D-Scythe Posted December 21, 2006 Posted December 21, 2006 The radar is interfaced with other systems in the aircraft, the air data system for one, that tells the radar how fast the aircraft is moving and which way the plate is pointed. Stationary objects give a doppler shift which matches the speed of the transmitting aircraft, therefore they are filtered out by the receiver's processor. If the object is moving at a significant positive or negative closure speed in relation to the ground clutter, then it creates a doppler shift different from the speed of the transmitting aircraft, thus its detected. When the plate is pointed above the horizon (look-up), the radar then knows that doppler filtering of the incoming signal is no longer necessary, so the notch disappears. No - the notch NEVER disappears. I don't know where you're getting your info, but it's not right. You still need doppler look up - else your target can blanket everything in chaff and you're screwed.
GGTharos Posted December 21, 2006 Posted December 21, 2006 Ahem ... the doppler notch is measured with respect to a doppler shift created by reflection from clutter. What are you creating a notch with when looking up? Nada. If there's chaff, then you might need to re-instate some sort of notch, but it'll be with respect to the chaff's velocity, and only when it is within the radar beam, so ... ????????? [sIGPIC][/sIGPIC] Reminder: SAM = Speed Bump :D I used to play flight sims like you, but then I took a slammer to the knee - Yoda
D-Scythe Posted December 21, 2006 Posted December 21, 2006 Ahem ... the doppler notch is measured with respect to a doppler shift created by reflection from clutter. What are you creating a notch with when looking up? Nada. If there's chaff, then you might need to re-instate some sort of notch, but it'll be with respect to the chaff's velocity, and only when it is within the radar beam, so ... ????????? There's plenty of noise to filter out - you don't think all RF wavelengths just zip through the sky without generating any sorta clutter do you? Temperature, clouds, mainlobe, sidelobe, etc. Besides, how would the radar know when to turn on the notch and when to turn off the notch? Chances are there will always be some type of ground clutter in the any case, from the mainlobe or the side lobes (which is a SECOND notch). And then you factor in such things as multiple bandits, chaff, missiles, scintillation of radar - all things that would affect a radar tracking by RCS, but not by doppler.
nscode Posted December 21, 2006 Posted December 21, 2006 There's plenty of noise to filter out - you don't think all RF wavelengths just zip through the sky without generating any sorta clutter do you? Temperature, clouds, mainlobe, sidelobe, etc Those are a job for the signal processor. They differ so much from usefull stuff that they can be filtered out based on signal paramethers. Notching out is done in the data processor. Never forget that World War III was not Cold for most of us.
Dudikoff Posted December 21, 2006 Posted December 21, 2006 Ahem ... the doppler notch is measured with respect to a doppler shift created by reflection from clutter. What are you creating a notch with when looking up? Nada. Why would you need a reflection to read the doppler shift? By using the emitter's speed (+/- some value) you can calculate the frequency band which should be filtered out. i386DX40@42 MHz w/i387 CP, 4 MB RAM (8*512 kB), Trident 8900C 1 MB w/16-bit RAMDAC ISA, Quantum 340 MB UDMA33, SB 16, DOS 6.22 w/QEMM + Win3.11CE, Quickshot 1btn 2axis, Numpad as hat. 2 FPH on a good day, 1 FPH avg. DISCLAIMER: My posts are still absolutely useless. Just finding excuses not to learn the F-14 (HB's Swansong?). Annoyed by my posts? Please consider donating. Once the target sum is reached, I'll be off to somewhere nice I promise not to post from. I'd buy that for a dollar!
GGTharos Posted December 21, 2006 Posted December 21, 2006 Why would you need a reflection to read the doppler shift? By using the emitter's speed (+/- some value) you can calculate the frequency band which should be filtered out. I don't think how you measure this speed really matters ;) [sIGPIC][/sIGPIC] Reminder: SAM = Speed Bump :D I used to play flight sims like you, but then I took a slammer to the knee - Yoda
Guest IguanaKing Posted December 21, 2006 Posted December 21, 2006 No - the notch NEVER disappears. I don't know where you're getting your info, but it's not right. You still need doppler look up - else your target can blanket everything in chaff and you're screwed. Scythe, it seems as if you aren't quite understanding what I meant. I didn't say that doppler isn't used at all in a look-up situation, I said that doppler filtering isn't used. So, you can still differentiate targets of different velocities, but if one of them magically stops in mid-air, yes, you will STILL see him. It'll shoot your firing solution all to hell and you'll lose lock, but he won't just suddenly disappear. Also, I'm not sure why you keep separating RCS and doppler. Where do you think the doppler shift comes from? It comes from the reflected RF energy coming back to the receiver of the transmitting aircraft, which is directly related to the RCS of the target aircraft. If the target is moving toward you, the pulse width and pulse spacing both decrease, and if he's moving away from you they both increase. If his relative speed to you is zero, his reflected energy comes back in pretty much the same pulse width and timing it left with. Does that mean he disappears? That would be pretty cool if that actually happened, because all you'd need to do to keep from being locked is to fly in the opposite direction and match speed with the transmitting aircraft. Your radar will just show a 0 velocity on a target that is still quite visible. The RF signature of those transmitted pulses remains pretty much the same, and the radar's receiver will still recognize it as its own pulse train rather than noise. This is something that is done in the signal processor, just like nscode said. How does it know when to turn on the doppler filtering? Gyro stabilization. The radar plate drive and processor both receive inputs from the attitude gyro or AHRS system in the aircraft, so they know where the nose is pointed. There is even a certain degree of roll stabilization available so that your plate will still be scanning parallel to the horizon when you are rolled. Even civilian radar systems are capable of up to 30 degrees of gyro stab. Aside from all that, Rhen has already stated on this forum that there is no notch in a look-up situation, and the above explanation probably has something to do with why it is that way. ;)
GGTharos Posted December 21, 2006 Posted December 21, 2006 I don't think you'd lose lock just because doppler shift is zero, nor should it cause any issues with your firing solution - the target is along the same bearing line, and still bouncing back energy - there's simply nothing that could or should drive the antenna off-target :) As for knowing when to turn on doppler filtering - not only gyro stabilization, but even something as simple as 'number of targets' reflected, total amount of energy reflected back, and quality of signal (spectrum spread) [sIGPIC][/sIGPIC] Reminder: SAM = Speed Bump :D I used to play flight sims like you, but then I took a slammer to the knee - Yoda
nscode Posted December 21, 2006 Posted December 21, 2006 Yeah.... relaying on attitude information alone would be a huge simplification error. What if there'se a mountin behind your target and you are below it.. you are looking up, but you still need the filtering. There is plenty info in the signal alone (if you know how to read it ;)) telling you about the general situation you're in. Gyro info could be a factor in the decision algorythm, but not the most importan one. Further more, all the mechanical instruments you and ijozic are mentioning are too slow for getting time-sensitive data for such a process. And I meen mostly for the moment you are switching from one mode to another. Never forget that World War III was not Cold for most of us.
Guest IguanaKing Posted December 21, 2006 Posted December 21, 2006 True, but the attitude info opens the first gate, then the other processing circuitry starts doing its job...and so on. ;) Mechanical instruments too slow? Not really. Besides, I wouldn't quite call an RLG or QRS mechanical. ;) Know how we test gyro stab on the ground? By having one guy grab a wingtip and shake the aircraft. The numbers can be read on the radar indicator when its in test mode. The main, limiting factor to stab speed is how quickly the stepper motors in the plate drive can move. The processor has NO trouble whatsoever staying ahead of them. ;)
Guest IguanaKing Posted December 21, 2006 Posted December 21, 2006 I don't think you'd lose lock just because doppler shift is zero, nor should it cause any issues with your firing solution - the target is along the same bearing line, and still bouncing back energy - there's simply nothing that could or should drive the antenna off-target :) I meant a sudden change to zero doppler, GG. It'd throw a few things out of whack in the lock. However, now that you mention it, I'm not sure now that lock would be dropped. Its likely that the processor would figure things out and re-establish a lock before it even reached its time-out point. So the radar would have an instant of "whoops, WTF", but the pilot would probably never see it. That's assuming, of course that the enemy is flying an aircraft that obeys the laws of physics. :D
GGTharos Posted December 21, 2006 Posted December 21, 2006 Again - it doesn't affect the bearing line, so, I think the radar would go 'huh, HoJ MODE POWA!' at the worst - just IMHO ;) [sIGPIC][/sIGPIC] Reminder: SAM = Speed Bump :D I used to play flight sims like you, but then I took a slammer to the knee - Yoda
Guest IguanaKing Posted December 21, 2006 Posted December 21, 2006 I agree GG, that's actually what I was trying to say. The radar would know something was up, but it would correct it so quickly nobody would even know that it happened. :D
nscode Posted December 21, 2006 Posted December 21, 2006 The main, limiting factor to stab speed is how quickly the stepper motors in the plate drive can move. And in signal processing world that is a huge amount of time. In data processing you could cope with it... but again as I sayed you've got more reliable indicators. Never forget that World War III was not Cold for most of us.
Guest IguanaKing Posted December 21, 2006 Posted December 21, 2006 Yes, I know it is, that's why I said that signals from AHRS, the 429 databus, and the radar's processor have no trouble keeping up, when the limiting factor is the plate drive itself. The plate is the end-all, be-all of the system, and even the slowest of old-fashioned vertical gyro systems, along with the stab circuitry in the processor, are both looking at the plate drive, tapping their feet, going "Come on dude, try to keep up with us." When we get into AESA radar, THEN signal responses in the system will become more critical. ;)
nscode Posted December 21, 2006 Posted December 21, 2006 I'm not saying that processor is haveing trouble keeping up with the gyro stab... I'm saying that gyro is having trouble keeping up with the speed of light :) Never forget that World War III was not Cold for most of us.
Guest IguanaKing Posted December 21, 2006 Posted December 21, 2006 Yes...and the stepper motors in the plate drive are having even more of a problem keeping up. Am I being understood yet? It seems like we're going in a circle and ignoring the plate drive.
nscode Posted December 21, 2006 Posted December 21, 2006 and i'm saying that doesn't matter... much (in respect to notch singing "should i stay or should i go" :D) Never forget that World War III was not Cold for most of us.
Guest IguanaKing Posted December 21, 2006 Posted December 21, 2006 Unfortunately, the rest of the system does have to wait for the plate drive to catch up. Granted, it only takes milliseconds, but it is the slowest part of the system, not the gyro/AHRS. That's all I've been trying to say.;)
nscode Posted December 21, 2006 Posted December 21, 2006 does not :P :D Never forget that World War III was not Cold for most of us.
Guest IguanaKing Posted December 21, 2006 Posted December 21, 2006 Does too, times infinity! :P :D :D
nscode Posted December 21, 2006 Posted December 21, 2006 infinity*does = infinity and there is no "does" in infinity so it is not does... so it does not :P :D anyway, we are waaaaaaaay ot and not going anywhere.. so let's just stop here :) Never forget that World War III was not Cold for most of us.
Guest IguanaKing Posted December 21, 2006 Posted December 21, 2006 Good plan. :thumbup: What can I say? I'm bored today, buried in snow. :cold:
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