German radar vendor says it tracked the F-35 jet in 2018

Discuss the F-35 Lightning II
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Unread post08 Oct 2019, 13:17

usnvo wrote: Frequency matters, at 10GHZ the wavelength is 3cm, at 2.5GHZ it is 12cm, but at 200MHZ it is 1.5m!

Yes, if the wave is longer than the width of the 'mirror' (reflector surface) then the wave is not going to reflect or interact as well from it. It will still reflect and interact some with the reflector but the reflected gain drops away as the wave gets longer than the object doing the reflecting.

For instance, a tuned resonant full-wavelength (1 meter long) receiver antenna gathers more 1m wavelength gain (for the receiver to amplify) than does a smaller tuned resonant 1/2 wave dipole 1m antenna (0.5m long), but a 1/2 wave antenna can still gather more reflected gain energy than a tuned resonant 1/4 wavelength 1m dipole (0.25m long), etc.

A similar decrease in gain reflected, will likewise affect the ability of increasingly longer waves to reflect off an object. The wave reflection interactions will become lower in gain as the wave used gets physically larger than the size of the reflector.

Thus as the wavelength gets larger than the reflector more illumination gain is required to get sufficient reflected energy for it to be detected at useful ranges.

So ...

UHF would have the best passive LOS reflection range

VHF would have relatively degraded passive LOS reflection range

HF should exhibit nearly useless passive LOS reflection range, unless HF were scattered off a very powerful distant first-bounce OTHR source, that is.

Now, if you look at the physical size of the HensoldT antennas, and their mix, the company is clearly aiming for contacts within the uppermost VHF and lower-most UHF bands. They apparently think this area offers the best mix of transmitters and the best potential to reflect off the size of the objects they want to track. Plus you cans see they covered all polarizations, and have omni-directional gain coverage with that antenna design. It's basically just a radio direction-finder system that detects reflectors, instead of emitters.

I personally don't doubt that a distributed network of these will detect reflections off aircraft in ideal conditions. And it should not be dismissed as a potentially very useful tool, but a stealth aircraft defeating mechanism it isn't.

It may not be militarily useful in a fight (for several reasons already mentioned) but it will be a tactically useful technology for passive surveillance in conditions short of actual air battle, and more importantly, to help deter and complicate stealthy intrusion, and to detect and quasi-'track' such intrusions. So it isn't without tactical consequences.

In fact this seems to be its biggest claim to fame, that it can track a stealth aircraft without the aircraft's avionics knowing anything about it, or being able to predict the range of detection or to tell the pilot (or drone or a missile) where to fly to avoid the threshold of detection. So is this a proof-of-concept or an ad campaign?

Would you want that sensor as a part of your EW capability prior to a conflict? Or during peace-time?

Yup, it's small, it's cheap, it's easy to setup, generally no one will know where it is, and it can not be detected, hunted, or kinetically killed-off easily. It could turn out to be a desirable EW and monitoring sensor.

For example: Let's say your border was the border of Saudi Arabia and southern Kuwait, and you have positioned a kilowatt range FM radio repeater every 5 km along your border and a HensoldT passive antenna every 2.5 kilometer apart, all along that border fence's length. I dare say you could have and probably would have passively detected a series of low flying cruise missiles stealthily crossing that border at around 3 AM on the 14th of September 2019. That could constitute a very useful civil band EW trip-wire detector, if you can devise a cheap national FM radio band repeater package to go with it to get coverage where military grade radar can't affordably provide full national border coverage.

You'd certainly be a lot more aware of activity around your border area (smuggling drugs, money or weapons with drones for instance).

There's real potential to leverage this technology for particular limited purposes, and it does erode stealthiness to some extent, so as much as I doubt its hype in defeating a VLO jet force any time soon, it was a good advertising and awareness demonstration. Yes, they had to correlate it with ADS-B and 2D radar return to define the reflected energy 'track', but they said their correlation was detectable for 150km, which means an improved antenna, better signal amplifiers and better processing may resolve this 'correlation' (presumably statistical), into something less touchy-feely, ambiguous or hyped.

A potential is there, maybe not for combat, but the approach still bears paying close attention too, as a potentially useful cheap detection means, IMO.
Accel + Alt + VLO + DAS + MDF + Radial Distance = LIFE . . . Always choose Stealth


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Unread post08 Oct 2019, 19:53

ricnunes wrote:
Ok, your explanation does indeed make sense. Thanks!

But I couldn't help to notice that the example that you gave - Chaff - is "detached" from the aircraft but Luneburg lens aren't and thus they are a "continuity" of the aircraft itself (of some sorts). Shouldn't this change somehow the facts above?

Another question: What's the size of a Luneburg lens fitted on the F-35?

I am not sure how big the lenses are on the F-35s reflectors or even there orientation. Even if I did know, I probably wouldn't be able to say. From the pictures, I would assume there is a row of Luneburg Lenses oriented at various angles to cover all angles. Civilian and Military ATC radars operate at 2.7-2.9MHZ, but I would guess they are somewhat larger than required for that frequency to allow them to be effective radar reflectors for long range air surveillance radars.

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