SUBMICROSECOND COMPARISONS OF TIME STANDARDS VIA THE NAVIGATION--FT~C(,APR 80 .J BUISSON, T MCCASKILL, J OAKSUNCLASSIFIED NRL-8395 SSIE-AO-EOO0 45 NL

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NRL Report 8395

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20. Abstmt (Continued)

"- U.S. at the Goddard Space Flight Center (GSFC), National Bureau of Standards (NBS), NavalResearch Laboratory (NRL), and the Naval Observatory (USNO).

The results obtained by satellite were compared to those obtained by portable clocks. At allstations the differences were less than 0.75 %. The RMS of the data from nine stations is less than0.4 /a . . .... . .-

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CONTENTS

INTRODUCTION................... 1

TIME DIFFERENCE MEASUREMENTS .................... 1

SPACECRAFT FREQUENCY STANDARDS ................. 2

TIME TRANSFER TECHNIQUES ......................... 2

TIME TRANSFER RESULTS ............................ 4

SYSTEM CLOSURE ................................... 8

CONCLUSIONS ....................................... 9

ACKNOWLEDGMENTS ................................ 10

REFERENCES ....................................... 10

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SUBMICROSECOND COMPARISONS OF TIME STANDARDS

VIA THE NAVIGATION TECHNOLOGY SATELLITES (NTS)

INTRODUCTION

The major objective of the experiment was to perform an interim demonstration ofthe time transfer capability of the NAVSTAR GPS system using a single NTS satellite.Measurements of time difference are made from the NTS tracking network and at theparticipating observatories. The NTS network measurements are used to compute the NTSorbit trajectory. The central NTS tracking station has a time link to the Naval ObservatoryUTC(USNO,MC1) master clock. Using measurements taken with the NTS receiver at theremote observatory, the time transfer value UTC(USNO,MC1) - UTC(REMOTE, VIA NTS)is calculated. The goal for the NTS effort was to achieve worldwide time transfer of lessthan 1.0ps accuracy. Comparison of the satellite measurements to travelling clock observa-tions showed differences no greater than 0.7 ps.

A second objective was to compute weekly worldwide intercomparisons of theobservatory clock offsets using predicted values of satellite clock offset and ephemeris.Each participant entered appropriate measurements into computer files for later processing.Other objectives include colocation at laser sites and the use of the observatory time scalesin evaluating the spacecraft clock performance.

For GPS, a similar procedure could be followed using simultaneous measurementstaken between the user and four GPS satellites. With the four GPS pseudo-range (timedifference) measurements taken at an unknown location the user may solve for three posi-tion coordinates in addition to time offset with respect to GPS time.

TIME DIFFERENCE MEASUREMENTS

Time difference (pseudo.range) measurements are made between the spacecraft andthe user by side tone ranging [1]. The NTS.2 spacecraft also has a GPS pseudo-randomsequence transmitter. All measurements presented in this paper were made using the sidetone ranging system, which has a resolution of 1.56 ns (48 cm). Measurements of timedifference may be converted to pseudo-range by multiplying by the speed of light in avacuum. Units of time are used in this report to facilitate comparisons with the PTTIcommunity.

The time difference measurement is composed of the difference between the satelliteclock and the user clock, plus satellite transmitter delays, propagation path delay, iono-spheric delay, tropospheric delay, user antenna delay, cable delay, and receiver delay. Allof these factors must be measured or estimated. In addition to the above factors, the

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BUISSON, ET AL

spacecraft clock is influenced by the relativistic frequency shift, magnetic fields, energeticparticles, and small variations in temperature and drive level.

Receivers of two designs were employed in making the measurements. One receiver 121made measurements at a nominal UHF frequency of 335 MHz. The second receiver wascapable of making measurements at the L band frequency of 1580 MHz in addition to theUHF frequency. The two channel receiver measurements were combined, by software, tocorrect for the first order ionospheric refraction.

SPACECRAFT FREQUENCY STANDARDS

Timing signals transmitted from NTS-2 are derived from a cesium frequency standard;NTS-1 employs both rubidium and quartz oscillators. Frequency stability results have beenpreviously reported [3,41 for one of the NTS-2 cesium standards and for rubidium andquartz oscillators.

The NTS-2 cesium standard was used to measure the relativistic frequency shift [51 atthe GPS constellation altitude. The NTS-2 cesium output frequency was adjusted so thatthe received frequency is near that of UTC(USNO,MC1). In contrast, the NTS-1 quartzoscillator is periodically adjusted in frequency and time. The maximum frequency excur-sions of the quartz varied from + 2 X 10 - 9 to - 4 X 10 - 9 with respect to UTC(USNO,MCI). Noteworthy is the fact that the ease of operation is superior with cesium, inasmuchas comparatively large periodic adjustments are required with the quartz frequency standard.

TIME TRANSFER TECHNIQUES

The time transfer to a remote location is obtained by four time links to UTC(USNO,MCI). The four links are (a) from the remote user clock to the spacecraft clock, (b) thespacecraft frequency time update for the time difference between observations obtained atthe remote site and the central site, (c) from the central station clock to the spacecraftclock, and (d) from the central station clock to UTC(USNO,MC1). Figure 1 depicts the fourlinks used in this procedure. This procedure incorporates the short to medium termstability of the spacecraft and control station clock with the long term stability of the U.S.Naval Observatory multi-clock time scale.

Measurements of [UTC(USNOMC1)-UTC(REMOTE, VIA NTS)] may be taken witha variety of frequency sources of varying stability. The major observatories participating inthis experiment possess frequency standards and time scales of proven accuracy, withsufficient difference in geographic location (Fig. 2), to check the time transfer at differentpositions of the spacecraft orbit.

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NRL REPORT 8395

SATELLITE--- CLOCK NTSUPDATE SATELLITE

STTON

Fig. 1 - Time transfer configurion

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BUISSON, ET AL

TIME TRANSFER RESULTS

Figures 3 through 12 present time transfer results as determined from the NTS space-

craft. The figures are similar in format inasmuch as each remote observing station is

referenced through the NTS central ground observing station located at Chesapeake Bay

Division (CBD) of NRL. The CBD site is linked to the USNOMC by a series of portable

clock closures to an accuracy of 10 to 20 ns.

EPOCH days# 156500

-140 EPOCH Idey| 186.00 4 OFFSET (06) 1.574

OFFSET it,) -160.734 FREQ (10- 1) -. 006

PREO (1 "" -. 246 RPS (psi 319

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Fig. 3 - Time transfer results from Royal Green- Fig. 4 - Time transfer results from Paris OP

wich Observatory (USNO,MC1)-(RGOJP) (USNOMC1)-(OP)

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90OCH Cays? t30.003 OFF39T (psi .99S -8 EPOCH (days) 16.00

(pal -. OFFSET Upsl -10.277

FRu g0 1 1 .04 FREO 10-11) -. 017

RMS | . - o RMS (psi .377

-110

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126 127 126 129 I30 131 132 33 134 35 us it 10 2 0 n60

DAY Will DAY 16'5

Fig. 5 Time tnder remults from Cerga, France Fig. 6 - Time transfer results from the Institute(USNO,MC1)-(CERGA) for Applied Geodesy, Wettzell, West Germany

(USNOMCI )-(IFAG)

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NRL REPORT 8395

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EPOCH (dayVs 186.00200 OFFSET (ts) 158.902 EPOCH (divs) 30300

FREO (10-11

1 -329 -9 OFFSET t%) -18050

RMS (u) .458 FREO (10-l1

) -. 010190-12 RMS (0$) .862

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130 1 80 1 I 1 1 -30 4

100 130 160 190 220 250 20 26 300 305 310 31 320 2S

DAY 1I'M DAY 1978

Fig. 7 - Time transfer results from the Division of Fig. 8 - Time transfer results from RRL, JapanNational Mapping, Australia (USNOmMC1)-(AUS, (USNO.MCI)-(RRL)DNM)

-36 I 4

EPOCH dys) 304.00 EPOCH (days) 151.00-39 OFFSET ps) -47.829 3- FFSET Jos) -474

FREG (M)") .003 FRED I10- 1 -. 014

-42 RMS (0s) .996 2 RMS (ps) 398

-45- I

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-54- -2

..--- PORTABLE CLOCK

-57 - 3

-40 i I 2 -4 WO295 30 5 0 I 1S0 10 210 240 210

DAY 19 7 DAY 11

Fig. 9 - Time transfer results from NRLM, Japan Fig. 10 - Time transfer results from NBS in(USNO,MC1)-NRLM) Colorado, U.S. (USNOMC1)-(NBS)

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BUISSON, ET AL

EPOCH (dys) 186.00

EPOCH days) 186.00 3 OFFSET (W, - 036OF FSET (as) -3.716 FREG ( , 0-1 1 ) .00

FREO 10- 1 1

) -. 005 RMS (s) 171- RMS ( s .152 2

-2 1

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110 140 170 20D 230 230 90 120 10 180 210 240 270

DAY 1978 DAY 1978

Fig. 11 - Time transfer results from National Fig. 12- Time transfer results from USNOResearch Council, Canada (USNO,MC1)-(NRC) (USNOMC1)-(USNO,MC1,NTS)

Table 1 presents the phase offset and frequency difference of each remote scation clockagainst the USNOMC for a given epoch time which is nominally placed in the middle of theobserved data span. In addition, the RMS of a straight line least squares fit to all satellitepasses observed by the remote station is presented as a measure of the noise in the timetransfer values.

Table 1 - NTS Time Transfer ResultsUTC(USNO,MC)-UTC(remote, NTS)

Epoch Phase Offset Frequency RMSRemote Site (day

1978) (ps) (pp10 1 1 ) (ns)

RGO (JP) 186 -160.734 -0.245 369BIH (OP) 156 1.574 -0.006 318CERGA 130 0.995 -0.049 324IFAG 186 -10.277 -0.017 377DNM (590) 186 158.902 0.329 458RRL 303 -18.050 -0.010 862NRLM 304 -47.829 0.003 998NBS 151 -0.474 -0.014 398NRC 186 -3.716 -0.005 152USNO (MC) 186 -0.036 0.000 171

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NRL REPORT 8395

From Table 1 it can be seen that the two Japanese remote sites (RRL and NRLM)exhibit a higher noise level than the other observing stations. These higher noise levelmeasurements were the result of using predicted satellite position ephemeris. Furtheranalysis will be performed using observed orbital trajectory.

Also plotted in Figs. 3 through 12 are the results of portable clock closures performedby personnel from the USNO. These portable clock closures are used as "truth" orabsolute accuracy tie-in for the NTS results.

Figures 13 through 15 present time transfer results from the NTS remote observingstation located at the Panama Canal Zone (CZ) site. Results in this data span wereobtained with both NTS2 and NTS1 spacecrafts. The NTS2 data included observationsavailable at both 335 MHz and 1580 MHz, allowing for a first order ionospheric delaymeasurement.

-22

-2D EPOC4 4ays) iT1.00

EPOCH (days) 119.00 -23 OFFSET (pa) -26.293-21 OFFSET (0S -23.882 FREG (10"

1) -. 093

%b oo FREO (10-11) -. 052-22 RS (s .330 -24 RMS (a) 063

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DAY 197 DAY W978

Fig. 13 - Time transfer results from Panama Fig. 14 - Panama resultsUSNO,MC1)-(PMA) (12 day segment)

-22

EPOCH (days) 170.00-3 OFFSET Ws) -26.357

FREO (10-11) -. 094-24 RMS (psi .009

-26

-26 Fig. 15 - Panama results°27 a (same successive revolution)

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BUISSON, ET AL

The NTS1 data used or.ly single frequency measurements at 335 MHz. Table 2summarizes the CZ results in a similar fashion to Table 1.

Table 2 - NTS Time Transfer ResultsUTC(USNO,MC)-UTC(CZ)

Epoch Phase Frequency RMS(Day, 1978) (/Is) (pplO 1) (ns)

119 -23.882 -0.052 330

177 -26.293 -0.093 63170 -26.357 -0.094 9

Figure 13 presents the entire data span consisting of both NTS2 and NTS1 measure-ments. Figure 14 presents only NTS2 data. The improvement in noise level was from 330to 63 ns. This improvement was the result of two major advantages of the NTS2 space-craft over the NTS1 spacecraft: first, the use of a cesium oscillator in space (NTS2) asopposed to a quartz oscillator (NTS1) and, second, the ability to correct for the ionosphericdelay by dual frequency measurement (NTS2).

The additional improvement in noise level between Figs. 14 and 15 (from 63 to9 ns) is the result of a systematic effect in the orbit determination method whichcorresponds to the 2 rev/day orbit configuration. Figure 15 uses observations obtainedfrom the same side of the orbit each day. This noise level of 9 ns is considered to beindicative of results which can be attained in the full operational GPS constellation.

SYSTEM CLOSURE

Figure 12 presents the time transfer results for a receiver located at the U.S. NavalObservatory with a direct input from UTC(USNO,MC1). It can be seen that the noise levelis 171 ns with an offset of -36 ns at the epoch presented.

Time comparisons for five of the major observatories are presented in Fig. 16. Theinset in Fig. 16 presents the offset of three of the observatories to permit relative frequencycomparison.

Noteworthy is the line for UTC(USNO,MC1) via NTS; a small slope on the order of afew parts in 1015 is present which is not statistically significant.

Table 3 presents the differences for the NTS1 time transfers with respect to theinterpolated portable clock measurements. The average accuracy indicated by the portableclock is- 0.06 ps. This table links the entire experiment to the absolute or "truth" valuesas determined by the DOD master clock.

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NRL REPORT 8395

Fig. 16 - Time comparisons via NTS0

2i -1 S; TEATION OFFSET(US)

IFAG {W, t

-2 NRC (C) -3OP ') INOS (N) 0

100 120 140 6o 9o 200 220 240 260 260DAY 1970

Table 3 - Summary of Portable ClockClosures vs NTS Time Transfer Results

Portable Clock-Station Day NTS Time Transfer

(1978) (US)

BIH 124 -0.57CERGA 117 0.70DNM 282 0.09IFAG 199 0.03NBS 221 0.19NRLM 299 -0.53RGO 115 0.44RRL 303 0.13USNO 186 0.04

CONCLUSIONS

The following items are summarized as a conclusion for the six nation time transfercampaign:

* Time transfers via NTS satellites of better than 1 js accuracy have beendemonstrated.

* Simulated single satellite GPS operation has been demonstrated.

* A 9 ns time transfer noise level over a 12-day span has been demonstrated as apossible best value of results.

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ACKNOWLEDGMENTS

Acknowledgment is given to the contributors of the NTS Data: Dr. G.M.R. Winkler,K. Putkovich, and A. Johnson from the Naval Observatory (USNO), Washington, D.C.,U.S.A.; D.W. Hanson, National Bureau of Standards (NBS), Boulder, Colorado, U.S.A.;Dr. C.C. Costain, National Research Council (NRC), Ottawa, Canada; Dr. B. Guinot,Bureau International de I'Heure (BIH), Paris, France; Dr. P. Morgan, Division of NationalMapping (DNM), Queanbeyan, N.S.W. Australia; Dr. Y. Saburi, Radio Research Laboratories(RRL), Tokyo, Japan, T. Inouye, National Research Laboratory of Metrology (NRLM),Tokyo, Japan; Dr. J. Pilkington, Royal Greenwich Observatory (RGO), East Sussex,England; Dr. J. Kovalevsky, Group de Recherches de Geodesie Spatiale (GRGS), Grasse,France; Dr. K. Nottarp, Institute fur Angewandte Geodasie (IFAG), Wettzell, Germany.

REFERENCES

1. P.G. Landis. I. Silverman and W. Weaver, "A Navigation Technology Satellite Receiver,"NRL Memorandum Report 3324, July 1976.

2. L. Raymond, J. Oaks, J. Osborne, G. Whitworth, J. Buisson, P. Landis, C. Wardrip,J. Perry, "Navigation Technology Satellite (NTS) Low Cost Timing Receiver Develop-ment," Proceedings of the Eighth Annual Precise Time and Time Interval (PTTI)Applications and Planning Meeting, November, 1976.

3. T.B. McCaskill, J.W. White, S. Stebbins, and J.A. Buisson, "NTS-2 Cesium FrequencyStability Results," Proceedings of the 32nd Annual Symposium on Frequency Control,1978.

4. T.B. McCaskill, and J.A. Buisson, "NTS-1 (TIMATION-II) Quartz and RubidiumOscillator Frequency Stability Results," NRL Report 7932, December 12, 1975.

5. J.A. Buisson, R.L. Easton. and T.B. McCaskill, "Initial Results of the NAVSTAR GPSNTS-2 Satellite," Proceedi of the Ninth Annual Precise Time and Time Interval(PTTI) Applications and Planning Meeting, March 1978.

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