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80th Annual Precise Time and Time Interval (PTTI) Meeting 

PERFORMANCE OF GPS 

ON-ORBIT NAVSTAR FREQUENCY STANDARDS AND 
MONITOR STATION TIME REFERENCES 

Orville Jay Oaks 
Thomas B. McCaskill 
Marie M. Largay 
U.S. Naval Research Laboratory 
Washington. D.C. 

Wilson G. Reid 
SFA, Inc. 

Washington, D.C. 

James A. Buisson 
Antoine Enterprises Inc. 

Alexandria, VA 


Abstract 

The U.S. Naval Research Laboratory (NRL) conducts comprehensive analyses of the Global Positioning 
System (GPS) atomic frequency standards under the sponsorship of the GPS Joint Program Office (JPO) and 
in cooperation with the 2nd Space Operations Squadron (2SOPS) at the Master Control Station (MCS) in 
Colorado Springs, Colorado. Included in the analysis are the on-orbit Navstar space vehicle clocks and the 
ground reference clocks at each of the five Air Force and seven National Imagery and Mapping Agency 
(NIMA) GPS monitor stations. A presentation will be made of the performance of the Navstar clocks currently 
operating in the constellation, which are characterized through the use of phase, frequency, drift and stability 
histories in addition to frequency stability profiles based on the Allan and Hadamard variances. Clock 
performance is analyzed using a multi-year database comprised of pseudorange measurements collected by 
each of the 12 GPS monitor stations. Results of these analyses are routinely used by the MCS in optimizing the 
q’s in the Kalman filter. 

Continuous 15-minute measurements of the phase offset of each monitor station time reference from the DoD 
Master Clock are obtained from Linked Common-View Time Transfer from DoD Master Clock, which is the 
reference clock at the NIMA Washington, D.C. monitor station. The method is extended to obtain continuous 
15-minute measurements of the phase offset of each active Navstar space vehicle clock from the DoD Master 
Clock. Hence, the performance of all space and control segment clocks is referenced to the DoD Master Clock. 

Discontinuities in the phase and frequency of the clocks are removed to yield the unperturbed performance of 
the clocks. The corrections, together with the probable cause of the discontinuity, are summarized. Examples 
of the frequency history and the exhaustive calculation, for every multiple of the sample period of 15 minutes 
from 15 minutes to 18 days, of the frequency stability profile for several Navstar space vehicle clocks and for 
the time reference at two of the GPS monitor stations will be presented. Analysis of the performance of the first 
on-orbit Block IIR operational rubidium clock will also be presented. 


135 






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1. REPORT DATE 

DEC 1998 


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3. DATES COVERED 

00-00-1998 to 00-00-1998 


4. TITLE AND SUBTITLE 

Performance of GPS On-Orbit Navstar Frequency Standards and 
Monitor Station Time References 

6. AUTHOR(S) 


7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 

U.S. Naval Research Laboratory,4555 Overlook Ave 
SW,Washington,DC,20375 


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Approved for public release; distribution unlimited 

13. SUPPLEMENTARY NOTES 

See also ADA415578. 30th Annual Precise Time and Time Interval (PTTI) Systems and Applications 
Meeting, Reston, VA, 1-3 Dec 1998 


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see report 


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INTRODUCTION 


The pseudorange measurements upon which this analysis is based were collected at the five Air 
Force and the seven National Imagery and Mapping Agency (N1MA) monitor stations using dual¬ 
frequency GPS receivers. Figure 1 presents the information flow from a single Navstar space 
vehicle to each of the GPS monitor stations. The use of dual frequencies enabled ionospheric 
corrections to be based on the measured ionosphere. The pseudorange measurements were collected 
every six seconds synchronized to GPS time and smoothed to one point every 15 minutes. Clock 
offsets were computed using the NIMA post-processed ephemerides and observations that were 
collected at the 12 GPS monitor stations. All monitor station clock performance was computed 
using Linked Common-View Time Transfer from the NIMA Washington, D.C. site. 

A key feature in the NRL Clock Analysis Software System (CLASS) is the capability to detect 
phase and frequency discontinuities, to solve for the discontinuity, and to correct the clock data. 
Correction of the data makes possible the analysis of long-term clock, system, and environmental 
effects[l]. The results of the analysis are included in NRL Quarterly Reports to the GPS Joint 
Program Office (JPO) and to the Master Control Station (MCS), as well as to other interested 
members of the scientific community. 

Other measures of performance are determined, such as the total operating time for each operational 
Navstar space vehicle and the operating time for each Navstar clock. Included are histories of the 
phase, the frequency, and the frequency stability. 

CONSTELLATION 

The constellation as of 30 September 1998 is shown in Figure 2. This table shows by plane and by 
position in the plane each of the Block II/IIA/IIR Navstar space vehicles in the constellation and the 
type of clock that was operating. Of the active cesium clocks, Frequency & Time Systems, Inc. 
manufactured all but the Navstar 30 cesium clock. Kemco, Inc.manufactured the Navstar 30 cesium 
clock. All Block II/IIA rubidium clocks were manufactured by Rockwell, Inc. The Block HR 
rubidium clock on Navstar 43 was manufactured by EG&G. Seven of the 27 clocks operating were 
rubidium, while twenty were cesium atomic frequency standards. Three of the six planes have four 
Navstars, while the other three planes contain five Navstar space vehicles (SV) each, although the 
SVs are not evenly spaced in the planes. 

The total operating time for each of the Navstar space vehicles since the space vehicle was inserted 
into the constellation is presented in Figure 3. Thirteen of the space vehicles have been in operation 
for six years or more, which exceeds the mean mission duration specification. 

The number of clocks that have been placed in operation on each space vehicle is presented in 
Figure 4. Eight of the space vehicles are operating the first clock, thirteen are operating the second 
clock, four are operating the third clock, and two are operating the last clock. Navstar 43 is 
operating its second clock, but one of the clocks was activated as a test and could be used again 
when needed. 


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NAVSTAR CLOCKS 

The operating time or length of service, of the clocks that were operating as of 30 September 1998 
is presented in Figure 5. The shaded bars correspond to the cesium clocks and the unshaded bars to 
the rubidium clocks. Nine clocks, all cesium frequency standards, have exceeded five years of 
continuous operation. Two clocks, both cesium frequency standards, have exceeded eight years of 
continuous operation. Two of the rubidium clocks have attained three years of continuous operation. 

The operating times of Block II cesium and rubidium clocks respectively both active and 
deactivated clocks are presented in Figures 6 and 7. The data are presented in order of activation. 
The shaded bars correspond to the active clocks, while the open bars correspond to the deactivated 
clocks. The comparison shows that the Block II cesium clocks have an average operating time on- 
orbit of 3.7 years,while the rubidium clocks have an average operating time on-orbit of 1.3 years. 

The one-day frequency offset history for the clocks in plane B of the GPS constellation as of 30 
September 1998 is presented in Figure 8. NRL analyzes each clock in each plane in the 
constellation, but only plane B is presented for brevity. The history of the clocks in all planes is 
included in the NRL quarterly reports. 

The two-day frequency drift for each of the clocks in the constellation, presented by Figure 9, 
shows that the rubidium clocks exhibit typically large drift rates, which are characteristic of 
rubidium frequency standards—the largest being Navstar 38 with a drift of-5.0^_pl0l3 per day. Six 
rubidium clocks exhibit negative drift, excluding the rubidium in Navstar 34,which was activated on 
September 14,1998 and did not have sufficient data to be included. Figure 10 is a plot on an 
expanded scale of the two-day average drift of the cesium clocks. Ten of the cesium clocks exhibit a 
negative drift, while the other ten exhibit a positive drift. All but one of the cesium clocks exhibit a 
drift with a magnitude below 5 /TplO^ per day, which is two orders of magnitude less than the 
largest drift rate reported for the Block II rubidium clocks. 

Figures 11 through 14 are examples of the frequency stability profile for four of the 27 Navstar 
clocks. Figure 11 shows the stability of the Navstar 19 rubidium clock, which is typical of the 
performance of the rubidium clocks manufactured by Rockwell Corporation. Figure 12 shows the 
effects of a 10-15 nanosecond oscillation at the orbital period in the phase offset of the Navstar 36 
cesium clock manufactured by Frequency and Time Systems. The cause of these oscillations which 
affected a number of clocks has not been identified. Figure 13 shows the performance of the best 
cesium clock in the constellation, the Navstar 30 alternate-source cesium clock manufactured by 
Kemco. The stability of this clock for a sample time of one day was estimated to be 7 pp\Q^^. 
Figure 14 shows the performance of the best rubidium clock in the constellation, the Navstar 43 
Block HR clock manufactured by EG&G. The stability of this clock at one day was estimated to be 
near the noise threshold of the receiver at 2.4 pp\<4^^. 

The frequency stability profile, made using the precise ephemerides, for the 27 clocks in the 
constellation that were operational on 30 September 1998 is presented in Figure 15. All but seven of 
the clocks are evenly distributed between 7 1014 and 1.4 pp\0^ at one day. Figures 16 and 17 

show the ranking of the estimates of the frequency stability for a sample time of one day using the 
Allan and Hadamard deviations respectively. In Figure 16 , three of the six rubidium clocks for 
which the stability was estimated rank last because of the large drift typical of rubidium clocks. In 
Figure 17 , based on the Hadamard deviation which adaptively corrects for the drift [2], all six of the 
rubidium clocks rank before the cesium clocks with values of stability measured in ppiQ^ 

The frequency stability estimates for each of the Navstar clocks for a sample time of one day using 
the post-fit ephemeris are presented in Figure 18. Estimates of the frequency stability were made 


137 











using the Allan deviation without any correction for aging. Superimposed on Figure-18 are dashed 
lines corresponding to the GPS system specifications of 2 /?pl0l3 and 5 pp\0^ for the one-day 
stability of the cesium and rubidium clocks respectively. The frequency stability of all Block II 
space vehicle clocks can be seen to meet the specifications. Eleven of the Block II clocks—nine 
cesium frequency standards and two rubidium standards—show stability at or below 1 pp\0^. 

TIME TRANSFER 

Linked Common-View Time Transfer is a special case of GPS time transfer [3], which uses 
simultaneous measurements by two users of a Navstar space vehicle clock when the space vehicle is 
in view of both users. Each of the two users computes his clock offset with respect to the Navstar 
clock at the same epoch. Then, the difference between their respective clock offsets with respect to 
the Navstar clock yields the offset between the two user clocks. This procedure results in a 
measurement which is independent of the Navstar clock, but which retains the difference in the 
measurement errors. 

The precision of a single common-view time transfer measurement was first demonstrated using 
common-view time transfer measurements taken over a 20-day time span with a single space 
vehicle. Recently, the precision of the common-view time transfer measurement was definitively 
determined through the use of multiple common-view measurements taken at the same epoch [4], 
This process was made possible with the full constellation of GPS space vehicles. The estimated 
precision of a single 15-minute interval has been determined to be between 1.4 and 2.7 
nanoseconds. Using the measurements ffom all Navstar space vehicles in common-view during the 
15-minute interval, typically 3-7 space vehicles, improves the estimate of the precision of the time 
transfer measurement to between 0.65 and 1.13 nanoseconds. This level of measurement precision 
results in the capability to determine the frequency stability of a remote clock (with respect to the 
DoD Master Clock) anywhere on Earth to within 2 pp\ 0l2 for a 15-minute sample time and 2 
pp 10^ for a one-day sample time. 

A Linked Common-View Time Transfer measurement results by linking two or more remote sites 
that are in common-view with either the same or another Navstar space vehicle. The precision of 
the Linked Common-View measurements can be estimated by considering the special case of the 
sum of stationary random variables with mean zero and standard deviation equal to the precision of 
a single common-view time transfer measurement. It is expected that the precision of the Linked 
Common-View Time Transfer will grow as the square root of the number of links multiplied by the 
precision of a single common-view measurement. However, in the analysis of the stability of a 
remote clock, it is possible that other factors such as the quality of the receiver and short-term 
environmental effects could have a greater influence on the precision of the measurements than the 
effect of multiple links. 

MONITOR STATION CLOCKS 

Figures 19 and 20 show the one-day average frequency offset six-month history of the ground 
reference clocks from the DoD Master Clock for the Air Force and NIMA GPS ground tracking 
stations respectively. These results were obtained using Linked Common-View Time Transfer[4], 
The performance of the ground reference clock at the Colorado Springs Monitor Station is superior. 
It is the Alternate Master Clock #1 which is a hydrogen maser steered to UTC (USNO) by two-way 
satellite time transfer [5], The performance of the ground reference clocks at the remaining four Air 
Force stations, which are equipped with HP5061 cesium beam tubes, has more noise than that of the 


138 




ground reference clocks at the NIMA MS, which are equipped with HP5071 high performance 
cesium-beam tubes. The superior performance of the Colorado Springs monitor station time 
reference can be seen in the frequency stability profile in Figure 27, where its performance is 
dominated by white phase noise as far out as the profile was estimated, i.e. 18 days. On the other 
hand, the best NIMA time reference was at the Quito, Ecuador monitor station (Figure 22),which 
shows a flicker floor of 1 pp 10^4 being achieved at about five days. This station had the lowest 
short-term noise of all the monitor stations, whereas the Colorado Springs monitor station had the 
highest. In addition, the time reference for the Colorado Springs monitor station showed a cyclic 
component at the fourth harmonic of the orbital period (2.99 hours). The cause of this cyclic 
component has not been determined. The performance of the time reference at each of the ten 
monitor stations is compared in Figure 23, which presents the frequency stability profile for sample 
times of one to 18 days. The Colorado Springs monitor station is clearly superior, followed by the 
five NIMA monitor stations, and finally by the remaining four Air Force monitor stations. 

CONCLUSIONS 

Thirteen of the Block II space vehicles have been in operation for six years or more and have 
exceeded the expected mean mission duration. An average of two Block II Navstar clocks per space 
vehicle has been activated. Twenty-one of the space vehicles have at least two spare clocks 
available to complete the design lifetime. Fourteen Navstar clocks—more than one-half of the 
operational constellation—are performing with an estimated one-day frequency stability of 1 pp\0‘3 
using the precise ephemerides and based on the Hadamard deviation. The time reference at each of 
the GPS monitor stations exhibited a frequency stability for a sample time of one day of between 3 
jqpl0l4 to 4.2 jqplC)14. 

REFERENCES 

[1] McCaskill, T. B., Reid, W.G., Oaks, O.J., Beard, R.L., U.S. Naval Research Laboratory, and 
Buisson, J. A. and Warren, H. E., SFA, "Performance of Global Positioning System (GPS) On-orbit 
Navstar Clocks," 1995 IEEE International Frequency Control Symposium}, 31 May—2 June 1995, 
pp 133— 139. 

[2] Hutsell, Steven H., "Relating The Hadamard Variance to MCS Kalman Filter Clock 
Estimation" Proceedings of the 27th Annual Precise Time and Time Interval (PTTI) Applications 
and Planning Meeting}, 29 November—1 December 1995, pp291—301. 

[3] Buisson, J. A., McCaskill, T. B., Smith, H., Morgan, P., and Woodger, J., "Precise Worldwide 
Station Synchronization via the Navstar GPS Navigation Technology Satellite (NTS-1)", Proc. 8th 
Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting, November 30 — 
December 2, 1976, Washington, D.C., pp.55-84. 

[4] Reid, W.G., McCaskill, T. B., and Oaks, O.J., U.S. Naval Research Laboratory, and Buisson, J. 
A. and Warren, H.E., Sachs Freeman Associates Incorporated. "Common View Time Transfer 
Using Worldwide GPS and DMA Monitor Stations", Proceedings of the 27th Annual Precise Time 
and Time Interval (PTTI) Applications and Planning Meeting, November 29—1 December 1995, 
ppl45—158. 

[5] Breakiron, Lee A., "Frequency Steering of Hydrogen Masers", Proceedings of the 50th 
Frequency Control Symposium, 5 June 1996. 


139 




Figure 1. Clock Analysis Data Flow 



NAVSTAR NUMBER 


Figure 3. Total Operating Time of Current Navstar 
Satellites 

30 September 1998 



Figure 5. Operating Lifetime of Current Clocks 
30 September 1998 


Plane 

Slot 1 

Slot Z 

Slot 3 

Slot 4 

Slot 5 

A 

39 

.... 25 

27 ’ 

19 

38 

B 

22 

30 

13 

35 . 


C 

36 

33 : 

31 

37 


D 

24 

75 

17 

34 


E 

74 . 

21 

16 

'■ 23 : ' 

■u 

o 

F 

32 

26 

. 18 : 

29 

43 


Cesium Clock 


Rubidium Clock 


Figure 2. GPS Constellation Space Vehicle Location 
and Clock Type 
30 September 1998 



NAVSTAR NUMBER 


Figure 4. Number of Clocks Operated Since Insertion 
30 September 1998 



ORDER OF ACTIVATION 

Figure 6. Lifetime of Block 11 and IIA Cesium Clocks 
in Order of Activation 
30 September 1998 


140 


Nova! Research labotatot 























LIFETIME (YEARS) 



Figure 7. Lifetime of Block II, 1IA, and HR Rubidium 
Clocks in Order of Activation 
30 September 1998 


Figure 8. One-Day Frequency Offset History of 
Navstar Clocks from DoD Master Clock 
Plane B 




Figure 9. Two-Day Average Frequency Drift of 
Current Clocks 

1 June 1998 to 30 September 1998 



NAVSTAR NUMBER 


Figure 10. Two-Day Average Frequency Drift of 
Cesium Clocks 

1 June 1998 to 30 September 1998 



0.001 0.01 0,1 1 10 100 
SAMPLE TIME {DAYSI 

Figure 11. Frequency Stability of Navstar 19 
Rubidium Oscillator (No. 53) 

1 April 1998 to 1 October 1998 



0.001 001 0.1 1 10 100 
SAMPLE TIME (DAYS) 


Figure 12. Frequency Stability of Navstar 36 
Cesium Oscillator (No. 46) 

1 April 1998 to 1 October 1998 


141 












































SAMPLE TIME (DAYS) 


Figure 13. Frequency Stability of Navstar 30 
Cesium Oscillator (No. K3) 

1 April 1998 to 1 October 1998 


Figure 14. Frequency Stability of Navstar 43 
Rubidium Oscillator (No. 06) 

1 April 1998 to 1 October 1998 




RANKING 


Figure 15. Frequency Stability of Navstar Clocks 
1 April 1998 to 30 September 1998 


Figure 16. Ranking of One-Day Frequency Stability 
Estimates of Navstar Space Vehicle Clocks Using 
Allan Deviation 

1 June 1998 to 30 September 1998 




Figure 17. Ranking of One-Day Frequency Stability Figure 18. One-Day Frequency Stability Estimates of 

Estimates of Navstar Space Vehicle Clocks Using Navstar Space Vehicle Clocks Using Allan Deviation 

Hadamard Deviation 1 June 1998 to 30 September 1998 

1 June 1998 to 30 September 1998 


142 


































§ 


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APR MAY JUN JUL AUG SEP OCT NOV DEC 


210 240 270 300 




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MODIFIED JULIAN DATE 

Figure 19. Frequency Offset of Air Force Monitor 
Station Time Reference from DoD Master Clock 


AV". AVVVV*. '•W^W ► 


50900 50950 51000 51050 51100 51150 

MODIFIED JULIAN DATE 

Figure 20. Frequency Offset of NIMA Monitor Station 
Time Reference from DoD Master Clock 


MINUTES -1 [- HOURS -1 I- DAYS 



r— MINUTES j— HOURS —1 r-- DAYS 



SAMPLE TIME (DAYS) 


Figure 21. Frequency Stability' of Monitor Station Time 
Reference with Respect to DoD Master Clock via 
Common-View Time Transfer 
1 April 1998 to 1 October 1998 


Figure 22. Frequency Stability of Monitor Station Time 
Reference with Respect to DoD Master Clock via 
Linked Common-View Time Transfer 
1 April 1998 to 1 October 1998 


V.-.- CAYS 



SAMPLE TIME (DAYS) 


Figure 23. Frequency Stability' Profile Comparison of 
Time References with Respect to DoD Master Clock via 
Linked Common-View Time Transfer 
1 April 1998 to 1 October 1998 


143 




























Questions and Answers 

DENNIS McCarthy (USNO); To what extent does the quality of the orbits that you are using affect the 
analysis of the satellite clocks? 

JAY OAKS (NRL): This sounds like a loaded question? Obviously, it plays an important part. That is 
why we use the N1MA precise orbit, which is reported to be accurate within centimeters. What we see here 
is mostly dominated by the clock, but there are some anomalies that we investigate,like the orbit where it 
might be showing up. 

SIGFRIDO LESCHTUTTA (TEN): Could you please show us one of the last vugraphs concerning the 
behavior of the USNO clock monitor station frequency stability profile? 

JAY OAKS: Was it the monitor station clock frequency? 

SIGFRJDO LESCHTUTTA: Yes. 

JAY OAKS: This one? We have one like this for the monitor station frequency stability profile, one for 
monitor station clocks, and one for the space vehicle clocks. 

SIGFRIDO LESCHTUTTA: The one concerning your two-way link. 

JAY OAKS: That was this. What I had said is that the Colorado Alternate Master Clock, which is shown 
here, is a hydrogen maser steered using measurements made once an hour using two-way satellite time 
transfer measurements between the Naval Observatory and the Colorado Station. That is Steven HutselTs 
algorithm and if you have some questions about that, he would probably be happy to answer them. Is that 
what you were asking? 

SIGFRIDO LESCHTUTTA: Yes. 


144