Your RF Cafe
February 1967 Electronics World[Table of Contents] People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Electronics World was published from May 1959 through December 1971. All copyrights are hereby acknowledged. See all Electronics World articles.
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 of an 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.
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.
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.
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.
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.
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.
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.
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 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 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.