February 1967 Electronics World
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
They 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
AN/FPS-16 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.
Radar Signature Analysis
By 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.
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.
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
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
Fig. 1. Since the radar beam is much wider
than the target, exact shape and size of target may be unresolved.
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,
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 target measurements.
Although the 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.
Fig. 2. A compound body along with its radar
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 craft.
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.
The target end of Bunker-Ramo Corp.'s microwave
anechoic chamber. Foam plastic pyramids lining chamber absorb
Signature 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 strip-chart.
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 target.
Satellites such as Mariner IV have a more
complicated radar signature on account of the solar-cell paddles
that are used.
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.
Fig. 3. Typical patterns produced by symmetrical
Fig. 4. Patterns produced by rotating plate
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 more difficult.
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.
Thus 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
By using appropriate formulas and by counting the number
of lobes in one period, one can find the length and radius of
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
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.