The present status and future expectations of four satellite tracking systems, satellite-to-satellite tracking, lasers, very long baseline interferometry (VLBI) and geoceiver are briefly discussed

Smith, D. E.

English

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PRECISION TRACKING SYSTEMS OF TEE IMMEDIATE FUTURE: A DISCUSSION I0
David E. Smith
Geodynamic s Branch
Trajectory Analysis and Geodynamics Division
Goddard Space Flight Center
Greenbelt. Maryland , _7"3'" 1 5 _ _
INTRODUCTION
This paper discusses briefly the present status and future expecta-
tions of four satellite tracking systems, satellite-to-satellite tracking_
lasers, very long baseline interferometry (VLBI) and geoceiver. None of
these techniques are being fully exploited at the present time but all
can be expected to provide measurements in the next few years of a
quality that will contribute extensively to studies of Earth and
ocean dynamics.
SATELLITE-TO-SATELLITE TRACKING
This technique has been discussed extensively during the last few
years as a possible means of maintaining current orbits on active space-
craft and also as a precision tool for studying the perturbations in
satellite motion for Earth physics. In its simplest terms satellite-to-
satellite tracking is an electronic tape measure connecting a master
satellite, usually depicted in a high stable orbit, with a relatively
low altitude, strongly perturbed spacecraft. The big advantage for
Earth physics of satellite-to-satellite tracking compared to normal
tracking is that extensive coverage both geographically and in time is
possible with nearly all proposed systems. Indeed, it has been argued
that this technique is probably the only way many of the objectives
described in the Williamstown Report on Solid-Earth and Ocean Physics
(ref. l) can be achieved.
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Although satellite-to-satellite tracking is not yet a reality the
technique has essentially been explored and applied to problems of the
lunar gravitational field. The way lunar satellites are tracked from
the Earth is es'sentlally the same method and, furthermore, it has been
successfully spplied to the mascon problem where the perturbations, of
too high a frequency for adequate representation by spherical harmonics,
were shown by Muller and Sjogren (ref. 2) to be correllated with topo-
graphical features.
The type of spaceborne tracking system that will be used for the
Earth physics investigations is expected to be similar to the Goddard
Range and Range-Rate System (GRARR) which operates at S-band. The
present accuracy of the range-rate measurements based on an averaging
time of i0 seconds is about 0.3 mm/sec compared with the 0.03 mm/sec
which will be actually required for Earth physics investigations.
Improvement of the system to the required level is not expected to be
a major problem.
LASER TRACKING
Of the four systems being discussed the laser technique has probably
been in operation longer than any of the others. However, only a limited
amount of operational experience has been gained with this system, and
most of that has been gained during the last year. In concept, the
laser is probably the simplest of all measuring devices since, like
radar, it sends a pulse of energy towards the spacecraft which is
reflected and received back down near the transmitter. The real measure-
ment is the roundtrip travel time which is then turned into a range.
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Systems of this kind have been operating in a network configuration
only recently and then with only a few stations and with systems not
fully tested. Furthermore, the best way of analyzing these data for
Earth physics purposes has not yet been determined and, in this respect,
puts this system in essentially the same position as the others.
In the United States it is Goddard Space Flight Center and the
Smithsonian Astrophysical Observatory who have been the main organiza-
tions responsible for the development of laser tracking of artificial
satellites and for investigating and fostering the application of these
systems to Earth physics. "The present systems probably have an accuracy
of about 50 cm but this is a rather arbitrary figure because there is no
absolute scale by which their quality can be judged and for this reason
a laser system tends to be judged by the rms range noise about some
reference, such as an orbit. In some respects, however, this is not an
undesirable parameter, provided it is not confused with accuracy,
because the noise is an indication of the stability, albeit short-term_
of the system, and for many investigations this maybe even more
important than accuracy.
The quality of the laser in the next few years is expected to improve
to about the 5 to lO cm level. This is not a great deal better than is
currently being claimed for the present lunar laser ranging system and
is, in fact, a little worse than is expected of future lunar systems.
The major disadvantage of the laser over competitive techniques is that
it is a fair weather instrument. The operation of the systems during
both day and night has been routine for several years but little can be
done to overcome clouds or heavy fog, except perhaps, by choosing a more
favorable site location. This question of site selection is one that can
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h_ expected to be of considerable importance for all the precise tracking
systems of the future because their quality will make them sensitive to
t_e very small changes in station position associated with such move-
ments as creep, subsidence, tidal loading and fault slips. The question
of weather is just one more factor in an area which can be expected to
become increasingly complex.
Finally, a problem that is unique to the laser, and for which very
little information is presently available, is the restrictions that may
be placed on the operation of the system near highly congested air-
traffic lanes. At present, safety regulations require that an observer
keep watch for aircraft and that the system cease operation, when
necessary. However, the locating of stations on the west coast by GSFC
is necessitating the addition of radars to the systems that will detect
aircraft at much greater distances and the impact of this m%y be to
1_duce the overall effectiveness and efficiency of the system. These
questions will only be answerable after a period of operation under
these conditions.
VERY LONG BASELINE INTERFEROMETRY (VLBI)
It is only recently that the technique of very long baseline inter-
ferometry has been applied to the problems of geodesy, notably, base-
line determination. However, VLBI has been successfully demonstrated as
a radio astronomical tool as far back as 1967 with many measurements of
:.r_j_lar resolutions smaller than a thousandth of a second of arc
( 3).
Of the techniques being discussed here this is the only one which
:_.u.the ability to make measurements fin an inertial frame of reference
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and this is an important advantage. With this capability VLBI can be
used to monitor the motion of the Earth in space and on its axis in
addition to the ability to measure chord lengths. However, the prime
consideration here is for the interferometer to be used to track a
spacecraft. Recently VLBI experiments conducted jointly by SAO and GSFC
on an ATS spacecraft from sites at Rosman and Mojave have detected
fringe rates that correspond to changes in the velocity of the spacecraft
as small as I mm/sec and with a resolution of the order 0.I mm/sec.
These velocity measurements can greatly enhance the position determination
capability of synchronous spacecraft, thereby enabling the recovery of
low degree and order geopotential coefficients to be significantly
improved.
One of the major problem areas of VLBI is in the atmospheric dis-
tortion of the ray paths to the source. For very short baseline systems,
such as Minitrack, the ray paths are almost identical i'or both antennas
and consequently the atmosphere is not of major importance. However, as
the baseline increases to inter-continental distances the ray paths
through the ionosphere can differ by several meters even at C-band
frequencies. Atmospheric modeling can be applied to the measurements
but to achieve accuracies of i0 cm will require models accurate to a
few percent and this may not be practical.
GEOCEIVER
The Geoceiver is a continuously integrated doppler system, similar
in design to the TRANET doppler stations but considerably smaller and
simpler yet designed to produce position determinations of equal quality.
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The Department of Defense is apparently obtaining over thirty of these
systems and the first few production models are already being tested.
One method of using the Geoceiver system is in a small dense
network of, say 7 or 8 stations on a continent with baselines of about
lO00 km. Studies of the potentialities of such a network (ref. 4)
indicate a position determination of the spacecraft to be about lO cm.
Now .the Geoceiver is not, inherently, a ranging system but when used in
groups the biases for each of the systems can be recovered enabling space-
craft position to be determined with considerable precision. However,
individually, the stations cannot be used as ranging systems and this is
probably the biggest disadvantage of Geoceiver.
The production models of Geoceiver are reportedly exceeding the
specifications (ref. 5) and under test the rms of the deduced range
residuals are about 5 cm. These tests suggest that a geometric deter-
mination of the spacecraft position.with respect to the ground stations
is probably of the same order and that the absolute accuracy of the space-
craft position while being tracked is largely due to the errors in the
lo_%tions of the ground stations and unmodelable atmospheric effects.
Furthermore, it seems highly probable that with sufficient data the
station positions relative to each other could be improved upon to the
level of a few tens of centimeters and in a geocentric system to at
].east one meter.
The possible application of these systems to the calibration of a
spaceborne altimeter are obvious but with our present gravitational
models it would be impossible to extend the same quality of calibration
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ito areas over which the spacecraft was not being continuously tracked
by several of these systems.
DISCUSSION
During the next few years the four systems that have been discussed
can be expected to approach the quality required for the Earth physics
investigations of the kind described in the Williamstown report (ref. 1).
However, it is very doubtful at the present time if the software availale
for _ndling these data and our knowledge and ability to model the ef-
fects of the atmosphere are anywhere near adequate. Consequently, even
if these systems provide measurements of lO cm precision_ it is doubt-
ful that we shall be able to make proper use of the data unless consider-
able effort is expended on improving orbital perturbation theories,
numerical integration systems, atmospheric and gravitational modeling,
etc.
Table 1 is an attempt to summarize the present capabilities of the
four systems that have been described, together with an estimate of
their precision in two or three years time. Some of the advantages and
disadvantages of each of these systems are also given but it should not
be inferred that the systems should be competitive 3 but rather comple-
mentary. When working at the l0 cm level it will be dangerous to take
for granted the results of any one system for very many years until it
has been fully proved. The confirmation of geophysical measurements
by at least two systems employing different techniques and data handling
metnods should be a major aim of this new work for many years to come.
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."The Terrestrial Environment: Solid-Earth and Ocean Physics"
NASA CR-1579, April 1970.
Muller, P. M. and W L. Sjogren, "Mascons: lunar mass concentrations",
Science, Vol. 161, p. 680, 1968.
Cohen, M. H., D. L. Jauncey, K. I. Kellerman and B. G. Clark,
"Radio interferometry at one-thousandth second of arc", Science,
Vol. 162, p. 88, 1%8.
Brown, D. C., "Near term prospects for positional accuracies of
0.i to 1.0 meters from satellite geodesy", D.B.A. Systems, Inc.,
Final report on contract number F19628-69-C-(_264 for AFCRL,
Report Number 70-0501, August 1970.
Private communication with Duane C. Brown, D.B.A. Systems,
Melbourne, Florida, Sept. 1971.
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Precision tracking systems of the immediate future: A discussion

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