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The SATRACK System: Development and Applications
Thomas Thompson, Larry J. Levy, and Edwin E. Westerfield
ATRACK has been a significant contributor to the development and operational
success of the Trident Weapon System, and it continues to provide a unique
monitoring function that is critical to the maintenance of the U.S. sea-based strategic
deterrent. This article reviews the background and evolution of this unique Global
Positioning System user application (the first committed user) and discusses its support
of other systems of national importance. Its connection to APL’s pioneering
contributions to the development of the technology and methodology supporting
successful deployment of the sea-based strategic deterrent is reviewed. Basic concepts
and implementation specifics of the evolving system design are described, and its
extended use and performance improvements over the past 25 years are presented.
(Keywords: GPS translators, Missile system test and evaluation, Satellite positioning.)
INTRODUCTION
SATRACK was developed to validate and monitor
the Trident missile guidance error model in the System
Flight Test Program. It is the primary instrumentation
and processing system responsible for accuracy evaluation
of the Navy’s Strategic Weapon System. Instrumentation
and processing systems available when the
Trident Development Program began could not meet
this need. APL conceived and led the development of
the SATRACK system to fulfill this requirement. Prototype
instrumentation required for the missile and
ground station data collection functions were developed
at APL to validate the concept, and we generated
specifications controlling the development of the operating
missile and ground station hardware. We also
developed and operate the SATRACK processing
facility, which includes a unique preprocessing hardware
and software configuration and an extensive
postprocessing analysis capability. Additionally, the
APL satellite tracking facility has operated as a backup
SATRACK recording site for all East Coast test flights
since 1978.
SATRACK fully met all its guidance subsystem
evaluation requirements and also provided weapon
system error model insights that would not have been
possible without its unique ability to detect and allocate
small error contributors to miss distances observed
in the flight test program. SATRACK not only validated
the Trident system accuracy, but the test-derived
and -validated system error models have allowed the
command authority to confidently assign and allocate
targets to sea-based strategic resources.
The next section will discuss the background leading
to SATRACK’s development as a natural extension
from APL’s role in the development of the Fleet
JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 4 (1998) 437
THE SATRACK SYSTEM
Ballistic Missile Evaluation System and the Navy Navigation
Satellite System (Transit). Then, following a
discussion of the basic concepts, this article will trace
the system’s evolution:
• SATRACK I—A technology project to develop and
demonstrate the instrumentation and processing system
using the Trident I missile (1973–1983)
• SATRACK II—The operational system designed to
meet system requirements for the Trident II missile
(1983–present)
• Other applications—Uses of SATRACK for Army
and Air Force missile test applications (1983–present)
• SATRACK III—Current system upgrade and future
applications
BACKGROUND
From 1967 through early 1971, APL conducted a
series of studies to support concept development of the
Defense Navigation Satellite System (DNSS). These
studies addressed a variety of configurations capable of
meeting DNSS requirements and eventually led to a
concept proposal called Two-In-View (TIV) Transit.
The name was chosen to indicate that DNSS requirements
could be met with only two visible satellites (in
contrast to the alternate concepts that required four
visible satellites) and that it could evolve naturally
from the already operational Transit system. We chose
this concept because it was the lowest-cost approach
to meeting DNSS requirements.
The ability to provide three-dimensional positioning
with the required accuracy using two satellites is
possible only because their motion relative to a user
is significant. The alternate concepts were based on
simultaneous range measurements to four satellites.
Since these concepts were not benefited by high
relative motion, they incorporated satellite constellations
at higher altitude. The higher-altitude constellations
were selected because they achieved the
required global coverage with a smaller number of
satellites. This choice was partly motivated by the
incorrect assessment that system costs increased as the
required number of satellites increased. However, fewer
satellites were possible because the area that each
served expanded in direct relation to the reduction in
the number of satellites, and costs for signal services
tend to have a direct relationship to service area, not
to the number of satellites. To a first-order approximation,
positioning satellite system costs are independent
of constellation altitude. Further discussion of these
topics and the prevalent views of the time is available
from Refs. 1–3.
The last performance study of the TIV Transit
system addressed its ability to provide trajectory measurements
of SLBM test flights. The study showed that
SLBM measurement objectives could be met, and an
interim report published in late 1971 formulated the
tracking concepts and missile and ground station capabilities
needed to support TIV Transit measurements
of SLBM flight tests. However, by mid-1972 it was
clear that none of the then-proposed DNSS concepts
would be developed, and the missile tracking concept
was temporarily forgotten.
In a parallel chain of events, the Navy’s Strategic
Systems Programs organization was asked to address
the suitability of the Trident Weapon System for more
accurate targeting requirements. Several studies were
initiated to consider this question. The primary issues
concerned possible system improvements to achieve
the desired accuracy. An important secondary concern
regarded the method by which the accuracy of the new
system would be validated.
It was soon evident that the impact scoring techniques
used for Polaris and Poseidon evaluations would
not be adequate. A new methodology that provided
insights into major error contributors within the flight
test environment would be needed so that accuracy
projections could be based on a high-confidence understanding
of the underlying system models. The
technique of comparing observed test impact statistics
with results computed from models used for development
(i.e., “shoot and score” approach) was unacceptable.
Assessing performance models in the flight test
environment requires guidance-independent measurements
with sufficient precision to separate out the
important contributors to system inaccuracy. The
existing range instrumentation (missile tracking and
trajectory estimation) was largely provided by radar
systems, and it was not clear that they could provide
the needed measurement accuracy or coverage in the
broad ocean test areas.
In early 1973, we initiated a study to compare current
range radar with TIV Transit measurement capabilities
in relation to needed SLBM accuracy evaluation
objectives. The study results showed that only a
satellite-based measurement system could meet future
requirements. APL presented the SATRACK concept
to the Navy’s Strategic Systems Programs staff in May
1973. The proposed system was based on a custom
satellite design patterned after TIV Transit satellites,
but simplified by the removal of any requirement for
real-time positioning service. A six-satellite constellation
would support two flight test windows per day.
These concepts were proposed to minimize costs.
Missile hardware and ground support capabilities were
unchanged from the concepts defined in 1971.
The proposal was accepted and preliminary development
was initiated. However, 1973 was also the year
that the Global Positioning System (GPS) development
began. With the emergence of GPS, we were
asked to consider its use in place of the specialized
satellite constellation. Our studies indicated .