and ?

and another example from the last page
That should be pretty clear, as the basics are rather simple. It’s the practical part that’s complex, since you need to do it in real time and adapt the jamming in power and direction (sending back accurate signal so to cancel your aircraft’s signature from that particular angle, not to speak if you’re manouvering, as you’d need to adapt your jamming as your radar “signature” varies while your aircraft turns).

bet you wont find this in a decent link

you need links to get what I’m talking about? ok, let’s see:

you could start around here:

http://en.wikipedia.org/wiki/Radar_cross-section

just to get an idea about the fact that your radar signature isn’t the same depending on the angle the radar waves hit your aircraft (very basic thing in fact, since an aircraft is of complex shape), if you need more explanation, then this discussion is already too complicated for you.

now let’s try to think a bit (you know? using the thing named “brain” in your head), you want to replicate the wave that’s echoed by your aircraft out of phase to cancel it (basics here: http://en.wikipedia.org/wiki/Active_noise_control as it works for any waveform) To do so, you must be able to replicate the “image” (wave intensity ciombined by eventual deformation through doppler effect and parasitic reflections between different parts of your airframe) your aircraft will send back to the emitter (same waveforms, but inverted – half wavelength out of phase, which is school grade physics).

As you turn, you don’t present your aircraft under the same angle to the emitting radar (you get the idea?), so, what do you do to keep jamming (think?)? you adapt the signal to what you aircraft will reflect under the angles you present to the radar.

Now, what’s complicated in that (conceptually speaking)?

It’s just the basics which, if you have them, make easy to understand the whole idea. Of course, if you don’t understand squat, you don’t get it and try to hide in some nice speech talking about pointless things like “near noise level emissions” and other similar crap

as you said ww:

“It’s not necessary to continuously jam, or even desirable if you’re trying to maintain a discrete profile. You just need to jam long enough to break a lock while performing an evasive maneuver, or to provide bogus info to the source.”

what’s more, as I said previously, the computing power available even from civil processors (one can imagine that the military ones, developed specifically will be even better at it) would allow to adapt your jamming extremely rapidly to any signal that’s in your “jamming range” for as long as you manage to program correctly the jammer (which is the complicated part I talked just before).

Basically, the problem is more in software than in hardware today.

So rather than cancelling radar freq. you need to cancel (for example) all X-band. So stil – you have to divide power between wide spectrum and then you’re not as effective as jamming just the freq. of the radar (plus you shine like a beacon).

what are you talking about? Your RWR will listen to a frequency range, but your jammer won’t emit in all frequencies simultaneously. you jam only the frequencies where you get a peak in power and only in the direction of the emitter.

That should be pretty clear, as the basics are rather simple. It’s the practical part that’s complex, since you need to do it in real time and adapt the jamming in power and direction (sending back accurate signal so to cancel your aircraft’s signature from that particular angle, not to speak if you’re manouvering, as you’d need to adapt your jamming as your radar “signature” varies while your aircraft turns).

I do simplify for explanatory purposes, but not so much, in fact. the basic principle is perfectly valid

only a small fraction of the energy bouncing on the target will go back in the direction of the emitter. now, if the waves hitting the aircraft are at the level of the ambient noise, there will be nothing detectable coming back to the emitter who’ll receive noise only. and only way to overcome that is to make lots of long emissions and then try to filter out the “irregular stuff” (noise). But you’d need the other guy to stay still, since his movement would prevent you from superposing successive “shots”. You have similar phenomenon in astro-photography, when you want to eliminate your sensor noise from a picture by making lots of long poses and then combining them to get a clear picture.

The problem with the radar is that you’re the one to emit, and if you want anything to come back your way, you need to send much more than what comes back.

As for “coding”, it all nice, but means nothing. the jammer has to reproduce a wave, not decrypt what it says, like if you tried to spy on a conversation.

For example, take ANR headsets for pilots. Do you thing the headset understands when someone talks and makes a disctinction with the ambient engine noise? it could care less. any wave coming in is cancelled by generating its own image and delaying it for 1/2 wavelength, period. If someone tries to talk, he can shout as much as he wants, the headset will just cancel the sound. the only way to talk to a guy wearing one is through the microphone, since the comms, of course, aren’t cancelled. For the jammer, it’s the same story, if it recognises a radar wave (relatively “easy, since it has to be a power peak, and at a given frequency range which is rather specific), and wants to jam it, it just has to reproduce it, not interpret it

It’s not just X number of watts emitted though. It’s antenna gain/sensitivity, filtering, signal processing power, available memory, the amount of time required to detect a recognizable pattern as being a hostile signal, and then being able to successful degrade it enough to break the kill chain.

I think we have a little misunderstanding.

when I said you needed “X watts to detect” I meant that X was the minimum amount of energy you had to emit to get a detectable and recognisable signal in return (regardless of your fiddling with gain, filtering etc…).

I could have used numbers to illustrate.

say, imagine that your radar can detect only signals above 1W coming back (all are imaginary numbers given simply for illustration purposes) since any less would be undetectable because of environment noise, and that with the weather conditions, 50% of emissions get lost for 100 miles travel (it’s D1 above and, again, fictional numbers for illustration pruposes)

in such scenario, with a target with “5% reflectivity”, you’d have to emit 80W of power to get 1W in return

Target: 100 miles out
power emitted: 80W
power received by target (X-D1): 80W-50%=40W
signal reflected: 40W-95%=2W
return signal received: 2W-50%=1W

you do see the signal, but the target has been illuminated by a much stronger signal and, what’s more, the jammer would need to emit, in your direction, only 2W of power (same that the fighter reflects) and, eventually, even less… since it doesn’t need to cancel the signal entirely, but just to weaken it sufficiently so that it becomes undiscernable noise for your radar once the remaining signal comes back.

waht’s more, the reactivity of the jammer will depend on the electronics used. Programmed correctly (that’s the complicated part) a RISC processor (kind used for their speed in real time applications) will generate a return signal in one or (maximum) two clock cycles. if you use just civilian market RISC processors, running at 600MHz, that would give a reponse time of about 0.0017-0.0034µs. Not that much time left for the emitter to analyze before the signal “disappears”.

Of course, it sounds simple when said like that, the complicated part is that you need to have a perfect RCS image of your aircraft under all angles, with all sorts of emports and then program your jammer to be able to reproduce that image, to perfection under any precise angle, with the appropriate frequency (the image varies with the frequency, plus doppler effect, etc…) and then, emit the signal towards the radar.. very precisely (that’s where active array comes into play). Do it sligthly out of phase and you’ll “shine” on hunter’s screen, and do it in a wrong direction, even a little, and you may send a good strong signal giving your azimuth to another bad guy out there.

Here’s the thing though- how quickly can the filter identify a hostile signal vs. the LPI radar finding its target? It doesn’t take long to acquire a target track with an AESA radar, and if the targets are at long range they only get periodic updates until they get close enough to be more of a threat. Then factor in third party targeting, so not every platform is emitting(i.e. the emitter can be the hunter, while the other birds are the killers).

thrid party targeting is another thing. and besides the point while discussing radar/jammer relations.

the emitter (we’re in no awacs scenario, or the fighters would not need their radars at all since they’re all L16 equipped) is flying in a hostile area and has to count on his radar to see what’s ahead of him. LPI or not, how do you decide at what power you emit? if you haven’t detected anything yet, you don’t know whether there’s empty sky ahead of you or an aircfraft that’s simply too far out to be detected with its RCS caracteristics.

If you have a scenario where you need say, X watts of power emitted to see an aircraft. retaking my previous example:

-you emit X watts
– on the way towards the target, you have a certain percentage of dispersion and signal loss, let’s call it D1
– you have (X-D1)/20 watts reflecting back towards you
– and you have again the dispersion and signal loss percentage of the return signal, we’ll call it D2
– the minimum return signal you detect, will be (X-D1)/20-D2

now, while you get that small return, the receiver will be illuminated by a signal that’s (X-D1) and that’s it. which, in that example, is over 20 times stronger than what your radar will get in return. So, while LPI sounds nice on paper (signal barely above noise level), it tells only half of the story.. it’s barely above noise level for the hunter, but the guy it’s aimed at gets a much stronger signal, and that’s the point I was making. It could work against a target that uses outdated ECM suite, but a more modern one, especially if it managed to record a pattern of your radar, could become quite difficult to detect. And that’s also why the USAF agreed to go against the french in the UAE on condition that it be a WVR fight with F-22 radar off and SPECTRA off… the last thing the US would want is SPECTRA recording F-22s radar patterns and include it in its librairies.. if it was ever sold to a country the US declare war a few years later, the thing may prove quite costly for american fighters

agreed, MSphere. what’s more, how low can one go in power output to look for unknown targets?

you won’t need the same power output to get an echo from a B-52-like bomber at 50 miles and a LO fighter at 100. at the same time, the fighters with “all ears out” will have good chance of detecting a wave peak coming from a specific direction.

For the sake of illustration, if you have 5% energy hitting the aircraft return in the direction of the emitter, that means that the energy hitting the target will be 20 times higher than the energy reemitted to the source. Now, even if the source want a very low level coming back, it still has to travel all the way back an be sufficiently powerful to be detected above the noise (so, the signal hitting the aircraft in such case would need to be at least 20 times stronger than the environment noise, and you have to add some more since the return signal has to have the energy to travel all the way back to the emitter again).

from the nov. 2008 senate report, the total estimated price is 39.6 billion euros for the whole rafale program (including buying 286 airframes), which, depending on the euro/dollar parity which vary on a daily basis, can be considered as going from ~30 billion up to ~60 billion dollars

http://secretdefense.blogs.liberation.fr/defense/2009/06/rafale-la-france-a-fait-le-bon-choix-celui-de-voler-de-ses-propres-ailes.html

according to this site:

http://www.flightglobal.com/blogs/the-dewline/2010/05/benchmark-contest-f-35-cost-vs.html

by the end of fiscal year 2011, the F-35 program would have costed 67.9 billion dollars, for about 101 contracted airframes and between 28 and 58 built