Stealth fighters and LPI radar

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Insiders at times tell outsiders that all those public discussions about the merits of stealth fighters are ridiculous. Outsiders would simply lack the necessary insider information.

Well, on the one hand I'm an outsider, on the other hand I don't push mainstream stories about what stealth fighters can and would do in war. My position is therefore in between; I'm positive that the insiders weren't thinking of my stuff, but on the other hand I still don't know (much) insider stuff.


One piece of stealth fighters is their radar: Their whole low radar reflexivity is of little purpose if enemies detect them (or at least the direction to the fighter) by simply processing their radar signal.

The countermeasure of stealth fighter developers was the low probability of intercept (LPI) radar. There are apparently millions of stealth fighter fanbois in the world, and their reading of the "low" in LPI is quite often the phonetically similar "no". That doesn't change reality much, though: LPI radars are not NPI radars.

I'll use info from the popular book (well, popular in an electronic warfare context) "EW 102 - A Second Course in Electronic Warfare" to shed some light on LPI radars.

A number of measures can be taken to make a radar less subject to detection. One is to make the signal so weak that the ESM cannot receive it. This is difficult for the radar because the radar must receive enough energy after the round trip to the target (...) to detect the target. The receiver encounters only a one-way path loss (...).
A second way is to narrow the radar beam (thus increasing the antenna gain) or to suppress antenna side lobes. This makes it more difficult for a receiver not located at the target to intercept the signal, but does not impact a receiver located on the target.
A third way to reduce the interceptability of a radar relative to its performance is to give the radar a processing gain not available to the ESM receiver.

None of these approaches is magically enough to make a LPI radar invisible to modern ESM (electronic support measures; in this case a radar warning receiver, passive radar) tools.
(1) is quite impractical, (2) has an obviously limited growth potential and no real effect on the target's ESM and (3) is obviously an approach that depends on the modernity of the tool. A fine system with approach (3) can be great when it enters service and ineffective against modern ESM just a couple years later.

Radars can be thought of as having three levels of LPI:
• The radar is easily detectable but not easily identifiable—called an LPID radar (...).
• The radar can detect a target and is not detectable by an ESM receiver at the same range but outside its main beam (...).
• The radar can detect a target and is not detectable by an ESM receiver located on the target—a “quiet radar” (...).

Only the third - and most demanding - grade is what stealth fighter fanbois seem to think about when they read "LPI radar".

The math and technical details about LPI mentioned in the book aren't really useful for most readers, so let's just sum up:

In all cases, the radars’ level of LPI is described in terms of range detection ratio with various engagement parameters specified (for example the target cross section). They are also described in terms of “warning time” which is the time between the detection of the radar by a hostile receiver carried by a target and the detection of that target by the radar. Again, engagement parameters must be specified (e.g., target approach speed and radar cross section and the type of receiver employed).

It's a much, much more complicated thing than "an undetectable radar beam".
LPI radars are just like many other combat aviation systems - subject to a competition. They may have (had) a supreme time, but any such advantage is fleeting. It's no silver bullet technology, instead it's adding complexity to the high-tech competition in air war. An opponent with an adequate technology base can defeat the "LPI" characteristic, and this gets more and more easy as LPI radars become older and wait for more advanced upgrades or replacements.

Again, I don't trust any assertion that technology provides a huge and robust advantage. The historical pattern predicts rather a competition between action and countermeasure at ever growing complexity.
High-profile, high-cost advantages don't last for long.. Only small, underestimated or culture-specific improvements provide a lasting advantage. LPI radars are just tools that provoke an effective countermeasure.

S O

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