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Location: UFOUpDatesList.Com > 1999 > Jan > Jan 9

Space Command Surveillance 'FENCE' Revealed

From: UFO UpDates - Toronto <updates@globalserve.net>
Date: Sat, 09 Jan 1999 21:43:34 -0500
Fwd Date: Sat, 09 Jan 1999 21:43:34 -0500
Subject: Space Command Surveillance 'FENCE' Revealed

Source: Sightings On The Radio


The US Space Command Surveillance 'FENCE' Revealed
By Dr. Paul Schumacher
Technical Advisor
Naval Space Command
Logistics and Information Systems Division

From Michael Theroux <director@borderlands.com>


For more than 35 years, the U.S. Space Command (USSPACECOM) and
its predecessor organizations have maintained a continuous
surveillance of space and a complete inventory of trackable
Earth-orbiting objects. This space surveillance mission is
rooted in the military's need to know when hostile satellites
might be in position to target operational units or collect

But, as the number of objects - satellites and debris - orbiting
the Earth has grown, the 'space watch' has also become
increasingly important in protecting the safety of manned and
unmanned missions into space.

Today, in one of the largest-scale tracking enterprises ever
undertaken, more than 9,500 objects are tracked daily by a
network of more than 20 sensors spread around the globe.
'Objects' includes active and inactive satellites, as well as
assorted 'space junk.' And the total trackable population has
been growing at a long-term rate of about 250 objects per year.

Of particular concern in recent years is the planned deployment
of large new satellite constellations such as Iridium and
Teledesic, which will increase the trackable orbiting population
by hundreds of objects in a short period of time. Additionally,
concerns about the risk to payloads from small space debris have
led to proposals to expand the catalog to include objects as
small as 1 centimeter in size - potentially over 400,000 in
number. These trends threaten to overburden our current space
surveillance system.

However, a number of initiatives by Naval Space Command
(NAVSPACECOM) will help ensure that the United States will be
able to maintain a much larger and more rapidly growing space
catalog in the years ahead.

The Surveillance Challenge

USSPACECOM's satellite catalog is a database of orbital elements
from which it is possible to calculate position and velocity at
any time for any trackable object in Earth orbit. Predicted
positions of these objects are essential data for all analyses,
including threat assessments. But since the prediction accuracy
of each element set degrades with time from the last contact
with the object, all objects must be tracked continually and
element set updates calculated frequently if the threat
assessments and other analyses are to be valid.

The operators of active payloads usually use some kind of
cooperative tracking scheme to determine the orbits of their
satellites. Radio beacons, on-board sensors, laser
retroreflectors, on-board GPS receivers and other systems are
commonplace. They provide accurate orbits very economically.
Cooperative tracking also neatly identifies which satellite is
being tracked in a constellation of several objects, since each
payload can, for example, be assigned a unique frequency.

However, for space surveillance, non-cooperative tracking is
mandatory, for at least two reasons. First, one cannot assume
that enemy satellites will emit signals that are usable for
orbit determination by friendly forces. Second, 90 to 95 percent
of cataloged space objects are inert - dead payloads, empty
rocket bodies, a variety of objects deliberately jettisoned from
payloads and rocket bodies, and miscellaneous debris from
satellite explosions and other inadvertent causes. It is
essential to track and maintain current orbital elements for
every object that has a reasonable probability of detection,
including debris, if the catalog is to serve its purposes.

Non-cooperative tracking tends to be more expensive and less
accurate per observation than a cooperative method, especially
for radars, the mainstay of surveillance tracking. Moreover,
with non-cooperative tracking there is the problem of
determining from which object a given measurement originates.
This problem of data association is, in fact, one of the most
difficult aspects of multitarget, multisensor tracking. It is
often the limiting factor in overall system performance,
especially if the principal means of target identification is
only the positional tracking data themselves, the case in space
surveillance today.

In recent years, multitarget, multisensor problems other than
space surveillance have spawned a vast and sophisticated
literature. One of the first lessons of research related to
ballistic missile defense was that the processing needed for
data association and sensor coordination (command and control)
always increases exponentially with the number of objects being
tracked, and rapidly becomes unfeasible for any current
technology when the number of targets rises to a few thousand.

100 Fly Balls

There is an easily visualized baseball analogy. Imagine the
difficulties faced by 100 outfielders on the same field trying
to catch 100 simultaneous fly balls. Space surveillance is
actually easier than the outfielders' problem for a number of

A complete catalog has been maintained since the first satellite
was launched. Satellite motion can usually be predicted much
more accurately than the motion of other high-interest objects
such as missiles, aircraft and baseballs. Very few satellites
maneuver, split into several distinct satellites or try to evade
detection. The space environment provides a low-clutter
background against which to track most satellites, so that the
false-detection rate is extremely low. Large as it is, the
current catalog size is small enough to prevent frequent
confusion of targets by the sensors. Since cataloging was
started almost 40 years ago, computer and communication
capabilities have always grown faster than the catalog size.

The essential problem of space surveillance is data association.
The space object catalog is maintainable only to the extent that
sensor observations can be associated with the correct

All of the above factors combine to simplify the problem of data
association enough for straightforward processing techniques to
work. The current system does, in fact, depend critically on
these simplifying factors.

If any of the above fortuitious circumstances were to change for
the worse in the long term, we would have to both re-think the
current concept of operations and invest in major technical
improvements throughout the system.

Sometimes one or more of these circumstances does change
temporarily and in those cases we do face special difficulties
maintaining the catalog. For example, solar storms disturb the
upper atmosphere and temporarily degrade our ability to predict
near-Earth orbits accurately. On at least one occasion during
March 1989, the integrity of the catalog was endangered because
of unpredictable atmospheric perturbations affecting an
unprecedented number of orbits.

In another example, satellite break-ups may instantly increase
the trackable orbiting population by 500 or 600 objects. A
substantial fraction of the objects detected by a sensor may be
uncataloged and therefore unknown to any other sensors.

Once the break-up pieces have separated sufficiently, typically
after a few hours, we can begin to determine orbits for all
trackable pieces. It usually takes from a few days to several
weeks to find all the pieces, depending on the size of the

The Surveillance 'Fence'

The space surveillance sensor operated by NAVSPACECOM is unique
in the worldwide inventory of space tracking assets. It is
particularly valuable for catalog maintenance and high-altitude
unalerted detection.

The 'fence,' as it has come to be known, is a continuous-wave,
multistatic radar interferometer deployed along a great-circle
arc across the southern United States. Raw signals detected at
the receiver stations are sampled at an effective rate of 55 Hz
for up to one second as a satellite passes through the sensor
beam, and then forwarded in real time to NAVSPACECOM's
computational center at Dahlgren.

Although most satellites are in the near-Earth regime, the fence
routinely makes detections at ranges of more than 25,000
kilometers and occasionally at ranges of more than 40,000
kilometers. Well over 60 percent of the entire space catalog is
visible to the fence.

Objects in low-inclination orbits, or extremely small objects,
are essentially the only ones not routinely detected, and at
present there are more than 100 satellites that no other sensor
but the fence detects routinely.

Predictions based on the most recent element set for each
satellite are computed for 36 hours into the future every time
the element set is updated, or at least once per day. These
predictions are sorted in time order and merged with the
predictions for all other satellites in a single prediction
database. The fence data association then proceeds in time order
as the observations arrive.

Normally, at least 97 percent of the fence observations can be
associated automatically with known satellites. This is a
noticeably higher percentage than can presently be associated by
the other tracking sensors in USSPACECOM's Space Surveillance
Network, which makes the NAVSPACECOM combined fence-database
system an especially important asset for analyzing any
observation or track that has not been associated with a known

Known as 'uncorrelated targets,' or UCTs, these observations
must be disposed of by either associating them with a cataloged
element set or an existing analyst element set, or by creating a
new analyst element set from the UCT data.

Satellite break-up events present special challenges because
many unassociated observations and an unknown number of new
satellites are involved. Whenever the automatic system cannot
associate observations with known satellites, orbital analysts
must use special software tools and manual processing to make
the proper data association.

Recently, advanced computation and parallel processing
techniques have made it possible for analysts to consider many
more association hypotheses than in the past. Although the
analyst work is inherently slower than real-time, it is still
essential to make the correct data associations as soon as
possible. Human expertise built up through long experience has
always been indispensible for this analysis.

Near-Term Improvements

In order to improve our cataloging process, we must improve data
association. This, in turn, calls for better accuracy and
precision of observations, better accuracy of predictions and
faster processing to accommodate the growing catalog. Some
aspects of these problems are now being addressed in research
efforts at NAVSPACECOM.

The distributed computing system at NAVSPACECOM has the basic
computing capacity to use a fairly sophisticated special
perturbation (SP) numerical integration orbit model for catalog
maintenance. Most satellite programs use SP models for
operations because they are the most accurate methods of
prediction, though they are computation-intensive.

Space surveillance is one of the few space activities that still
relies mostly on general perturbation (GP) analytical models
which can execute much faster than SP models, though with
reduced prediction accuracies. Though NAVSPACECOM,s computer
network was not specifically designed for the extra processing
load of SP, it is a very capable system and efforts are underway
to maximize the network,s capacity for such computation.
Parallel processing seems to be the most practical and
economical way to achieve this.

For the past several years, NAVSPACECOM has been developing, in
collaboration with researchers at the Naval Research Laboratory,
parallel versions of three important space surveillance

* COMBO (Computation of Miss Between Objects) computes when
close approaches between selected satellites will occur. It has
several uses, though perhaps the most important is for collision
avoidance for the Space Shuttle. To date, during more than 500
days of on-orbit Shuttle operations, there have been seven
occasions when cataloged objects came within a 'warning zone'
extending 4 kilometers radial, 10 kilometers along-track and 4
kilometers cross-track, centered on the Shuttle. On four of
those occasions, the Shuttle has maneuvered to avoid the risk of

COMBO analysis is the means by which U.S. Space Command provides
notice to NASA sufficiently far in advance for the Shuttle to be
able to maneuver. The parallel COMBO algorithm successfully
demonstrated at NAVSPACECOM uses up to 23 medium-performance
workstations to analyze the list of all U.S. satellites of
military interest (198 satellites at this writing) for close
approaches to any cataloged object for a period of seven days,
in a total run time of well under an hour.

* SAD (Search and Determine) is used by NAVSPACECOM orbital
analysts to associate sparse UCT data that are widely separated
in time and to generate candidate element sets from the database
of UCT observations. The software works by generating candidate
element sets from every possible pair of UCT radar observations
and comparing each of these candidates against the entire
database of observations.

A parallel version for up to 23 workstations is now available to
NAVSPACECOM analysts and has been in use for several months. It
is capable of processing 30 days' worth of UCT data in about six
to 12 hours. However, some work remains to improve the
efficiency of the implementation and to integrate it into
routine operations.

* Parallel SP-based catalog maintenance is in development, and
initial demonstration runs on the NAVSPACECOM network have

Additionally, the problem of parallel database management to
support all these processes is receiving intense investigation.

Far-Term Improvements

In the more distant future, we will have to consider how
advanced computation - especially parallel processing - allows
us to re-engineer the whole catalog maintenance system. For
example, should the catalog have to include small debris, we
will need a more robust catalog maintenance process, and we will
need it before the catalog begins to grow rapidly, because of
the large numbers of UCTs that will have to be processed.

Ultimately it is desirable to be able to reconstitute the entire
catalog without any prior element sets or data associations. At
present and for the forseeable future, we have neither the
resources nor the know-how to do this task, meaning that the
satellite catalog should be considered a national treasure.

Rebuilding the catalog ab initio on demand in a reasonable
amount of time is the 'grand challenge' problem of space
surveillance. It is probably out of reach to solve this problem
in practical terms anytime soon, but by pursuing it we will
develop the more robust catalog maintenance process that we do
need soon.

In particular, we need detailed modeling and simulation of the
entire space cataloging operation in order to estimate system
sensitivities, risks, performance boundaries, failure modes and
relative merits of proposed improvements. High-fidelity
simulation of the whole system at both sensor level and
command-and-control level is probably the only way that a full
reconstitution capability can be developed and tested.

Author Dr. Paul Schumacher is technical advisor for Naval Space
Command's Logistics and Information Systems Division.

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