz: relative velocity, attenuation rate, and excitation, The values obtained were, respectively, 1.0003; 1.6 dB/Mm; -1.3 dB at 6.3° for the first mode and 1.0303; 8.8 dB/Mm; and 1.4 dB at 20° for the second. i 20 o c 15 2 z 10 - > & & 5 = << e 0 = va < =5 a > 5 8 -i0 lu Ly e = 5 | -20 0 1000 2000 3000 4000 5000 PATH LENGTH (NAUTICAL MILES) Figure 4. Phase perturbation over short paths at night. (After Wait & Spies, 10.2 kc/s.) applied. Nonetheless, there is a rather surprising agree- ment between the computed effects and those actually observed. Charleston, S. C., is 675 n.mi, (1250 km) from Short path Forestport, while Curacao is 660 n.mi. (1220 km) from effects observed Summit, so that measurements at these sites would be expected to have +6.5-cec residual discrepancies in table 1. The actual discrepancies average 4.3 cec. The theoretical work indi- cates that an additional delay of 12 cec should be expected over the 395 n.mi. (731 km) path from Forestport to Norfolk, Va. The sense of the measurement indicates that a residual of -12 cec would be expected. The average residual is about -9 cec. Because of the effects of higher-order modes, the Ibe Arctic propagation 18 theoretical model may not apply over shorter paths such as those from Criggion to the Netherlands sites. While the phase differences observed in Holland cannot be explained at present, the path lengths, 285 and 281 n.mi. (528 and 521 km), are comparable as are the residual discrepancies. * The foregoing is not an adequate sample from which to verify the theoretical model. However, it is at least en- couraging and may constitute an initial understanding of Omega operation near transmitters. It is noteworthy that both the observed and theoretical discrepancies are small at moderate distances from stations. Exclusive of sites where higher modes may be significant, the rms discrepancy of the observed data in this region is 7.2 cec if no second mode corrections are made. A consistent picture of arctic propagation cannot be expected from brief monitoring at only five locations. However, some of the measurements in the Canary Islands and the Azores are of considerable value since the residual errors can confidently be said to include the effect of a de- crease in the propagation velocity of the Haiku signal in the vicinity of Hudson's Bay and Labrador. In particular, the propagation paths may be compared with a recent conduc- tivity map by E. L. Maxwell** from which it is found that about 3000 km of propagation occur over ground with a con- ductivity of one millimho per meter or less, nominally averaging 0.6 millimho per meter. While theoretical esi- mates of the additional phase delay expected for a 10. 2-kc/s signal propagated over this path are not directly available, *Criggion-Summit phase differences in Portland, U.K., re- flect propagation over a very short path. **Furnished in a personal communication to the author, 9 June 1965. Daytime propagation Computer predictions a crude estimate is possible using the work of Wait and Spies. °° A delay of 10 to 20 cec may be expected. The effect is neither confirmed nor rejected by the observed phase differences because of the brief monitoring and also because of the high scatter resulting from a poor signal-to- noise ratio. The measurements at Den Helder include propaga- tion directly over the Greenland ice cap and also propagation at extreme northern latitudes. No attempt at analysis is justified at the present time. In fact, the lane assignment for this particular measurement is only tentative. Phase-difference measurements during the day are more complicated to analyze than those at night, since phase velocity varies with the solar zenith angle and exhibits related seasonal changes. In general, for normal illumina- tion of the ionosphere, phase prediction is more easily accomplished during the day than at night. In particular, propagation during the day is less sensitive to geomagnetic path orientation, latitude, and the effects of the second waveguide mode. Prediction methods have been discussed elsewhere and widely applied.*’’ The basic prediction coefficients have been in use since December 1963. Day and night prediction coefficients have been combined in a computer program together with transition criteria to predict the phase difference observed at any location at any time of day. While neither the daytime pre- diction coefficients nor the transition criteria have been changed since the inception of the computer program, changes in the computation procedure at night were made early in 1965 and can affect the predictions at all times of day. Pre- sent computer results therefore differ slightly from those provided for navigational use during the early part of the operation and those used in an earlier analysis.® Also, an offset erroneously present in the Criggion-Forestport cor- rections has been corrected. Comparisons of observed 19 PHASE DIFPERENCE (CEC) 0000 0400 0800 1200 1600 2000 2400 phase differences with predicted diurnal variations are shown in figures 5 to 9. A complete set of such curves has been compiled as a supplement to this report.” The computer predictions are no more accurate than the basic prediction model employed and hence the limi- tations previously discussed apply; the effects of the second and higher propagation modes should be considered when using a nearby transmitter. In practice, operation within 1000 n.mi. of a station was not advised although the correc- tions provided were valid first-mode predictions to 840 n. mi. TIME (GMT) Figure 5. Bathurst, Gambia: Forestport-Summit, 10.2-kc/s Omega phase difference (4 to 6 Dec 63). PHASE DIFFERENCE (CEC) Within 840 n.mi. the computations were invalid for naviga- tion but of some interest for analysis. Recently the computer program was modified to approximate corrections near transmitters. The modification uses simplified transition criteria and still does not include the effects of the second and higher-order propagation modes. Where applicable, comparisons with observed data have been made with modi- fied corrections. The supplement indicates that the cor- rections may be adequate for some types of navigation even using nearby transmitters. 2 60 40 20 00 0000 0400 0800 1200 1600 2000 2400 TIME (GMT) Figure 6. Bridgetown, Barbados: Criggion-Summit, 10.2-kc/s Omega phase difference (30 Dec 63 to 2 Jan 64). 21 Results 22 PHASE DIFFERENCE (CEC) NO TRANSMISSIONS FROM 0835 TO 1650Z 60 40 20 0000 0400 0800 1200 1600 2000 2400 TIME (GMT) Figure 7. Charleston, South Carolina: Haiku-Forestport, 10.2-kc/s Omega phase difference (8 to 11 Mar 65). Comparison of the observed and computed phase differences permits a system evaluation in the area. * Three periods will be considered separately, namely: night (when all component propagation paths are dark); day (all illuminated); and transition (mixed). During each period, each observed value has been compared with the *Because of present prediction limitations, measurements including propagation over paths less than 700 n.mi. or in arctic regions have been excluded from the analysis. PHASE DIFFERENCE (CEC) 0000 0400 0800 1200 1600 2000 TIME (GMT) Figure 8. Den Helder, Netherlands: Forestport-Summit, 10.2-kc/s Omega phase difference (16 to 26 Nov 64). corresponding prediction and an rms value obtained. * The results are summarized in table 2. The discrepancies re- flect errors of all types but especially the stability of the propagation medium and present prediction errors. The results are consistent with previous estimates. However, the rms discrepancy during the day has been seriously affected by prediction errors and may be expected to 2400 *See ref. 9 for a description of the computations and more detailed summary of the results. 23 PHASE DIFFERENCE (CEC) 60 40 NO TRANSMISSIONS FROM 0900 TO 1330Z 20 00 0000: 0400 0800 1200 1600 2000 2400 TIME (GMT) Figure 9. Las Palmas, Canary Islands: Criggion-Forestport, 10.2-kc/s Omega phase difference (10 to 14 Feb 65). TABLE 2. TYPICAL DISCREPANCIES BETWEEN OBSERVED AND PREDICTED PHASE DIFFERENCES Period RMS Discrepancy (cec) Day 6.7 Night 8.1 Transition 10.8 Overall 9.2 Probable fix accuracy during survey if using present predic- tions: 1-1/2 n.mi. Accuracy of implemented system: Day 0.7 n.mi. Night 0.8 n.mi. Transition 1.1 n.mi. Overall 0.9 n.mi. substantially improve when the daytime propagation coeffi- cients have been revalued. * The overall accuracy which would be actually ob- tained using present prediction in the entire North Atlantic area may be estimated from figures 1 and 2 and the overall rms discrepancy. Apparently, a typical circular error probable on the order of 1-1/2 n.mi. is appropriate for the 24-hour day. However, results during the day would be better than nominal while results at night and during transi- tions would be worse; and, of course, the expected accuracy varies considerably with position. In practice, actual re- sults might be either better or worse. The earlier skywave corrections provided to the ships were not as refined as those used in the present analysis. Computations for Criggion-Forestport were in error. At best, the spatial density of corrections was rather sparse, thus presenting a possibility for interpolation errors. However, adjustment of the computec diurnals by local monitoring in port may have considerably reduced the effect of prediction errors. The results may also be used to evaluate the accu- racy available in an implemented Omega system. The appli- cation is not direct but depends on assumptions of eventual system geometry, possible effects of spatial correlation, and the effect of operating in the absolute mode instead of operating with some stations actually synchronizing as slaves to a master station. It has been shown that an approxi- mate conversion from LOP error in the synchronized mode to fix error in an implemented system may be made by mul- tiplying the present results by 0.1 n.mi./cec.** Hence, the *See footnote on page 8 of reference 7 and also reference 4. **See references 2, 10, and 11. In particular, multiplica- tion by 0.106 n. mi. /cec yields rms fix error while multi- plication by 0.093 n.mi./cec yields the c.e.p. 25 CONCLUSIONS 26 observed phase differences are indicative of an implemented system accuracy of 0.7 n.mi. during the day, 0.8 n.mi. at night and 1.1 n.mi. during transitions, based on present radiated powers and predictability. Omega phase differences on various lines-of- position were measured over ten million square miles during a period of two years. Most data are well explained by pre- sent propagation theory while the additional data measured should serve as a useful basis for extending the theory to arctic and short propagation paths. The results continue to indicate that a 24-hour accuracy of about 0.9 n.mi. will be obtained in an imple- mented Omega system. The operation was also noteworthy from the stand- points of reliable equipment operation for an extended period and of excellent and mutually beneficial international coop- eration. This Laboratory is especially appreciative of the fine cooperation received from the entire complements of both ships. REFERENCES Canada. Hydrographic Service. NAVADO III, NAVADO Ill H. Neth. M.S. SNELLIUS, by M.M. DeLeeuw, 14 September 1965 Navy Electronics Laboratory Report 1188, Estimating the Accuracy of Navigation Systems, by E.R. Swanson, 24 October 1963 Navy Electronics Laboratory Technical Memorandum 781, Present Predictability of Omega 10.2 kc/s Nighttime Signals, by E.R. Swanson, 18 March 1965 Navy Electronics Laboratory Report, Propagation of 10.2 kc/s Electromagnetic Waves, by E. R. Swanson (In preparation) National Bureau of Standards Technical Note 300, Characteristics of the Earth-Ionosphere Waveguide for VLF Radio Waves, by J.R. Wait and K.P. Spies, 30 December 1964 Wait, J.R. and Spies, K.P., "Influence of Finite Ground Conductivity on the Propagation of VLF Radio Waves," Journal of Research of the National Bureau of Standards. Section D: Radio Science, v.69D, p.1359-1373, October 1965 Navy Electronics Laboratory Report 1226, Omega Navi- gation Capability Based on Previous Monitoring and Present Prediction Ability, by E.R. Swanson, 5 June 1964 27 REFERENCES (Continued) 8. 10. ibe 28 Navy Electronics Laboratory Letter Report 103, Omega Operations Aboard H. M.S. ViDAL (1963-1964), 25 September 1964 Navy Electronics Laboratory Technical Memorandum 897, Data Supplement to NEL Report 1350, by E.R. Swanson, 10 January 1966 Navy Electronics Laboratory Report 1185, Accuracy of the Omega Navigation System, by M.L. Tibbals and D.P. Heritage, 2 October 1963 Navy Electronics Laboratory Report 1305, Omega Lane Resolution, by E.R. Swanson, 5 August 1965 Note: Since this report was prepared, the work by E. L. Maxwell referred to on page 18 has been published in: Deco Electronics, Inec., Report 54-F-1, OMEGA Navigational System-Conductivity Map, by R. R. Morgan and E. L. Maxwell, December 1965 UNCLASSIFIED Security Classification DOCUMENT CONTROL DATA - R&D (Security classification of title, body of abstract and indexing annotation must be entered when the overall report is classified) 1. ORIGINATING ACTIVITY (Corporate author) 2a. REPORT SECURITY CLASSIFICATION U.S. Navy Electronics Laboratory Unclassified 2 ea ae 3. REPORT TITLE OMEGA IN THE ATLANTIC (1963-1965) 4. DESCRIPTIVE NOTES (Type of report and inclusive dates) Research and Development; October 1963 - December 1965 5. AUTHOR(S) (Last name, first name, initial) Swanson, E. R. and Davis, W. E. 6. REPORT DATE 7a. TOTAL NO. OF PAGES 7b. NO. OF REFS 10 January 1966 28 ial 8a. CONTRACT OR GRANT NO. 9a. ORIGINATOR'’S REPORT NUMBER(S) b. PRosEcTNo. OO 161 001, Task 6101 1350 (NEL A10461) 9b. OTHER REPORT NO(S) (Any other numbers that may be assigned this report) 10. AVAIL ABILITY/LIMITATION NOTICES Distribution of this document is unlimited. 11. SUPPLEMENTARY NOTES 12. SPONSORING MILITARY ACTIVITY Bureau of Ships, Department of the Navy, Washington, D. C. 20360 13. ABSTRACT Measurements were made of 10.2-kc/s phase difference in 24 ports of call in the Atlantic area. The measurements are generally well explained by present propagation models which indicate that an implemented Omega system will have an accuracy of about 0.9 n. mi. (c.e.p.) over the 24-hour day. DD .528. 1473 101-807-6800 UNCLASSIFIED Security Classification UNCLASSIFIED Security Classification KEY WORDS Very Low Frequencies Phase-Difference Measurements Omega Navigation System NAVADO Oceanographic Surveys HMS VIDAL HR. MS. SNELLIUS INSTRUCTIONS \. ORIGINATING ACTIVITY: Enter the name and address of the contractor, subcontractor, grantee, Department of De fense activity or other organization (corporate author) issuing the report. 2a. REPORT SECURITY CLASSIFICATION: Enter the over- all security classification of the report. Indicate whether “Restricted Data’’ is included. 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