Electronics: Signatures in the Sky

Though the Russians reveal no details about their satellites before they are launched — and precious few afterward — they cannot keep their secrets for very long. Soon after a Soviet space craft has gone into orbit, U.S. Air Force scientists not only record its speed and plot its orbit but determine its size and shape and often deduce its mission. Their spatial detective work is made possible by radar signature analysis (RSA), a little-known technique that may some day be used to save the U.S. from a sneak attack.

With RSA, scientists can reconstruct the characteristics of a foreign satellite from the pattern of radar pulses it reflects back to a tracking station. Largely by measuring the amplitude, or strength, of the reflected pulses, they can calculate the satellite’s size; by analyzing the variations in pulse amplitude caused by the satellite’s rotation or merely by its passage across the sky, they can determine its shape with remarkable precision. By determining the time it takes the pulse pattern to repeat itself, they can learn how fast the distant space craft is tumbling, rolling or spinning around its axes.

Space Graphology. For such satellites as the U.S. Geminis or Agenas —or, indeed, for intercontinental missiles — their shapes are a dead giveaway. When, for example, the conical nose of a tumbling projectile-like satellite is pointed directly at a ground radar station (see diagram), the radar “sees” only a small cross section; the reflected pulse is scattered in all directions, and the radar reading is relatively weak. As the projectile begins to swing broadside to the radar, however, its radar cross section increases; reflections become stronger. When the satellite’s flat rear surface turns to face the radar antenna, the reflected pulses become even more intense. The changing pattern that forms on the radar screen is unique for that tumbling projectile, a distinctive signature that scientists can use to identify and describe it—much as a graphologist uses handwriting to identify the man who handled the pen.

Once the radar signature of a U.S. satellite has been determined, it is relatively easy to detect changes in the spacecraft’s known configuration. In June, RSA was employed to discover which of four solar panels on a secret Air Force satellite had not flopped into place. When telemetry failed to confirm that a boom on a gravity gradient satellite had extended, RSA recognized a change in the radar pattern that proved the boom had stretched into place. A study of the radar echoes reflected from the first Nimbus weather satellite provided tumble and spin data that were unavailable from telemetry.

Analyzing Soviet satellites is more of a test. “You haven’t the foggiest notion of what they look like when you begin,” explains Electrical Engineer Charles Brindley, head of Radio Corp. of America’s RSA research program. Despite the difficulties, an RCA scientist managed to use radar signature analysis as early as 1958 to describe Sputnik 2. When the Russians finally displayed a model of the satellite, it was confirmed that the sketch was remarkably accurate. It even included Sputnik’s special radar reflectors—which led the U.S. to the conclusion that the Soviet tracking network included many low-power World War II radars. Refinement of RSA equipment and technique now allows analysts to make considerably more sophisticated, but highly classified, conclusions about Soviet spacecraft and intercontinental ballistic missiles.

Cones, Cylinders & Spheres. So many satellite analyses have been made that RCA is now compiling a catalog of the radar signatures of known satellites. Scientists working for the Air Force have also been bouncing radar signals off an assortment of complex, spacecraftlike shapes on test ranges to establish their characteristic radar patterns. Even more important, they are deriving a series of mathematical formulas that match radar signatures to specific satellite shapes. These, they hope, can some day be used to program a computer to recognize and identify radar signatures more quickly than human operators. Their work is proceeding slowly. “Once you get past a few simple shapes like cones, cylinders and spheres,” says Brindley, “the mathematical analysis goes to hell and becomes incredibly difficult.”

Despite the math problems, the Air Force is determined to develop computerized RSA, or at least a combination of human analysts and computers, as quickly as possible. In the event of a nuclear war there would be little time for human analysts to leaf through a radar signature catalog in an effort to differentiate between an incoming ICBM warhead and its decoys. Only a computer could spot the authentic warhead radar signature quickly enough to order its interception and destruction by defending missiles.