NRL Memorandum Report 1661

Dispersion of Frequencies in theOmega Navigation System

J. W. BRGG[A:N AND A. N. DUCKWORM-1

Radio Natigatiow BranchRadio Division

CLEAfl NO 10USEFOR F&DERAJ-, NTLrfc 1_ID

November 3, 1965

'1~JAN 1DDC.IRA

U.S. NAVAL RESEARCH LABORATORYWuhington, D.C.

DIITRIBIUL!ION OF THIS ?) X;! ENT IS ! NI.IMITFID.

CONTENTS

ABSTRACT ii

PROBLEM STATUS ii

AUTHORIZATION ii

INTRODUCTION 1

METHOD OF MEASUREMENT 3

DATA ANALYSIS 4

CONCLUSIONS 5

FUTURE PLANS 5

REFERENCE 6

FIGURES 1 THROUGH 8 7

ABSTRACT

It has been proposed to solve the lane identification problem ýnthe Omega navigation system by the transmission of signals at two ormore frequencies to produce beat frequencies. A frequency of 13.6kiloHertz (kHz) will be used with the basic 10.2 kHz transmissions

to obt, Jn a three-to-one increase in the width of the ambiguities.The phase velocities of waves at 10.2 kHz and 13.6 kHz are not thesame, so a four-to-three relationship of wave lengths will not existand correction factors will be required to obtain lane identification.

Observations were made in an aircraft flying from the transmittingstations out to ranges of 4000 miles. The results show that calculateddifferences in velocity of propagation will be accurate enough to obtainreliable lane identification. The necessary corrections could be pro-vided for the navigator in the form of simple tables.

PROBLEM STATUS

This is an interim report; work on this problem is continuing.

AUTHORIZATION

NRIL Problem 54R04-11

DUSWPS ProJect 88 161-001-6154

it

INTRODUCTION

The basic frequency in the Omega navigation system is 10.2kiloHertz (kHz): therefore, a line-of-position (l-o-p) will have anapproximate eight nautical mile ambiguity on the baseline. Thenavigator can resolve the ambiguity in several ways, one being jto set the receiver to the correct lane number at a known pointand then the correct lane number will be indicated automaticallyas the receiver moves through an operating area. If the signalor power to the receiver is lost, the correct lane number can berecovered by dead reckoning or by other means that the navigatormay have at his disposal.

The zone of ambiguity can be increased by the transmissionof additional signals at different frequencies and using the beatfrequency (reference (a)). This report is concerned with the useof 13.6 kHz in conjunction with the basic Omega frequency of10.2 kHz.

If 13.6 kHz is used with 10.2 kHz, a beat frequency of 3.4kHz is obtained. This frequency is one-third the basic Omegafrequency; aid if the velocities of propagation are the same, amoire" pattern will be produced in which every third zero phasepoint of the 10.2 kHz signal will coincide with every fourth zerophase point of the 13.6 kHz signal (figure 1). Thus, the zone ofambiguity his been increased by a factor of three. The use ofother frequencies can be used to produce other beat frequenciesand further inc.rease the width of the ambiguous zones.

The velocity of propagation at 10.2 kl and 13.6 kls is notthe same, so in actual Practlse a tuPl ftee-to-four pattern asshown in figure I will not be obeitrod. The following equationdeveloped by Watt efereneo a)) shows the phase velocity as afunction of frequency.

Equation 1

Y&a 1- 0. 36 h/a + 2n -I-

h = effective height of ionosphere at 10.2 kHz

day height - 69. 5 kilometers 0Cm)night height - 86.5 km

1

At 13.6 kHz

day height - 70.5 kmnight height - 87.0 km

a = radius of earth - 6.4 x 106 meters

%g and 0i = ground and ionosphere reflection phase lag coeffi-cients (radians)

Vp = phase velocity (meters per second)

Vo = velocity of light

f frequency in Hertz

n = propagation mode

There are other methods for the calculation of phase velocity butsince what is desired In this cese is the difference in phase volocit;"this method appeared as satisfactory as any.

The values of Vp/Vo obtained were as follows:

DAYTIME - Sea Water

1.0026 for 10.2 kHz - eastward0.9996 for 13.6 kHz - eastward1.0033 for 10.2 kHz - westward1.0000 for 13.6 kHz - westward

DAYTIMB - Land

1.0023 for 10.2 kHz - eastward0.9994 for 13.6 k•z - eastward1.00,0 9a 10.2 k•z - westwardo.9"1 for 13.6 kMz - westward

NIGHTTIME - Sea Water

X.9991 for 10.2 kHz - eastward0.9973 for 13.6 kHz - eastward0.9996 for 10.2 kHz - westward0.9976 for 13.6 kHz - westward

NIGHTTIME - Land

0.9988 for 10.2 kHz - eastward0.9971 for 13.6 kHz - eastward0.9992 for 10.2 kHz - westward0.9974 for 13.6 kHz - westward

2

While these values may be somewhat different than those thatwill be used in the charting of the system it is the differences in thevelocities that are of interest here and it is expected that errors inthe calculated differences should be negligible.

The result of this difference in velocity is shown in figure 2 whichis a plot of the difference as referred to wave lengths of 10.2 kHz orlanes on a radial basis. It can be seen that after approximately 100lanes (wave lengths) at 10.2 kHz have been traversed a difference ofone-third of a coarse lane will be accumulated. This corresponds toan error of one wave length at 10.2 kHz with reference to the trans-mitter. On a hyperbolic grid this would be an indicated 200 l-o-p onthe baseline. This curve is based on eastward propagation over seawater during the daytime.

METHOD OF MEASUREMENT

An ITT Federal Laboratories type AN/URN-18(lN-1) Omega monitorreceiver was modified by changing the gear head in the servo motorsfrom a ratio of 22325:1 to 4143:1. This enabled the equipment to trackat speeds of 200 knots with a lag error of less than two CEC8. OneCEC is equal to 0.01 wave length.

An adaptor was developed at the U. S. Naval Research Laboratoryto permit the AN/URN-18 or AN/WRN-2@CN-1) equipments t simulta-noously receive and record info:maticn bern 10.2 kiz and 13.6 kHzsignals. This equipment and & frequency standard manufactured bySulzer Laboratories were installed in a type 30-1211 aircraft SUNO135753).

The method of measurement wee to fly away f*o (or toward)an Omega station and count the number of wave l ths at eachfrequency. This Is basically wha Is dose In m- Omega.The counting is accomplialhd by compang the Phase ot each signalwith the frequency standard and recording the results on a solp-chartrecorder.

The first investigation was conducted ovsr the Atlantic Oean usingsignals from the Forestport transmitter. A flight from Patuxent Naval AirStation to Mildenhall Air Force Saw, England, was made. Stops weremade at the Naval Air Station, Argentia, Newfoundland, and Keflavik,Iceland, going over and at Lajes Air Force Base, Azores, andT KindleyAir Force Base, Bermuda, on the return trip. The trip was planned inthis way to have virtually all over-water daylight paths. The pailimumspeed of the aircraft and the tracking ability of the equipment dade itimpossible to make the trip in one daylight flight.

3

Tha signals from the Foestport station were the only ones thatcould be used because the attenuation at the Greenland icecep madethe signals from Haiku unusable over much of the trip and becauseonly one frequency was being transmitted from Criggion, Wales.

A second trip was made to Hickham Air Force Base, Hawaii, witha stop in California at the Naval Air Station, Moffett Field, on theoutbound trip and at the North Island Naval Air Station on the returntrip. Data was obtained from the Haiku, Hawaii, and the Forestport,Now York, Omega stations. These data gave information on westwardand overland propagation. It wah not possible to have all daylightconditions on this trip. Therefore, the calculations were made fornighttime conditions when applicable.

DATA ANALYSIS

Figure 3 is a plot of the data obtained on the trip to Mildenhall.The aircraft was flown close to the Forestport stad/on and this pointwas used as the starting point. Since only daylight paths were in-volved the data were referenced to take-off to the same reading aswas obtained on landing at each stop. The variations shown forapproximately thi first 40 wave lengths (620 miles) are caused bymode interference. The observed data follow the calculated valuesvery closely uttil the aircraft was about 115 wave lengths from thestation. Ner there is an umexlained deviation from the calculatedvalue. This oourved at I#=} so night-tn-day transition effects canbe ruled out. The valration as dhe arcrast got further from the sta-tion was due to the poor slosal-to-noise ratio. The receiver did notuse aided eck• lind, ft d hfie * a wider bandwidth then is used inthe &abma ft uoeirers was require.

we wver a ft"ae show that fte calculated velocity difference

timi, with doe wmwption of the close-ir. areas and where the signalstrengths wer margmu~l.

Similar data were obtained on the return trip (figu 4). Theseagain show that tho computed velocity difference would be accurateenough to provide lane reqolution. Slightly bettor results were ob-tained in the fringe area of the system besause of lower noise.

Figures S. 6, 7 and 8 show results obtained on the westwardtrip using both the Haiku station and the Forestport station. Thepropagation paths are more involved than those on the trip to Milden-hall. Long overland paths existed and it was not possible to makethe flights so no transition or nighttinme pericds would exist. TheF! 4

1*calculated curves were, therefore, plotted according to the existingconditions; i.e., da: or night, land or sea. The take-off times wereunder conditions of all-daylight paths, while the time of landing wasunder conditions of nighttime propagation. Therefore, the data attake-off from Muffett Field and North Island Naval Air Station do notshow the same reading as was obtained on landing, as a correctionwas put in for the diurnal variation.

The same variations due to mode interference again were obtainedfor approximately the first 50 wave lengths as was seen on the trip toMildenhall.

CONCLUSIONS

The data show that the calculated difference in the velocity ofpropagation will be accurate enough to provide lane identificationcorrection factors. Improved results can be obtained for transitionperiods after more experience has been obtained in choosing theconstants used in the calculations. The marginal operation of theequipment at extreme ranges was because the receiver did not userate-aiding and, therefore, a wider bandwidth was required.

FUTURE PLANS

Continued studies and investigations will be made using 11-1/3klIz signals end flight will be made Into te Arctic areas.

Investigations will be mAde to determine the effect of diurnalvariations on lane Identlficatlon at die venous Omga frequencies.

5

REFERENCE

(a) "Omega - A World-Wide Navigation System," System Specifi-cation and Implementation by J. A. Pierce, W. Palmer, A. D. Watt andR. H. Woodward, published oy Pickard and Burns Electronics, P ancd BPublication No. 886. dated 1 June 1964.

6

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6

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