In July 1994, the U.S. Naval Observatory (USNO) Time Service System Engineering Division conducted a field test to establish a baseline accuracy for two-way satellite time transfer synchronization. Three Hewlett-Packard model 5071 high performance cesium frequency standards were transported from the USNO in Washington, DC to Los Angeles, California in the USNO's mobile earth station. Two-Way Satellite Time Transfer links between the mobile earth station and the USNO were conducted each day of the trip, using the Naval Research Laboratory(NRL) designed spread spectrum modem, built by Allen Osborne Associates(AOA). A Motorola six channel GPS receiver was used to track the location and altitude of the mobile earth station and to provide coordinates for calculating Sagnac corrections for the two-way measurements, and relativistic corrections for the cesium clocks. This paper will discuss the trip, the measurement systems used and the results from the data collected. We will show the accuracy of using two-way satellite time transfer for synchronization and the performance of the three HP 5071 cesium clocks in an operational environment

Chalmers, David

Davis, Angela

Koppang, Paul A.

Kubik, Anthony

Powell, William M.

Wheeler, Paul J.

English

NASA Technical Reports Server

N95- 32321
HIGH ACCURACY TIME TRANSFER
SYNCHRONIZATION
Paul Wheeler, Paul Koppang, David Chalniers,
Angela Davis, Anthony Kubik and William Powell
U.S. Naval Observatory
Washington, DC 20392
Abstract
In July 1994, the US Naval Observatory (USNO) Time Service System Engineering Division
conducted a field test to establish a baseline accuracy for two-way satellite time transfer synchro-
nization. Three Hewlett-Packard model 5071 high performance cesium frequency standards were
transported from the USNO in Washington, DC to Los Angeles, California in the USNO's mobile
earth station. Two-Way Satellite Time Transfer links between the mobile earth station and the USNO
were conducted each day of the trip, using the Naval Research Laboratory(NRL) designed spread
spectrum modem, built by Allen Osborne Associates(AOA). A Motorola six channel GPS receiver
was used to track the location and altitude of the mobile earth station and to provide coordinates
for calculating Sagnac corrections for the two-way measurements, and relativistic corrections for
the cesium clocks.
This paper will discuss the trip, the measurement systems used and the results from the data
collected. We will show the accuracy of using two-way satellite time transfer for synchronization
and the performance of the three HP 5071 cesium clocks in an operational environment.
INTRODUCTION
The purpose of this experiment was to demonstrate our ability to accurately calibrate remote
precise time laboratories and Department of Defense (DOD) installations using two-way satellite
time transfer techniques |11. Although the USNO has participated in two-way experiments
for years, little work has been done in performing absolute calibrations of remote sites using
this technique. When the need for high accuracy calibrations to remote DOD sites arose, the
two-way satellite time transfer technique was selected because of its greater accuracy (+ 1 ns)
than the Global Positioning System (GPS). This accuracy of two-way has been demonstrated
at the USNO and other laboratories, but two-way accuracy had not been demonstrated in the
field, in partict, lar in the east-west direction where Sagnac effects are significant.
CLOCK TRIP
For this experiment a two-member team drove the USNO's mobile earth station from the
USNO in Washington, DC to Los Angeles, California and a second two-member team drove
51
"PA_ _ iNTENTIONALLY 8LAt,_K
https://ntrs.nasa.gov/search.jsp?R=19950025900 2020-06-16T06:11:16+00:00Z
it back to the USNO (see Figure 1). Two-way measurements were made each day to provide
a precise time link between the three cesium clocks in the van and the USNO. The trip was
started on July llth, 1994 and completed on July 22nd.
The USNO's mobile earth station is a Ford Econoline 350 van with a folding Ku-band 1.8
meter dish antenna on the roof, a generator, air conditioning and three equipment racks of
electronics to support the antenna and two-way operations. For this trip three Hewlett-Pack_rd
5071A high performance cesium frequency standards, a PC-based data acquisition system, and
a Motorola six channel GPS receiver were added.
The data acquisition system consisted of an industrial grade PC manufactured by Texas Microsys-
tems Inc. controlling a Stanford Research SR650 time interval counter, a Hewlett-Packard
3488 VHF switch and the GPS receiver. The GPS receiver was mounted inside the PC and
connected to one of the PC serial ports. The clocks and the GPS receiver 1 pps (1 pulse per
second) were intercompared every minute with the time interval counter. The GPS position
information was logged every 10 minutes. While the mobile earth station was at the USNO,
a 1 pps from the Master Clock was also connected to the VHF switch so that the clocks
were compared against UTC USNO. This was done before and after the trip to establish the
performance of the clocks while they were in the van.
In order to calibrate the two-way system, repetitive two-way measurements between our 4.5
meter base station and the mobile earth station were taken. A calibrated 1 pps and a 5 MHz
reference signal, both from the USNO Master Clock, were connected to the modem in each
earth station. With this setup the resulting measurement, one clock measured against itself,
should be zero, if the transmit and receive delays are identical through the modem, the earth
station, the satellite and back, and the cable delays from the reference clock are the same. In
reality this is not the case and the resulting measurement will be the calibration factor that is
applied to the measurements throughout the experiment. The calibration factor measured to
be 243.3 nanoseconds. After returning from the trip this calibration procedure was repeated
to verify that the delays through the two-way system had not changed.
The trip was started after a final calibration run. After arriving at a destination in the evening,
the van was fueled and parked in the hotel parking lot so that there was good satellite visibility.
The clocks and measurement equipment were transferred from inverter power to generator
which kept the equipment and air conditioner operating throughout the night. The satellite
was located and the equipment was set up in preparation for the time transfer measurement
in the morning. For this experiment and most of our domestic two-way operations we use
Satellite Business Systems satellite SBS-6.
In the morning the satellite was re-acquired and then, typically, three two-way measurements
were obtained. Each result consisted of 300 averaged 1 pps measurements. The AOA modems
used for this experiment operate in a sonrce/target configuration. The modem in the van was
operated as the source and the target was at the USNO. This gave the travelers control of
the sntellite link and the two-way measurements. The two-way measurement data from the
target modem was transmitted back over the satellite link to the source modem, allowing the
results of the measurement set to be calculated. This allowed the field members to evahlate
the measurements and to look for anomalies in the data being collected.
52
The two-way data collected each morning were used to meast,re the time of the three cesium
clocks in the van. The GPS receiver position data were used to calculate Sagnac corrections
for the two-way measurements and to calculate relativity corrections for the clock data. The
1 pps from the GPS receiver was used as a coarse rule to compare the clocks.
RELATIVISTIC EFFECTS
The data collected throughout this experiment needed to be corrected for relativistic effects.
There are many excellent references [1-51 on the derivation and theory behind these effects;
therefore, they will only be touched upon briefly in this paper. Due to the rotation of the
earth and the satellite, the path lengths (from one earth station to the satellite to the other
earth station and back) during a satellite two-way time transfer are not symmetrical. This
phenomenon is referred to as the Sagnac effect. The time difference caused by this effect
is given by 2coA/c 2, where c is the velocity of light, co is the Earth's rotation rate, and A is
the area defined by the projection onto the equatorial plane by the segments connecting the
satellite and the Earth's center to the two earth stations [11. Figure 2 is a plot of the Sagnac
corrections needed for the two-way data taken during the trip.
There are three main components of relativistic corrections that need to be addressed for the
clock trip elapsed time. These corrections are due to height (red shift), velocity (Doppler shift),
and east-west motion. The equation used to calculate these corrections is:
At= /pathds [1-g(c})h/c2 + _(v/c)2 + _2avEcosO ] (1)
where g(¢) is the acceleration of gravity, vE is the ground velocity of the clock having an
eastward component, h is the altitude above the geoid, co is the angular velocity of rotation of
the Earth, a is the Earth's equatorial radius, and ¢ is the geographical latitude [31.
The GPS receiver provided most of the information needed to solve the above equation. Every
10 minutes the position of the van (height, latitude, and longitude) and the time were gathered
from the GPS receiver and stored on the PC.
The height correction:
a( )h/c 2 (2)
of 82.57 ns turns out to be the dominant clock trip relativistic correction term in this experiment
due to the trip length of 11 days and the vast height differences encountered along the trip,
for instance Washington, DC is at 55 meters while part of Colorado is over 3000 meters(see
Figure 3). Since this term does not depend on velocity, but on height, it is continuing to
have an effect as long as there is a height differential. Therefore, the clocks were realizing
a relativistic change of rate even when the van was parked for the night at a location with a
different elevation than that of Washington, DC.
53
The east-west correction:
-- ,:os (3)C2ave
ends up being an integration of east(west) velocity over time, which is then just the distance
traveled east(west). Since this term turns out to be proportional to the distance traveled and
independent of velocity, it would be the same whether a van or, for instance, an airplane was
used for the clock trip. For a round trip (east-west, west-east) this term cancels out at the
conclusion of the trip, but gives a necessary correction to the data during the trip.
The Doppler term is:
2 (4)
Even though the van traveled very slowly compared to the speed of light, the 11 day trip was
long enough to give the Doppler term a non-negligible correction of 1.24 ns. Figure 4 shows
the three different contributions along with the total relativistic correction for the trip.
It is interesting to compare these clock trip data to what we would have seen on a airplane trip
from Washington, DC to Los Angeles and back. We will assume an average air speed of 550
miles per hour and an average altitude of 25,000 feet. The height correction for the round trip
calculates to be -28.2 ns, while the velocity correction would be 11.4 ns. Therefore the total
correction to the clock data would be -16.8 ns as compared to the -82.6 ns of the van trip.
DATA
The two-way time transfer method was used to compare cesium clock serial #254 vs. the
Master Clock at 12 different sites during the round trip. The phase data being logged between
clocks #254, #416, and #227 locally in the van (see Fig. 5) along with the two-way data
was used to calculate the differences between the Master Clock and the clocks #416 and
#227 during the trip. The relativistic corrections due to the clock trip were then made to the
cesium clock - Master Clock data after Sagnac effect corrections to the two-way data had
been taken into account. Figures 6-8 show the raw data and the data corrected for both the
two-way Sagnac and relativistic clock trip effects. In Figures 6-8 the data that are bunched
together at the beginning and the end of the plots were taken at USNO with the Master Clock
directly connected to the measurement system, while the individual points were obtained via
the two-way time transfer method.
The cesium clocks performed very well considering the less than ideal environmental conditions
inside the van during the trip. The clocks experienced approximately 40 degree temperature
swings and considerable vibration. The Allan deviations of the cesium clocks as measured in
the van during the trip were:
54
hr
1 1.14e-13
2 8.21e-14
4 5.93e-14
8 4.52e-14
16 3.18e-14
32 2.32e-14
64 1.83e-14
128 1.50e-14
#227-#416 #254-#227 #254-#416 #points
1.18e-13
8.06e-14
5.85e-14
3.81e-14
2.80e-14
2.46e-14
1.39e-14
7.38e-15
1.18e-13
8.44e- 14
6.38e-14
4.47e-14
3.59e-14
3.24e-14
2.38e-14
1.82e-14
415
207
103
51
25
12
6
3
Clock pairs 227-416 and 254-227 had stabilities that were below the specifications given by the
manufacturer for clocks tinder environmental control(see Figure 9).
CONCLUSION
This experiment has shown that the two-way time transfer method can be used to accurately
calibrate remote precise time laboratories and DOD installations using the necessary Sagnac
corrections to the data. Also, it is necessary to take into account the effects of relativity when
using a portable clock to do remote synchronization no matter what the mode of transportation
of the clock. The three HP 5071 clocks performed very well in less than ideal conditions.
REFERENCES
[1] D.W. Hanson, "Fundamentals of two-way time transfers by satellite", Proceedings of
the 43rd Annual Symposium on Frequency and Control, 31 May - 3 June 1989, Denver,
Colorado, pp. 174-178.
[2] G.M.R. Winkler, "Synchronization and Relativity", Proceedings of the IEEE, gol. 79,
No. 6, June 1991. (This paper gives references to many excellent papers on relativity.)
[3] N. Ashby and D.W. Allan, "Practical implications of relativity for a global coordinate
time scale", Radio Science, Vol. 14, 1979, pp. 649-669.
[4] J.C. Hafele and R.E. Keating, "Around the world atomic clocks", Science, Vol. 177, pp.
166-170, July 1972.
[5] N. Ashby, "RELATIVITY and GPS", GPS World, Nov. 1993. pp. 42-47.
55
40
39
38
UJ
£3
D 37
I--
I--
•_ 36
35
34
33
CLOCK TRIP
Los Angeles _
, il , i I I ,
80 90 100 110 120
LONGITUDE
Figure i. Clock trip as measured by the GPS receiver.
120
100
80
o(l) 60
E
40
20
0
TWO-WAY SAGNAC CORRECTIONS
,e ii ,I ,I ,,I ,,I
49546 49548 49550 49552 49554 49556
MJD
Figure 2. Sagnac corrections calculated for the two-way data.
56
3000
2500
2000
1500
1000
50O
0
CLOCK TRIP HEIGHT ABOVE GEOID
• . . , .... i .... , . , ...... A i .... , , , , , A , ,
49542.5 49545 49547.5 49550 49552.5 49555 49557.5
MJD
Figure 3. The height of the van above the geoid during
the trip as measured by the GPS receiver.
0
-2O
o -40{D
u)
t-
-6O
-8O
CLOCK TRIP:TOTAL CORRECTION
'_ east-west
,ota, \---". he h,
• - , . . , . i ..... , , I , I , . , . i .... , . • - | I ,
49542.5 49545 49547.5 49550 49552.5 49555 49557.5
MJD
Figure 4. The total relativistic clock trip corrections
along with the individual components.
57
LOCAL CLOCK COMPARISON
150
100
0
c/)
C
v
uJ 50
<
-r
a. 0
-5O
CS254-CS416
_k CS227-CS416
CS254-CS227
! , a
49540 49545 49550
I
49555 49560
MJD
Figure 5. The cesium clocks inter-compared as measured locally
in the van.
150
125
--- 100O
(D
u)
,- 75
v
I..IJ
O9
<
T
5O
25
0
-25
49540
CLOCK TRIP:CS254-MC
" • • • corrected
linear fit
i m
49545 49550 49555 49560
MJD
Figure 6. Shows the original cesium #254-Master Clock data along
with the data corrected for two-way Saguac and the clock
trip relativistic effects.
58
CLOCK TRIP:CS227-MC
49540 49545 49550 49555 49560
MJD
Figure 7. Shows the original cesium #227-Master Clock data along
with the data corrected for two-way Sagnac and the clock
trip relativistic effects.
0
-2O
o
-40(n
¢-
U.I
oo -60
<
-1-
a.. -130
-100
-120
Figure 8.
CLOCK TRIP:CS416-MC
e
• •
e
i .
49540 49545 49550 49555 49560
MJD
Shows the original cesium #416-Master Clock data along
with the data corrected for two-way Sagnac and the clock
trip relativistic effects.
59
2; 3
5
-14
2.10
-14
1.10
CLOCK TRIP:ALLAN DEVIATION
e
• •
• 254-416
227-416
• 254-227
m lab spec
............... 100.5 1 5 10. 50. 0.
HOURS
Figure 9. Stability of the cesium clocks as measured in
the van during the trip.
6O
QUESTIONS AND ANSWERS
DAVID ALLAN (ALLAN'S TIME): A question regarding the regression that you did to
determine the frequency during the trip from the data. As near as I can tell, it looks like you
did a linear regression to all of the phase points. Is that correct?
PAUL WHEELER (USNO): Yes.
DAVID ALLAN (ALLAN'S TIME): Given your sigma tau plot that the noise is white
noise frequency modulation, which is random walk of phase, the optimum interpolater for the
frequency while you were on the trip would be the end-point from the beginning and the
beginning point of the end, to give you a better estimate of freqt,ency than the linear regression.
Thank you.
SIGFRIDO M. LESCHIUTTA (IEN): Could you please elaborate to me concerning the
two calibration processes? One was done before and after the trip. What was done really?
The second, have you made any calibration before and after each session, calibration of the
orbital treatment?
PAUL WHEELER (USNO): The calibration -- we do it a couple different ways. For this
experiment, since the clocks in the van, we wanted to measure the clocks that were taken with
us the same way at the Observatory as we did in the field. So we did two-way time transfer
between the two stations right there at USNO, our base station being measured against the
USNO Master Clock. The mobile air station being measured against one of the clocks in the
van. Right after that session, we then measured that clock with a cable, to our acquisition
system, against the Master Clock and determined the difference between the two ways.
The second question, the answer is no. It was strictly before we left and when we returned,
and nothing in-between.
61/62



High accuracy time transfer synchronization

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