Radar Signature Analysis
February 1967 Electronics World
didn't know how good they had it in 1967. The story talks about the
nuisance of having to sift through "hundreds" of satellites, old rockets,
and assorted space junk" in order to search for and track potentially
threatening objects in orbit around the Earth. We're into the millions
of objects in 2012, and the potential threats are infinitely larger.
The article mentions the use of an
radar operating in C-band to detect and measure the returns and then
the results were analyzed in an attempt to determine the character of
the object. Open air test sites and anechoic chambers were used to measure
the radar cross section and characteristic signature of many shapes
to populate a database of recognizable returns that would help to determine
whether the space object was friend or foe.
February 1967 Electronics World
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Radar Signature AnalysisBy Edward A. Lacy
Every satellite and missile
produces a distinctive pattern of radar echoes. These can be employed
to deduce satellite size, shape, as well as motion.
When our satellite-tracking radars detect a new foreign space vehicle,
it surely must cause some worrisome moments for our intelligence experts.
For, after all, such a satellite could be anything from a harmless scientific
experiment to a surveillance vehicle or, worse yet, a satellite equipped
with a nuclear or biological warhead.
Typical of the radars used for signature analysis is this advanced
projects terminal measurements radar built by Raytheon for the White
Sands Missile Range in New Mexico.
With hundreds of
satellites, old rockets, and assorted space junk now in orbit and with
many of them passing over the continental United States, it has become
important to our military peace of mind to know the origin, capabilities,
and intentions of each of these objects. To determine this, the Air
Force is building a surveillance system to detect, track, identify,
and catalogue all objects in space on their first orbit.
Lest this sound like a simple matter, it should be noted that until
recent years it just was not possible for us to determine much, if anything,
about such objects. Of course, our radars told us the altitude, range,
and velocity of a given satellite, but even with the most precise radars
it was not possible to "paint" a picture of the satellite, that is,
resolve the target in angle. As shown in Fig. 1, a radar's beam is ordinarily
too wide to give any indication of the shape of a satellite which is
an important factor in determining its mission or intent. At a distance
of 100 nautical miles from the antenna, for instance, a radar beam may
be a mile or more wide. To use such a beam to paint a silhouette of
an object only a few feet in diameter is like trying to fill in a "paint
by number" drawing with a 6-inch brush.
While much of
this information is still "classified" by the military, enough has been
released to indicate how a new technique, called "signature analysis"
- a remarkable bit of engineering detective work, is being used to determine
satellite size, shape, and motion.
Over-all view of the control house at the Air Force radar target
scalier site (Rat Scat). The two antennas at the right are 10-ft
dishes, while two at the left are 6-ft dishes. These antennas are
elevated on individual tracks when they are being used for radar
Fig. 1. Since the radar beam is much wider than the target, exact
shape and size of target may be unresolved.
Fig. 2. A compound body along with its radar signature.
Fig. 3. Typical patterns produced by symmetrical bodies
Fig. 4. Patterns produced by rotating plate and cone.
The target end of Bunker-Ramo Corp.'s microwave anechoic chamber.
Foam plastic pyramids lining chamber absorb radiation.
Satellites such as Mariner IV have a more complicated radar signature
on account of the solar-cell paddles that are used.
new system hasn't been refined to that extent as yet, it is almost as
if each satellite or reentry body has its own radar "fingerprint", which
is a plot of the signal strength of the radar echo (as recorded in the
automatic gain control circuit) versus time. In this technique, plots
or signatures of the target echo are broken down into patterns that
represent the returns from objects of known shape. These shapes are
then put back together to define the complete shape of the satellite
- whether it is a cone, cylinder, sphere, or some combination of these
shapes (Fig. 2). Using other techniques of signature analysis, it is
then possible to determine the size of the satellite, its orientation
if it is not tumbling, and its tumble rate if it is tumbling.
Knowing these characteristics of the satellite, the analyst may then
be able to determine the satellite's intended mission. For example,
if the satellite is always oriented toward the earth as it passes over
us, then it could very possibly be a surveillance satellite. Particular
shapes are optimum for certain types of sensors used on surveillance
satellites. On the other hand, extreme altitudes would indicate that
the satellite probably is not spying on us. By using this information
and making deductions, we can obtain a pretty good description of the
Although plots of aircraft radar echoes have been
available for several years, it should be noted that signature analysis
really began only in 1958. In that year D. Barton of RCA was able to
deduce the contours of Sputnik 2 from the plots of echoes received on
an AN/FPS-16 radar. By this process it was shown that complex patterns
of radar returns could be resolved into combinations of returns representing
simpler shapes and then put back together to indicate the original shape.
In the RCA publication, "An Introduction to Target Recognition", from
which much of the information in this article was derived, Charles Brindley
reveals many of the techniques used in signature analysis.
analysis is based on radar cross-section: predicting it, measuring it,
recording it, and recognizing it. Radar cross-section is simply the
size of an object as it appears to a radar, irrespective of its actual
size. While there is no simple relationship between radar cross-section
and actual size, generally the larger the object, the greater its radar
cross-section or reflectivity.
Obviously, the greater an object's
cross-section, the easier it will be for the radar to see it. Conversely,
the smaller the cross-section, the harder it is for the radar to acquire.
The enemy takes advantage of this by shaping reentry bodies so as to
reduce their radar cross-section and by coating the vehicles with a
radar-absorbing material. Radar cross-section depends on radar frequency,
the angle at which the beam strikes the target, and the polarization
of the signal.
To obtain laboratory cross-section data
of actual satellites and other objects is a difficult matter: it is
hard to maneuver the satellite into known aspects, satellites are expensive,
and it is hard to repeat measurements. These difficulties have lead
to the development of test ranges, both indoors and out, for plotting
the cross-sections of various objects at rest.
In the indoor
test range, called radar or microwave anechoic chambers, scale models
of various shapes and sizes are observed with radars which are scaled
down in size and up in frequency. Special radar absorbing materials
are placed on the walls of the chamber to prevent unwanted reflections.
The scale-model test object is placed on a turntable so that various
aspect (viewing) angles may be obtained. The radar signal is bounced
off the object and the signal strength of the echo is recorded on a
Anechoic chambers have the advantage of being
immune to bad weather: you can use them when it is raining, something
you can't do with outdoor ranges since the rain absorbs too much of
the signal at the frequencies used on the model ranges. Such chambers,
though, can be an expensive proposition when waveguides and models are
built to small scale.
With outdoor test ranges the models do
not have to be nearly as small. Avco has a test range where 2500-pound
models can be suspended up to 300 feet in the air. At the radar target
scatter site (called "Rat Scat") near Holloman Air Force Base, New Mexico,
static cross-section measurements can be made on objects weighing up
to 8000 pounds at frequencies from 100 to 12,000 MHz. On outdoor ranges
such as these, special care must be taken to eliminate or discount the
return from the tower or other supporting structures on which the target
is placed since the tower may have a greater cross-section than the
Various Types of Signatures
Now let us consider the various types of signatures or returns which
we obtain for bodies of various shapes, based on test range measurements.
Figs. 3 and 4 show the returns for a sphere, cone, cylinder, and other
shapes. The returns shown are for rotating bodies at a fixed position:
the lobes may vary in width and number for moving bodies.
Since a sphere looks like a sphere no matter how you view it, its radar
cross-section will be a constant level with no variation because of
different aspect angles. The cone and cylinder have more complicated
returns because the strength of the echo will depend on the angle or
aspect at which the beam strikes the object. By the use of certain approximations,
most symmetrical bodies can be considered to be made up of combinations
of these basic shapes. If the satellite is not symmetrical (for example,
if it has solar cells mounted on paddles), the analysis problem becomes
In either a test range radar or an operational
radar, the target signature may be obtained from the automatic gain
control circuit or the video circuits of the radar receiver. The recording
of the a.g.c. voltage versus time is usually made with an analog strip-chart
recorder. While this technique gives a good indication of the average
strength of the return signal, it is being replaced at many stations
by a video tape recorder which furnishes much more information since
it records on a pulse-by-pulse basis.
Using the recording of
an operational radar and knowing the characteristic returns of certain
bodies obtained with a test range radar, the signature analyst can be
expected to come up with a reasonable approximation of the unknown body,
provided that both radars were looking at the target at the same angle.
Now that we've established the shape of the satellite, let's
consider its motion. While the more sophisticated satellites will be
stabilized, it is possible for the satellite to be tumbling which would
indicate either a failure of the satellite to perform as programmed
or a lack of engineering ability on the part of the designers and builders.
In either case, it is important to know if the satellite is tumbling
and, if it is, the tumble rate. This can be determined by observing
periodic repetitions of the same cross-section pattern.
far we have considered the cross-section just as a pattern and have
ignored units of measurement. To use cross-section to determine actual
size of a satellite, it is necessary to calibrate the radar, One method
that has been used is to track a 6-inch sphere suspended below a balloon
and then calibrate the relationship of the radar cross-section and the
radar return accordingly.
By using appropriate formulas and
by counting the number of lobes in one period, one can find the length
and radius of the object.
Reentry Body Study
Besides satellite target recognition, signature analysis is
being used to study reentry bodies. When a reentry vehicle enters the
atmosphere, the shock waves formed by the vehicle cause a plasma sheath
- a concentrated layer of electrons-to be formed on the vehicle. The
plasma sheath has a drastic effect on the cross-section - the reentry
vehicle may show a significant increase or decrease in cross-section
compared to its cross-section as measured in free space. The ionized
field - called the "wake" - which trails behind the vehicle will show
similar effects. Using signature analysis, we can then determine how
the vehicle is being affected by reentry.
In the military
area, anti-radar signature devices (decoys) use built-in electronics
to reshape the returning radar echoes so that a small, inexpensive decoy
can "look" like a larger reentering vehicle.
A more important
use of signature analysis is to be able to determine which reentry bodies
are enemy war-heads and which are merely decoys in the mass of reentry
bodies. Since the warhead must be identified in time for us to take
defensive action, a computer is necessary. A computer, however, tends
to take things too literally: if a return differs only slightly from
the description which was given to it, the computer will not recognize
the object. But with new computers which are capable of learning and
with improved optical techniques for pattern recognition, perhaps this
problem is closer to solution.