Monday, December 10, 2007

Stealth-detecting bistatic radar is back in the news

Posted by John Keller

I can remember back in the mid '80s when rumors first started circulating widely in print of a secret military aircraft under development that would be invisible to radar. During those Cold War days that meant that U.S. and allied aircraft might be able to have unlimited access to airspace over the Soviet Union.

Those rumors eventually morphed into the official announcement of the F-117 stealth fighter. Radar-evading technology developed for the F-117 was applied to the B-2 stealth bomber, but experts speculated that if we had this radar-evading technology, then our adversaries would have it, too.

With that realization came feverish discussion of how to defend against stealth technology. The discussion quickly included not only stealthy manned aircraft, but also low-flying cruise missiles that radar had a hard time picking up.

One of the counter-stealth technologies then was called hybrid bistatic radar, which eventually came to be known simply as bistatic radar -- a scheme where the radar transmitters and detectors were in separate locations.

Bill Sweetman wrote on the Aviation Week defense technology blog this month of a formerly secret project to defend Swedish airspace against stealthy cruise missiles with a bistatic radar approach called the Associative Aperture Synthesis Radar (AASR).

The AASR program, Sweetman wrote, was cancelled in 2000 because at the time experts no longer considered cruise missiles a threat. U.S. bistatic radar and other anti-cruise-missile programs have fallen by the wayside, as well, as research dollars have shifted to other priorities.

A similar casualty of post-Cold-War defense priorities was the so-called Over The Horizon-Backscatter radar system, or OTH-B. This experimental system was to bounce radar waves off the ionosphere -- just like shortwave radio signals do -- not only to extend radar defenses against cruise missile threats, but also to help radar pick up low-flying cruise missiles in ground clutter.

I always thought bistatic radar was a good and obvious idea, and not only for counter-stealth. Conventional radar is a double-edged sword; it can detect the enemy, but also gives away the location of the transmitter. It's like turning on a flashlight in dark woods. Everything in the vicinity knows you're there.

Not so with bistatic radar. If defense researchers could use it to make and deploy a practical system, then perhaps stealth fighters could use radar and still remain stealthy.

If bistatic radar research is going on, it's being done quietly. Depending on how military threats emerge in the future, I think we'll hear of this technology again.

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