With two geosynchronous relay satellites the tracking and data relay satellite system (TDRSS) can provide nearly worldwide coverage for communication between all near orbiting satellites and the satellite control center at Goddard Space Flight Center. Each future NASA satellite will carry a TDRSS transponder with which the satellite can communicate through a TDRSS to the ground station at White Sands, New Mexico. It is using this system that the ground station master clock time signal can be transmitted to the near Earth orbiting satellite in which a clock may be maintained independently to the accuracy required by the experimenters. The satellite time transfer terminal design concept and the application of the time signal in autonomously operated spacecraft clock are discussed. Some pertinent TDRSS parameters and corrections for the propagation delay measurement as well as the time code used to transfer the time signal are given

Chi, A. R.

English

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NASA
Technical Memorandum 80605
(NASA-TM-90605) SATELLITE TIRE TRARSF*R VIA 	 M80-16084
TRACKING AND DATA RELAY SATELLITE SYSTEM
(TDRSS) AND APPLICATIONS (NASA) 23 p
HC 102/t!E A01	 CSCL 22A	 Unclas
G3/15 11674
Satellite Time Transfer Via
TDRSS and Applications
A. R. Chi
DECEMBER 1979
r
National Aeronautics and
Space Administration
Goddard Space Flight Center
Greenbelt, Maryland 20771
SSE FAIIt ry 
PQ
Accts	
F
CONTENTS
PAGE
ABSTRACT
	
1
INTRODUCTION
	
1
TDRSS
	
2
SATELLITE TIME TRANSFER USING TDRSS
	
3
CONCEPT OF OPERATION
	
3
TDRSS PARAMETERS
	
4
TIME SIGNAL CORRECTIONS USING TWO WAY TIME TRANSFER VIA A SYNCHRONOUS
SATELLITE
	
4
ONE WAY TIME TRANSFER OPTION
	
5
SUMMARY
	
5
REFERENCES
	
5
SATELLITE TIME TRANSFER VIA TDRSS
AND APPLICATIONS
Andrew R. Chi
NASA Goddard Space Flight Center
Greenbelt, Maryland
ABSTRACT
the early 1980's NASA will enter a new era of space program, i.e.,
space transportation through Space Shuttle. It will have a new large
scale space communication system for satellite tracking and data
transmission known as the Tracking and Data Relay Satellite System (TDRSS).
With two geosynchronous relay satellites TDRSS can provide nearly
worldwide coverage for communication between all near orbiting satellites
and the satellite control center at Goddard Space Flight Center. Each
future NASA satellite will carry a TDRSS transponder with which the
satellite can communicate through a TDRS to the ground station at White
Sands, New Mexico. It is using this system that the ground station
master clock time signal can be transmitted to the near earth orbiting
satellite in which a clock may be maintained independently to the accuracy
required by the experimenters. This paper presents the satellite time
transfer terminal design concept and the application of the time signal in
autonomously operated spacecraft clock. Some pertinent TDRSS parameters
and corrections for the propagation delay measurement as well as the
time code used to transfer the time signal will be given.
INTRODUCTION
The use of satellites for time transfer began soon after the first
artificial satellite was placed in orbit. This was because the concept
as well as the instrumentation design was simple. It was done as an
experiment to demonstrate the capability in precision over long range
and the favorable geometrical configuration for the signal to propagate
through the medium. In comparison with the terrestrial propagated signal,
the signal transmitted from or transponded by a satellite is relatively
independent of the propagation medium. The stated precisions by investigators
in the last two decades using different carrier frequencies, techniques, and
satellites shows orders of magnitude of improvement, ranging from microseconds
to nanoseconds, over the conventional techniques. Although the capability
of precision time transfer using a satellite has been amply demonstrated.
the limitation of further improvement still lies on the ability to measure
the signal path delay. At present, this limitation is about 30 centi-
meters or 1 nanosecond.
Fortunately, the technology for the time generation and dissemina-
tion has been ahead of or at least in pace with the requirements [1].
As independence for timekeeping becomes an essential requirement
for on-board navigation and spacecraft autonomy [2] satellite time
transfer becomes a technology that is needed for immediate application.
*This paper was presented at the 11th Annual Precise Time and Time Interval
(PTTI) Applications and Planning Meeting, November 27-29, 1979, Goddard
Space Flight Center, Greenbelt, MD.
In this paper, I shall describe a particular satellite
which clock time can be transferred from the ground to
satellites in near earth orbit or on the ground. The
is called the Tracking and Data Relay Satellite System
system through
the users in
satellite s stem
(TDRSS) [3 .
TDRSS
The TDRSS is a new large scale space communication system to be shared
between government and commercial use. It is a new NASA tracking and
data acquisition and communication system. It also provides additional
capability for growing communication traffic in the private sector.
The system consists of 4 geosynchronous relay satellites. The first
two are for NASA, the third (the advanced Weststar) is for commercial
use, and the fourth is a common spare. The system concept is shown in
Figure 1. It is represented by the two NASA geosynchronous relay
satellites, 1300
 apart in longitude, and a ground terminal located at
White Sands, New Mexico. The system will be capable of tracking,
transmitting data to, and receiving data from user spacecraft over at
least 65 percent of the user orbit. The ground terminal at White Sands,
New Mexico is shown in Figure 2.
The satellite design is shown in Figure 3. Each satellite gene-
rates 1700 watts of electrical power from its solar arrays and trans-
mits and receives in 3 frequency bands (S, C, and K) from 6 antennas,ti	
3 of which are steerable. The weight is 2132 kilograms (4700 pounds)
and the size is 17 meters (57 feet) from tip to tip. The satellite
will be launched by the Space Shuttle in 1981 and 1982 and will have
a lifetime of 10 years.
The steerable S and Ku-Band, 4.9 meter antennas, are used to
provide communication service for the single access (SA) users, and the
S-band antenna array is used to provide communication service for the
multiple access (MA) users. The steerable K-band, 2.0 meter antenna,
is for the forward and return communication links between space (TDRS)
and the ground terminal. The two TDRS can support up to 4 S -band or
K-band single access users (SSA or KSA) and up to 20 MA users.
The advantage of such a space communication system [4] can be
seen in the next two figures. Figure 4 shows the present NASA track-
ing and acquisition network. There are 14 ground stations located
throughout the world. Figure 5 shows the post TDRSS NASA tracking
data acquisition and communications network. It shows 8 ground stations
including the Bermuda _.tation to provide only the launch support.
There is a 50 percent reduction in the number of the ground stations
which also serves the deep space probes and the highly eliptical orbit
satellites.
4
k
P
L
kl
SATELLITE TIME TRANSFER USING TDRSS
A user configuration of a satellite time transfer system using a
MRS is shown in Figure 6. A master clock is located at the White
Sands ground terminal. The user may be mobile, fixed on the ground
or in a satellite. The master clock is calibrated via a TDRS to a
national time standard such as the National Bureau of Standards (NBS)
or the U.S. Naval Observatory (USNO), since the NBS and USNO time scales
can be related to each other and to the Bureau International de 1'Heure.
The user's modes may be MA, SSA, or KSA, and the carrier frequencies
for each mode are shown in the table in Figure 6. The satellite cover-
age for the time users at 5° and 10° elevation viewing angles for TDRSS
at 41 0
 west and 171 0 west is shown in Figure 7.
CONCEPT OF OPERATION
The philosophy of operation is directed toward automation, that
is the clock time will be transferred from the White Sands terminal
via a TDRS to a user satellite by a command sent from the Proiect
Operations Control Center (POCC) at Goddard, Greenbelt, MD. The propa-
gation delay may be measured by a two-way time transfer technique or
maybe calculated based on the position information of the ground terminal
and the two predicted satellite positions, if the calculated delay accuracy
meets the time accuracy requirement. The received time signal in the
user satellite is measured relative to the on-board clock by a time
interval unit. After correction for the signal propagation path delay,
the clock error is transmitted via the TDRS to the ground for monitor-
ing and verification. The satellite clock is free running up to a pre-
set maximum clock error at which time, by on-board computer program
action, a step time or a step frequency correction is made. Should the
correction be deleted, a command signal is needed to override the
automatic clock correction. After such a command, a new value of the
maximum clock error must be re-set if the automatic clock correction
feature is to be maintained.
A functional block diagram of the ground station time transfer
terminal is shown in Figure 8. The time signal data is divided into
two parts. One part contains the grouped parallel binary time code
(PB5) [5] which is transmitted as data through telemetry. Only the
time unit in the time code that is larger than the propagation path
delay is of significance. Thus it is called the coarse time. The other
part contains a time epoch sequenced pseudo random noise (PN) code [6],
which is transmitted through the range channel on the forward link.
It has an ambiguity time of 85 milliseconds. It is to this time data
that the propagation path delay corrections must be applied. The data
processor is shown at the extreme right of Figure 8. The step time and
step frequency corrections are used to maintain the ground station clock
to that of a national time standard.
3
The functional block
shown . in Fiture 9. It is
The only exception is the
This feature is designed
time signal or to compare
transfer systems.
diagram of a satellite clock system [7] is
identical to the.ground station terminal.
Global Positioning System (GPS) receiver.
for a user satellite either to use the GPS
the time signals of the GPS and TDRSS time
TDRSS PARAMETERS
For detailed TDRSS parameters, signal characteristics and service
capabilities, the readers are referred to the TDRSS Users' Guide which
is available on request [8]. Some pertinent TDRSS parameters to
satellite time transfers are given in Figures 10 and 11.
TIME SIGNAL CORRECTIONS USING TWO WAY TIME TRANSFER VIA A SYNCHRONOUS
SATELLITE
In a two-way time transfer usin g a geosynchronous satellite, the
propagation path delay can be approximated as shown In the upper part
of Figure 12. This two-way delay, is 46 milliseconds (ms). For
simplicity of operation, the PN code period is considered to be longer
than the two-way path delay, i.e. 46 ms. Thus 85 ms ambiguity is used
for the PN code.
In a satellite-to-satellite time transfer, the relative satellite
motion of the two satellites must be considered. Based on. past data,
the doppler motion for all satellites falls in the range of 6 to 8 KM/S
which is equivalent to about 20 to 27 us/s rate. If the correction for
doppler motion is made for 1/4 of a second, the residual error is 5 to
7 us, as shown in the lower table of Figure 12. If the same correct-
ion time of 1/4 of a second is applied for satellite motion in a
geosynchronous orbit, the residual error is only 17 to 34 ns.
The propagation delay corrections due
medium depends on the assumed atmospheric
ical locations. Using the example worked
1970, as shown in Figure 13, the maximum e
at 2 GHz if the atmospheric correction is
to the composite atmospheric
model, season, and geo raph-
out by David Levine [93 in
rror is 65 ns at 8 GHz and 70 ns
not made.
ONE WAY TIME TRANSFER OPTION
For most space science users in the 1980's, the timing requirements
are in the range of 10 to 1000 microseconds. To meet these needs, one-way
time transfer via a TDRS is an attractive option. This is particularly
true if the user satellite can navigate on-board to achieve one kilo-
meter position accuracy. This is based on the capability of TDRSS orbit
and position data which can be provided in near real-time as shown in
Figure 14. Obviously, this service can be provided to a larger number
of users through the multiple access mode.
4
SUMMARY
TDRSS can be used after 1982 as an operational service to transfer
precise time by two-way or one-way technique. Using the two-w tech-
nique to measure the propagation path del, the precision of time
transfer, without corrections, can be of the order of hundreds of nano-
seconds and with correction to the order of nanoseconds. The precision
of one-way time transfer technique is limited by the accuracy of the
path delay calculations. This is generally in the order of microseconds.
Potential applications in addition to serving the satellite users
are for time comparison among navigation system clocks and the national
laboratory primary clocks and for cross-calibration of other equally
precise time transfer systems.
As in any system design, the accomplishment is the accumulated
results of many research and development programs. The author expresses
his appreciation without giving the names of those who have contributed
to the satellite time transfer technology.
REFERENCES
[1] Cooper, Robert S., and Andrew R. Chi, "A Review of Satellite Time
Transfer for Technology: Accomplishment and Future. Applications,"
Radio Science, vol 14, No. 4, pp 605-619, July-August 1979.
[2] Hearth, D.P., "A Forecast of Space Technology 1980 - 2000," NASA
Spec Publ SP 387, 1976.
Also, Ferris, A. G. and E. P. Green, "A Proposed Concept for
Improved NASA Mission Data Management Operations, NASA/GSFC Doc.
X533-76-81, May 1977.
[3] Carrier, Louis M. and Warren S. Pope, "An Overview of the Space
Shuttle Orbiter Communication and Tracking System," IEEE Trans.
Comm. vol com-26, no. 11, pp 1494-1506, November 1978.
[4] Schwartz, John and Eugene J. Feinberg, "STD! in the TDRSS and
Shuttle Era," IEEE Trans. Com . vol com 26, No. 11, pp 1506-1513,
November 1978.
[5] Chi, Andrew R., "A Grouped Binary Time Code for Telemetry and
Space Application," Session XIII Standards and Applications,
International Telemetering Conference, November 19-21, 1979,
San Diego, CA.
5
[6] Sabto, JCA, "Coding and the Spaceborne Atomic Clock," NASA/GSFC
Document X-521-71-81, Fegruary 1971.
[7] Chi, Andrew R., "Application of Satellite Time Transfer in
Autonomous Spacecraft Clocks," NASA Tech Memo 80572, October
1979.
Also presented in NBS workshop on "Impact of Improved Clocks and
Oscillators on Communications and Navigation Systems," November
18-20, Gaithersburg, MD.
(8] "Tracking and Data Relay Satellite System (TDRSS) Users' Guide",
Revision 4, NASA/GSFC Report STDN 101.2, Doc. Se*.., Code 863.2,
Goddard Space Flight Center, Greenbelt, MD
(9] leVine, David M., "Propagation Delay in the Atmosp!ere," NASA/
GSFC Document X-521-70-404, November 1970.
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Satellite time transfer via Tracking and Data Relay Satellite System (TDRSS) and applications

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