The Poor Man's Stealth Detector


Details of a formerly secret project to defend Swedish airspace against stealthy cruise missiles using a radical but inexpensive radar system were revealed at a conference in Oslo this week. The Associative Aperture Synthesis Radar (AASR) was approaching the hardware-test stage when it was cancelled in 2000 after eight years of work -- because there was no imminent cruise-missile threat any more. It has only recently been declassified and this was one of the first open, formal briefings on the project.

The AASR was designed to take advantage of the principle that a target's bistatic radar cross section -- where the radar receiver and transmitter are in different places -- may be affected minimally or not at all by stealth measures aimed at conventional radars. In particular, it exploits the "shadow" RCS behind the target, which depends entirely on the target's geometrical cross-section. The radar was also designed to operate in the UHF band where radar absorbent material (RAM) is less effective.

Developer Hans Hellsten of Saab Microwave Systems told the conference that the AASR used a number of novel techniques. Each transmitter would transmit on stepped frequencies so that receivers could tell where a signal came from. This made it possible to determine the length of the signal path, so that if a signal was picked up at several nodes it was possible to determine the target's location precisely.

One disadvantage: the transmitter and receiver had to be on opposite sides of the target, so it could not be detected until it had entered the defended airspace. To get around that problem and still intercept targets in a timely manner, Swedish planners expected to exploit the system's accuracy -- it could locate targets within 1.5 m -- and command-guide a high-speed missile on to the target.

But because the system used range rather than bearing to locate its targets, the antennas did not need to have accurate bearing resolution. Also, the system's use of UHF, its independence from target RCS and the fact that bistatic systems have long pulse times meant that the necessary power was modest.

The result was a price that caused sharp intakes of breath among the delegates. Each of the 900 nodes was expected to cost no more than 1 million Swedish kroner (about $156,000) and the entire system would be in the 1 billion kroner ($156 million) realm -- pretty much chickenfeed by defense standards.

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-- Christian

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