There are two hydrogen maser clocks located at each signal processing center (SPC) in the DSN. Close coordination of the time and frequency of the SPC clocks is needed to navigate spacecraft to the outer planets. A recent example was the Voyager spacecraft's encounter with Uranus in January 1986. The clocks were adjusted with the goal of minimizing time and frequency offsets between the SPCs at encounter. How time and frequency at each SPC is estimated using data acquired from the Global Positioning System Timing Receivers operating on the NBS-BIH (National Bureau of Standards-Bureau International de l'Heure) tracking schedule is described. These data are combined with other available timing receiver data to calculate the time offset estimates. The adjustment of the clocks is described. It was determined that long range hydrogen maser drift is quite predictable and adjustable within limits. This enables one to minimize time and frequency differences between the three SPCs for many months by matching the drift rates of the three standards. Data acquisition and processing techniques using a Kalman filter to make estimates of time and frequency offsets between the clocks at the SPCs and UTC(NBS) (Coordinated Universal Time realized at NBS) are described

Clements, P. A.

Kirk, A.

Unglaub, R.

English

NASA Technical Reports Server

TDA Progress Report 42-89 January-March 1987 
Results of Using the Global Positioning System to 
Maintain the Time and Frequency Sychronization 
in the Deep Space Network 
P. A. Clements and A. Kirk 
Communications Systems Research Section 
R. Unglaub 
DSN Control Center Operations Section 
There are two hydrogen maser clocks located at each signal processing center (SPC) 
in the DSN. Close coordination of  the time and frequency of  the SPC clocks is needed 
to navigate spacecraft to the outer planets. A recent example was the Voyager space- 
craft's encounter with the planet Uranus in January 1986. The clocks were adjusted 
with the goal of  minimizing the time and frequency' offsets between the SPCs at  en- 
counter. This article describes how the time and frequency at each SPC is estimated using 
data acquired from Global Positioning System Timing Receivers operating on the NBS- 
BIH (National Bureau of Standards-Bureau International de I'Heure) tracking schedule. 
These data are combined with other available timing receiver data to calculate the time 
offset estimates. The adjustment of the clocks is described. It was determined that the 
long range hydrogen maser drift is quite predictable and adjustable within limits. This 
enables one to minimize time and frequency differences between the three SPCs for 
many months by matching the drift rates of the three standards. The article will describe 
the data acquisition and processing techniques using a Kalman filter to make estimates 
of time and frequency offsets between the clocks at the SPCs and UTC (NBS) (Coordi- 
nated Universal Time realized at NBS). 
1. Introduction 
Each SPC (signal processing center) at  the Deep Space 
Communications Complex (three, one at each complex) 
contains a frequency and timing system which provides t h e  
required frequencies and timing pulses for all of the subsys- 
tems in that SPC. There are two hydrogen masers located at 
each SPC; the first acts as the master clock and provides t h e  
primary frequency source, and the second acts as a backup. 
The backup hydrogen maser is kept syntonized (that is, syn- 
chronized in frequency) to the primary, particularly during 
critical missions, so that a switch from primary to secondary 
would have a minimum effect on DSN users. 
A requirement exists that timing pulses and frequencies 
distributed throughout the SPC be coherent. This requirement 
precludes the use of microsteppers to keep the SPCs clocks 
synchronized 'to some uniform time scale. Therefore, the 
master clock exhibits the characteristic hydrogen maser drift 
in frequency. 
67 
https://ntrs.nasa.gov/search.jsp?R=19870014401 2020-03-20T10:26:34+00:00Z
The process of tracking spacecraft involves all three SPCs 
working in concert to continuously track spacecraft as the 
eaith turns. At Voyager’s Uranus encounter the round trip 
signal transmission time was very long (about five hours). 
Asignal, sent from one SPC tothe spacecraft, then transponded 
and sent back to earth, is received by a second SPC. The 
Neptune encounter, which is to occur in 1989, will require 
the transponded signal to be received by the second and third 
SPC. This procedure requires accurate synchronization and 
syntonization between SPCs t o  measure round trip signal 
transmission time and signal doppler shifts when communi- 
cating with the spacecraft. Other requirements such as Very 
Long Baseline Interferometry (VLBI) and radio science also 
require accurate synchronization and syntonization among 
the SPC clocks. 
Presently, the DSN Frequency and Timing system (DFT) 
requirements are to maintain synchronization between SPCs 
to within 220 microseconds with a knowledge of ? I O  micro- 
seconds. Syntonization between SPCs is to be maintained to 
within 1 X lo-’’ d f / f  with knowledge of f 3 X d f / f .  
It is possible that navigation at the Voyager Neptune encoun- 
ter will require knowledge of frequency to less than 1 X 
d f l f .  
It needs to be emphasized that the DSN is a user of time 
scales and not a producer. The DSN needs to adjust its clocks 
from time to time to keep them syntonized and synchronized 
with each other. It is convenient to use the UTC (NBS) time 
scale as a reference. First, it is a uniform and continuous time 
scale. Second, the proximity of the NBS Time and Frequency 
division to JPL (approx. 1000 km) has historically made 
NBS the most accessible time scale to the SPC-10 due to the 
short airplane flight to carry a clock. The use of the Global 
Positioning System (GPS) time transfer method has made the 
UTC (NBS) accessible to all SPCs, so we have created an 
ensemble of clocks using UTC (NBS) as the reference. JPL 
has a contract with NBS which provides JPL with access to 
UTC (NBS) and also to the cesium clock associated with 
WWVH (the NBS time broadcast radio station) in Hawaii, 
using the data from their GPS timing receiver. 
II. The GPS Time and Frequency 
Coordination System 
JPL has installed GPS timing receivers at the three SPCs 
[ I ] .  These receivers get a timing pulse from the master clock 
at each SPC which is the “on line” hydrogen maser. The data 
from these receivers and other data are combined using a Kal- 
man filter to produce estimates of time and frequency offsets 
of the SPCs with respect to UTC (NBS). JPL uses the tracking 
schedule published by NBS-BIH. This schedule allows mutual 
68 
views with receivers whose tracking data are posted on the GE 
MK 111 RC28 Catalog, which is administrated in the United 
States by the United States Naval Observatory’. Figure 1 is 
a schematic diagram showing the clocks used and the possible 
mutual views which are available. 
The data are gathered by telephone once per week from the 
receivers in the DSN from the database at NBS and from the 
GE MK I11 database using an IBM XT personal computer. This 
is certainly a change from just a few years ago when we 
depended on traveling clocks and LORAN. Now we are able 
to observe the clocks, control them, and provide a guaranteed 
high accuracy synchronization and syntonization for the users 
of the DSN. 
111. Operation of the Clocks 
During the initial installation of a hydrogen maser fre- 
quency standard at an SPC, a calibration sequence is performed 
that includes the following steps: 
(1) Verify environmental stability with unit in final 
(2) Set final operating parameters. 
(3) Degauss magnetic shields. 
(4) Set internal bias field to required value. 
(5) Spin-exchange tune the hydrogen maser cavity. 
(6) Calibrate hydrogen maser rate to NBS (synthesizer 
(7) Calibrate rate of backup standards. 
(8) Set master and backup clocks. 
position. 
calibration). 
Figure 2(a) is a quadratic least square fit to the time offsets 
of the SPCs clocks with respect to UTC (NBS), and Fig. 2(b) 
is a least square linear fit of the frequency of the clocks with 
respect to UTC (NBS). The maser frequency drift of approxi- 
mately 4.5 X lo-’’ df / f  per day is well behaved and could be 
matched even closer between the three hydrogen maser clocks. 
Knowledge of this drift allows us to maximize the time inter- 
val between rate adjustments. After the masers are calibrated 
the cavity is deliberately mistuned SO that the average fre- 
quency offset with respect to NBS is near zero between rate 
changes. The average time difference between the three SPCs 
clocks and NBS is minimized by choosing the appropriate 
initial time offset. By spin-exchange tuning the hydrogen 
’ Available from the Catalog Administrator, USNO Time Service Divi- 
sion, Washinton, D.C. 20390, USA. 
maser after one to  two years and recalibrating the synthesizer, 
we will be able to determine the long term drift that is due to 
mechanisms other than the cavity. 
The backup standards are monitored on site with respect 
to the prime standard so that traceability to NBS is main- 
tained. The rate of these units is manually corrected when 
necessary so that upon switching from prime to backup 
standard, the frequency shift is minimal. Figure 2(b) shows 
that this was not accomplished very well. From about day-of- 
year (DOY) 100 on, backup standards were shifted in and 
out for various reasons at SPC-60 and SPC-40. The GPS 
system gives near real time visibility of clock performance at 
the three SPCs and enables operational control personnel to 
evaluate and improve overall system performance. 
IV. Example of the Voyager Encounter 
As the Voyager spacecraft approaches the planet, calcula- 
tions are made to correct the estimates of the spacecraft’s 
location. The accurate measurements required for these calcu- 
lations cannot be made until about 10 to 20 days before 
encounter. With these measurements, adjustments can be 
made to produce the desired trajectory by the planet. By 
having the three SPCs’ clocks synchronized and syntonized 
through the encounter period, accurate measurements of the 
spacecraft’s location can be made. 
To satisfy encounter requirements it was decided to adjust 
the rate and offsets at each SPC so that at encounter the 
frequency and time difference between each station and 
NBS would be near zero. This was done about two months 
prior to encounter. 
Daily GPS common view time observations were made to 
monitor the clock performance at each station. Figure 3 
shows the time offsets with respect to NBS. The worst case 
station-to-station and station-to-NBS offset was less than 500 
nanoseconds during the encounter period. Note the several 
prominent time steps at SPC-60 on days 355 ,20 ,77 ,  and 87. 
The master clock at each SPC is driven by a 5 MHz “Fly- 
wheel” oscillator which is phase locked to the primary stan- 
dard 5 MHz output. The phases of the backup standards are 
not maintained at  zero with respect to the prime so that upon 
switching to the backup standard, a time change of up to 
+lo0 nanoseconds may result. This happened on the above 
mentioned days at SPC-40 when, for various tests and experi- 
ments, frequency standards were shifted. On about day 100, 
SPC-60 was directed to shift to a backup standard without 
first matching the frequency. A frequency change of about 
6 X df/f led to the rapid divergence of time from day 
100 on. 
V. Time and Frequency Offsets Using 
the Kalman Filter 
The difference of the data from the receiver pairs is taken 
to produce mutual view values for clock pair offsets, one 
value each day for each spacecraft that is used for a mutual 
view for a given receiver pair. Using space vehicle i, 
(Cb - ca + f l s v J i  = Ggps - c, + “)i - cgps - Cb -I- f l ) i  
where n is the noise, nsvi is the total common view noise, 
c,, cb are the ground clocks, and gps is the space vehicle clock. 
A mean Cba, of the set of mutual view differences is then 
calculated along with the mean time T.  
T = , C t i  1 
i 
where m is the number of daily observations, and ti is the time 
of the observation using space vehicle i. 
These mean values are used as inputs to the Kalman filter, 
which has the effect of giving each space vehicle’s common 
view measurement equal weight. This tends to ignore the bias 
problem which was described by M .  A. Weiss [2].  Also. if a 
measurement is not made for some spacecraft on a particular 
day, then the biases of the individual spacecraft measurement 
tend to offset the estimate for that day, an undesirable side 
effect. 
The Kalman filter is a classical design, written in BASIC, 
and run on an IBM PC-AT computer. The output of the 
filter is the time, frequency and frequency drift of each 
clock with respect to UTC (NBS). An estimate of the time. 
frequency, and frequency drift for any point between end- 
points of the data set can be made by smoothing to that 
point. The procedure is to smooth to 00 hours UTC each day 
and use that value of the state vector as the estimate of the 
time and frequency offset of the clock with respect to UTC 
(NBS). The data are handled in 30 day batches, with each 
new batch started off with the state vector and the covari- 
ance matrix from the previous batch. 
Figure 3 shows the detail of the time offset of each of the 
clocks through the encounter to about DOY 130. The data 
were generated by the Kalman filter and each day is a point 
smoothed to zero hours UTC. Each of the clocks is refer- 
enced to UTC (NBS). However, because the estimate is for the 
69 
same time, the relation between the clocks at any two SPCs 
can be obtained by subtracting the value of one SPC from 
that of the second. 
Figures 4,  5 ,  and 6 show the estimated value of frequency 
of the SPCs clocks before, during, and after encounter. On 
top of these smoothed values are estimates of frequency 
obtained from the mutual view data by making a linear least 
squares estimate of the data over about ten day groups of data. 
These estimates were done manually with judicious choice of 
data. 
Notice the boxed-in detail on Fig. 4, the frequency offset 
between SPC-10 and UTC (NBS). This indicates that there 
was almost 1 X df/f change in frequency over a period 
of 10 to 15 days. Unexplained frequency changes of this 
magnitude have been noticed in hydrogen masers during 
tests at JPL. However, the mysterious thing is that the other 
clocks, SPC-40 and SPC-60, showed similar frequency changes 
at the same time. The Kalman smoother output agrees with 
the hand calculations of frequency using the mutual view data 
which indicates the filter is not causing the appearance of 
similar frequency changes in the several clocks. 
One would conclude that the NBS time scale wiggled. 
Indeed, the wiggle seems to have been at NBS, but not the 
time scale. At that time JPL was not accessing the data from 
the time scale, but from clock nine at NBS. A check with 
NBS confirmed that clock nine experienced a similar frequency 
excursion at that time. Two conclusions can be drawn from 
this: the Kalman smoother produces good estimates of the 
clock performance and the need to make sure the data is from 
the UTC (NBS) time scale. This procedure was changed late 
in 1986; JPL presently accesses the UTC (NBS) time scale. 
Figure 7 shows the values of Allan variance of frequency 
estimates of the three SPCs’ clocks produced by the Kalman 
smoother. Nominal expected drift of the hydrogen masers 
which are deployed is a slope of 4.5 X lo-’’ df/f  per day. 
The SPC-10 line follows the performance of the field hydro- 
gen masers fairly well. At four days or less, the SPC-60 and 
SPC-60 Allan variance is considerably worse than hydrogen 
maser performance. The SPC-60 Allan variance settled down 
to hydrogen maser performance at about eight days. The 
SPC40  estimates never seem to settle down. 
VI. Conclusions 
Daily estimates of frequency with errors of less than 1 X 
df/f  are available on a routine basis. It appears that 
this can be reduced to several parts in df / f  in the next 
few months. Now it has been shown that DSN can be oper- 
ated continuously with clocks which are syntonized to within 
3 X df / f  and synchronized to within a microsecond. 
By matching the drift rates of the clocks it might be possible 
to routinely match the synchronization and syntonization 
even more closely. 
This year’s experience has shown that the development of 
better methods is needed to assure the secondary hydrogen 
maser is kept closely syntonized with the primary maser. 
Ref e rences 
[ l ]  P. A. Clements, S.  E. Borutzki, and A. Kirk, “Maintenance of Time and Frequency 
in the DSN Using the Global Positioning System” TDA Progress Report 42-81, vol 
Jan.-Mar., 1985, pp. 94-108, Jet Propulsion Laboratory, Pasadena, CA, May 15, 
1985. 
M.A. Weiss, Proc. 17th Ann. Precise Time And Time Interval Applications and 
Planning Meeting, pp. 261-273, Dec. 3-5,1985. 
[2] 
70 
r\ COLORADO 
SPAIN U 
Fig. 1. Possible GPS mutual views available to measure 
the DSN clocks 
CALIFORNIA 
AUSTRALIA 
SPAIN 
I I I .  I 1 1  I 
320 350 010 040 070 100 130 160 190 220 250 280 
1985 1986 DAY OF YEAR 
Fig. 2. Time and frequency offset with respect to UTC (NBS) 
v) 1.0 
n 
2 
# 0.8 
0 
0: 
0 
I 
8 
0.6 
0.4 
0.2 
.-: *.- 
: .. 
:*: 
320 350 010 040 070 100 1: 
1985 1986 DAYOFYEAR 
Fig. 3. Time offset with respect to UTC (NBS) (Output of 
Kalman Smoother) 
71 
c 
0 
X 
LL . LL 
a 
c 
0 
X 
U . LL 
a 
72 
5 - ~ " ' ' ~ ' ' ' ~ ] ' ' ~  " 1 ' 1  1 I I I " " ' I " " '  
4 -  
3 -  URANUS 
2 -  
- 
d VOYAGER - 
ENCOUNTER p....." 
0 ,.d i.c.,,-., - ? 1 -  - 
QZ' z x 0 -  
- 1  - 
a d 0 LINEAR FIT OVER 10 DAYS 
- . LL $..o...o..4-* - 
0 LINEAR FIT OVER 10 DAYS -2 l t 
320 350 
1985 
KALMAN SMOOTHER 
-5 
1985 1986 DAY OF YEAR 
010 040 070 100 130 
1986 DAY OFYEAR 
- 4  
* KALMAN SMOOTHER 1 
1985 I 1986 DAYOFYEAR 
Fig. 6. Frequency offset, Spain-UTC (NBS) 
Fig. 4. Frequency offset, California-UTC (NBS) 
NUMBER OF DATA POINTS 
-4 1 * KALMAN SMOOTHER 
7 t""S 
Fig. 5. Frequency offset, Australia-UTC (NBS) 
'days 
Fig. 7. Allan variance with respect to UTC (NBS) 


Results of using the global positioning system to maintain the time and frequency synchronization in the Deep Space Network

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