Stealth fighters and LPI radar

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 signal 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.

Sven Ortmann


  1. Not fully on topic, but isn't the usual tactic to use an awacs as radar and have the fighters shut off their radar system to use only esm and basically act as missile launcher? With this system a stealth plane has its advantages.

  2. Well, there's a reason why the Air Force thought it necessary to spend much on a good radar.

    There are dozens of problems associated with a reliance on AEW&C, and dozens of arguments for a fighter radar. In the end, there's a possibility to do what you wrote, but it's not a robust option.

    The German Cold War Lampyridae concept was based on such an approach, but canceled. You wouldn't need a large and expensive aircraft if you could rely often on it being merely a flying missile launcher.

  3. Sven,

    The countermeasures from stealth fighter developers isn't just LPI radars.

    Net-centric operations and passive detection systems are also tools that can be used to preserve stealth's advantages.

    Using net-centric tactics, one flight of Raptors could remain passive and stealthy, while receiving LPI datalink updates from another flight of Raptors who activate their radars.

    Passive ESM systems on stealth aircraft can permit them to wait for the optimal time to turn on their radars - essentially when it's too late for their opponent to do anything about it.

  4. The F-22 project had serious problems with establishing active datalinks at all. That capability wasn't integrated into at least the first production aircraft.

    A datalink from one F-22 to another is only under certain assumptions of use.
    The one with active radar needs to be out of weapons range from the enemy (or else it could shoot itself or get shot).
    That condition is difficult to meet if you need to fly almost head-on to the enemy to have him in your radar's relatively small-angle 'cone'. Now add in the factor of super-cruise and it becomes quite questionable whether this capability is regularly useful.
    The second, 'silent' F-22 needs to be closer or at similar range to make good use of the data, after all.
    You would need to have a multi-directional trap to fully exploit your plan - and that can be had without stealth as well due to the limited fov of enemy fighter radars.

    Overall, data links between F-22's sound more like a security against surprises (flying in a chain formation, for example) than like a tactical advantage for a sneak shot.

    Btw, simple datalinks date back to 1960's automated interceptor systems (crude forms date back to WW2!) and don't deserve to be called "net-centric".

    Passive ESM systems on stealth fighters may do what you expect them to do, but guess what; there's a similar concept built around air/air IRST such as on Russian (T-10 family, MiG-29) and European (Gripen, Typhoon, Rafale) fighters - the F-22 is missing such a system.

  5. APG-77 is, by most accounts, the most powerful radar yet installed on a fighter aircraft, and should be able to pick out any non-VLO target at very long range, especially if it is queued by AWACS.

    Fighter-to-fighter, net-centric datalinks, especially on stealth aircraft, are certainly still in their infancy. With luck (and funding) the F-22/F-35 datalinks will grow in sophistication and capability over time.

    Just because the F-22 can supercruise doesn't mean it has to. The "looker" Raptors could very well sit back at idle speeds to locate targets.

    Plus, even if it is too close, and someone takes a long-ranged missile shot via ESM, the Raptor just has to shut its radar off, turn, and supercruise out of the missile's sensor cone.

    I'm still skeptical of the value of IR sensors as wide area detectors. I can see their value for localizing passive ESM hits, but can they really search the volume needed in all weather conditions to be a reliable counter to stealth? Unfortunately, I just don't know enough about the state of the art in these systems.

  6. "APG-77 is, by most accounts, the most powerful radar yet installed on a fighter aircraft, and should be able to pick out any non-VLO target at very long range, especially if it is queued by AWACS."

    It's unlikely to be in a LPI mode if it exploits its emission power, though. That does effectively mean that both would likely see each other.
    And there's still the possibility of successful jamming, even against a powerful radar (that's actually mid-90's vintage; do you recall mid-90's computer tech?).

    "The "looker" Raptors could very well sit back at idle speeds to locate targets."

    That's quite risky, as it wouldn't be able to react much to any surprising problems while flying slowly.

    IRST is a tool for target identification and for sneaking up on enemies. It's stealth by no emission instead of stealth by little reflection. I mentioned it because it comes really into play against enemies who use passive radar only (as in your example).

  7. Sven,

    My money's still on the active Raptor "looker" loitering at 50k+ft, with its best in class performance, to get out of most trouble.

    APG-77V(1) uses the T/R modules from the F-35 program, so it's not just 90's technology.

  8. "Plus, even if it is too close, and someone takes a long-ranged missile shot via ESM, the Raptor just has to shut its radar off, turn, and supercruise out of the missile's sensor cone." - B.Smitty

    That is assuming the pilot detects the missile in time; anti-radiation missiles are passive, meaning that you won't get early warning from RWR-systems. Any detection of the missile will be either from the onboard IR systems or by the pilot himself, visually. And that still requires the appropriate course of action, IE shutting off the radar. If the radar is active, spoofing an ARM is much more difficult.

    I fully agree with Sven's post here; the F-22 is no 'silver bullet' at all. Throwing money and technology at the problem is in direct contradiction to the now accepted theories of John Boyd. He was still alive when the F-22 first flew, and according to those who knew him well, hated it with a passion.

    I hope it isn't a problem that I'm responding to a fairly old post. I read this blog regularly, and I think it's one of the most interesting on the web, let alone in this subject.

  9. A small correction to make; Boyd died a few months before the maiden flight of the F-22, but he was apparently familiar with the aircraft. Mea culpa.

    1. http://en.wikipedia.org/wiki/Lockheed_YF-22

    2. Ah, I hadn't thought of that. Thanks for the heads-up, Sven!


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