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US Army Developing Encrypted Radar Waveform (thestack.com)
An anonymous reader writes: The U.S. army is working on an innovative technology for masking radar emissions in contested territory and environments with heavily congested radio bands. Effective radar system performance is critical in military operations, yet remains a challenge in locations under attack or in areas of high traffic density. Army researchers have now developed a noise-encrypted radar waveform called Advanced Pulse Compression Noise (APCN), which can be tuned in real-time to allow users to adjust radar performance depending on their surroundings. Research scientist, Mark Govoni explained: 'Having the ability to transmit a radar waveform that's continually changing, one that never repeats itself, and looks like noise, is extremely difficult to intercept....and remains anonymous to radar detectors.'
It's been a long time since I was involved with Army radar and encrypted communications (in my case, merely humble air traffic control equipment), but the article intrigued me enough to do a very quick lookup. This article isn't very technical, but I can see how it's not simple spread-spectrum radio.
Remember that the information conveyed by radar microwaves is limited; we're primarily interested in reflections (this is "primary radar"; "secondary" radar actually does transmit information; IFF is a type of secondary radar). For a simple radar we know the radar echoes are ours because they come back to our own dish, and match the frequency that we transmitted. They're also incredibly easy to jam.
Frequency hopping on its own makes things harder to jam because the frequencies change in a cryptographic pattern. They can still be jammed if your broadcast a lot of noise over the entire spectrum, but then you limit your own communications. If you can detect the point source, though, you can broadcast a point source over the entire spectrum and still jam them.
What I think I understand about this is that it’s not merely frequency hopping, but the signal modulation is encrypted in a way to evade detection. With a receiver I can detect a typical radar’s 3.4 Ghz signal at -200db (numbers are made up), even if spread across the spectrum, because I know what a 3.4 GHz square wave looks like against the background noise, even if it only appears intermittently on the narrow frequency I’m scanning.
I could try to modulate the signal a different way; maybe a sawtooth, maybe a sine, but a repeating, predictable signal is observable, even with frequency hopping. However if I broadcast noise (and my receiver knows the noise’ pattern), then any listening equipment shouldn’t be able to pick out my microwave pattern from the background.
--Jim (me)
Will this avoid lock? Probably not. I would imagine that attackers will just lock onto maximum noise rather than maximum signal. May make attacking radars harder though. And I suppose standing near an arcing power line in a war zone might be a bad idea if this becomes common.
BTW, are these things going to play hell with other radio communications?
Won't it make identification problematic though? The big misconception about RW (Radar Warning) sensors is that it they only identify hostile radiation sources when in reality you want to know exactly what is lighting you up even if it is friendly. This has led to some unfortunate incidents. During the Iran-Iraq war the Iranians actually lost some F-14s to Iraqi fighters because the Iranian F-14s had a standard NATO RW unit that did not register the radars of Iraqi Mirage fighters and their Super 530 BVR missiles as a threat. In Europe this was OK since French fighters were not a threat but in the gulf, not so much. I expect the Iranians quickly figured out to change the threat classification of French radar signatures to 'Hostile'. The consequence is that firstly, once all radiation sources on the battle field look like noise, all you'll be able to tell after that situation becomes the norm, is that you are being lit up by an unusually strong source of radio noise. You won't be able to tell if it is friendly or not. Secondly I expect the current crop of anti radiation missile can be fired at a radar source and then lock onto another one if the primary target goes dead or the missile gets confused. For that purpose it would have to do some form of IFF, presumably based on the output of some derivative of a bog standard aircraft RW sensor, like those Iranian F-14s had, so that it does not accidentally choose a friendly radiation source when it picks it's alternate target so if radiation sources, friendly or hostile, all look like noise and the AR missile just gets locked onto strong sources of noise it would be unable to identify the operator of any alternate radiation source and thus unable to choose an alternate target without risking a blue-on-blue incident. I expect that the ROE for engaging radiation sources would be tightened up pretty severely, especially when firing AR missiles at AWAC aircraft.