Radar percormance

You're slightly confused (it's not an easy topic after all), the ground does have a relative speed to your own aircraft unless you're sitting on the ground at 0 kts. As a consequence, radar returns from the ground are compressed by doppler effect. Since the radar of your own aircraft knows at which speed it's currently flying, it knows how much of a doppler effect those ground returns will have, and that's how the ground returns get filtered out. The side effect is that any aircraft with the same relative speed as the ground (thus the same doppler effect) flying below the horizon gets filtered out too.

 

tl;dr When you fly at 500 kts, the ground has a relative speed of 500 kts, same as an aircraft trying to beam you.

I was going to jump in here and explain this as well, so I'll just concur. There seems to be a lot of confusion when it comes to notch when people think the notch is when the target is at 0 relative speed to the beam when its not, its at actually the same speed as the aircraft.

Cheers.

Exactly, timings between the different pulses (or their frequency if you will). The radar does not measure the frequency shift in the carrier wave. Probably because that shift is so miniscule for non-relativistic speeds. If you think the ground filter is wide for a pulse doppler radar, imagine what it would be for a "doppler radar". (It would most likely be unusable)

The radar does measure the frequency change in carrier wave, but it doesnt do it for processing doppler effect. It does it for pulse compression differentiation purposes.

Please read up on how pulse doppler radars actually work instead of just making stuff up.

 

 

 

As an example, in X-band (10 GHz), a target approaching at 50kt (25 m/s) will give you a frequency shift of 1,7kHz. Detecting this using FFT is trivial.

Well, I'm not pretending to be a radar engineer, but a radar engineer that has worked on fighter radars for most of his life told me that was how they worked. I really don't think I made this up. If I misunderstood him, please tell me how.

Now, I'm not really into signal processing, but I can imagine that detecting that 0.000017 % difference in frequency might not be so trivial after all when you also take into account that the ground return is many orders of magnitude larger than the return from an aircraft. But I was just assuming stuff, which I tried to indicate by writing "probably".

I understand the principle behind both methods. Which one is actually used, I'm in no position to say. A frequency shift or lack thereof is indeed the reason why an aircraft in the notch remains "invisible", as long as Doppler is used to analyse the return pulse or pulses.

Whether it is due to the carrier frequency change or due to the frequency change of multiple returns, I don't know. I'm an engineer, but not a radar engineer.

For me, it's a question of instrument tolerance and accuracy. Obviously, only one of the two above cases is correct, probably the timings one, but can anyone confirm and explain why? To the level that is allowed, of course.

The radar does not directly measure the carriers Doppler shift. Almost all radars will down-convert the received signals to baseband or IF.

Also, consider that each range bin will only have ~2 samples per pulse (sample interval is about half the pulse duration, and the range bin is equal to 1 pulse duration).

You won’t get much from a 2 point DFT (don’t think FFTs go below 8 point).. Especially given the poor SNR from a single pulse.

Thus, relative phase shift over a period of many pulses is needed to determine Doppler shift. An FFT will be performed once enough pulses have been integrated.

I can imagine that detecting that 0.000017 % difference in frequency might not be so trivial

Just for your interest, it actually is!

As Beamscanner mentioned, the signal is "downconverted" by mixing it with the original signal eg 10GHz.

That results in sum and difference signals, and by low-pass filtering you can get just the difference signal.

That gives you a simple low-frequency signal to measure (1.7kHz in the example), which an FFT can do very accurately.

Just for your interest, it actually is!

 

As Beamscanner mentioned, the signal is "downconverted" by mixing it with the original signal eg 10GHz.

 

That results in sum and difference signals, and by low-pass filtering you can get just the difference signal.

 

That gives you a simple low-frequency signal to measure (1.7kHz in the example), which an FFT can do very accurately.

Well, yes, but now you're describing an ideal situation and you intentionally left out the important part, the part about the ground return being so much stronger.

Are you saying that isn't a problem?

Well, yes, but now you're describing an ideal situation and you intentionally left out the important part, the part about the ground return being so much stronger.

 

Are you saying that isn't a problem?

No I didn't intentionally leave anything out...

As long as the dynamic range is large enough to handle the signal, and the noise floor low enough, FFTs are great at separating signals at different frequencies, even if the magnitudes differ by a lot.

I don't have experience with high power pulsed radars, only small CW radars, but the same principles apply when talking about this part at least.

You are correct that the large ground return causes a problem (large dynamic range of signals), but you design the signal processing chain around the expected signal magnitudes.

More modern technology = more processing power/lower noise = better separation of target vs ground clutter.

No I didn't intentionally leave anything out...

 

As long as the dynamic range is large enough to handle the signal, and the noise floor low enough, FFTs are great at separating signals at different frequencies, even if the magnitudes differ by a lot.

 

 

 

I don't have experience with high power pulsed radars, only small CW radars, but the same principles apply when talking about this part at least.

 

 

 

You are correct that the large ground return causes a problem (large dynamic range of signals), but you design the signal processing chain around the expected signal magnitudes.

 

More modern technology = more processing power/lower noise = better separation of target vs ground clutter.

But let me ask you this: Is a pulse doppler radar less sensitive to ground clutter than a continuous wave radar? And if so, why?

Ok, thank you. I think it's becoming more clear to me.

 

But let me ask you this: Is a pulse doppler radar less sensitive to ground clutter than a continuous wave radar? And if so, why?

Sorry I don't have enough knowledge about pulse doppler radar to answer that.

I suspect though, that due to the more complex signal processing that goes on in pulse doppler vs CW, the filtering can better remove unwanted signals such a ground clutter.

1. Determining a targets doppler is not always as easy as just doing an FFT. It can actually be very difficult in MPRF and RGHPRF, and almost impossible in LPRF. The reason is related to the low sampling rate I mentioned earlier and harmonics related to pulsed signals.

idk your background, but unlike doing an FFT on a CW signal, a pulsed emission will generate bi-directional harmonics in the freq domain that are equal to the radars PRF. Doing a single FFT will show a number of doppler frequencies per individual target. Additionally, all the side-lobe and main-lobe noise/clutter will also repeat itself with harmonics. The radar cant know which spike in doppler is real and which is one of many harmonics. In this case the doppler is ambiguous. The radar will have to change PRFs several times in order to resolve the real doppler frequencies.

In HPRF and CW emissions, determining doppler can be very simple though as the harmonics are pushed so far off from the carrier that its not a factor. However, HPRF and CW have the problem of having ambiguous range that needs to be determined in "slow time" (FMR dwell times for example) instead of "fast time" (single pulse timing)

2. If your goal is to reject ground clutter, CW is the simplest choice. Since CW doesn't have to deal with doppler harmonics, the ground clutter is only on one frequency (if you're not moving, the transmitters freq). It also has more data to sample, allowing larger FFTs, which enable greater doppler resolution.

In truth, a PD radar is trying to mimic CW by increasing the duty cycle of a pulsed radar, but with the added benefit of only needing one antenna. But 30% duty cycle is not the same as 100% duty cycle, and higher duty cycles are always better for doppler processing. Though there are ways to improve clutter rejection in MPRF/LPRF, they can be very complex and expensive. One example is "STAP".

1. Determining a targets doppler is not always as easy as just doing an FFT. It can actually be very difficult in MPRF and RGHPRF, and almost impossible in LPRF. The reason is related to the low sampling rate I mentioned earlier and harmonics related to pulsed signals.

 

idk your background, but unlike doing an FFT on a CW signal, a pulsed emission will generate bi-directional harmonics in the freq domain that are equal to the radars PRF. Doing a single FFT will show a number of doppler frequencies per individual target. Additionally, all the side-lobe and main-lobe noise/clutter will also repeat itself with harmonics. The radar cant know which spike in doppler is real and which is one of many harmonics. In this case the doppler is ambiguous. The radar will have to change PRFs several times in order to resolve the real doppler frequencies.

 

In HPRF and CW emissions, determining doppler can be very simple though as the harmonics are pushed so far off from the carrier that its not a factor. However, HPRF and CW have the problem of having ambiguous range that needs to be determined in "slow time" (FMR dwell times for example) instead of "fast time" (single pulse timing)

 

 

2. If your goal is to reject ground clutter, CW is the simplest choice. Since CW doesn't have to deal with doppler harmonics, the ground clutter is only on one frequency (if you're not moving, the transmitters freq). It also has more data to sample, allowing larger FFTs, which enable greater doppler resolution.

 

In truth, a PD radar is trying to mimic CW by increasing the duty cycle of a pulsed radar, but with the added benefit of only needing one antenna. But 30% duty cycle is not the same as 100% duty cycle, and higher duty cycles are always better for doppler processing. Though there are ways to improve clutter rejection in MPRF/LPRF, they can be very complex and expensive. One example is "STAP".

As a side question, how does an aircraft like the tomcat determine velocity in pulse STT? Since there's no doppler shift information in the return signal it has to use another method, right?

As a side question, how does an aircraft like the tomcat determine velocity in pulse STT? Since there's no doppler shift information in the return signal it has to use another method, right?

I dont know exactly what the AWG-9 did in Pulse STT (low PRF) mode. But I do know that it could not processes doppler in Pulse modes, hence why ground clutter cannot be filtered out in pulse modes.

A guess would be that the radar extrapolates target speed via range rate (relative change in position over time) and not velocity (Doppler). I'm sure the heatblur team knows the answer to this.

Yeah, that's what I figured. I'll ask them when they're a bit less swamped with work.

Also another question - if I understand correctly, zero doppler returns are always filtered out because the radar's PRF and its harmonics are filtered out to prevent the sidelobe return under the aircraft from cluttering, right? How do modern or semi modern radars such as the one in the Hornet deal with this? Do they PRF hop? You can certainly see near zero closure rate targets sometimes, but it's a lot less reliable.

Yeah, that's what I figured. I'll ask them when they're a bit less swamped with work.

 

Also another question - if I understand correctly, zero doppler returns are always filtered out because the radar's PRF and its harmonics are filtered out to prevent the sidelobe return under the aircraft from cluttering, right? How do modern or semi modern radars such as the one in the Hornet deal with this? Do they PRF hop? You can certainly see near zero closure rate targets sometimes, but it's a lot less reliable.

Yes, the altitude line is filtered out. Keep in mind that the mainlobe clutter, altitude line, and sidelobe clutter also exist in each PRF harmonic in the freq domain (doppler spectrum). So, there are many doppler blind zones at various doppler freqs. So modern radars have to change PRFs to move the doppler blinds zones around to see if a target is under them.

Generally speaking a modern fighter radar using MPRF will have to use many different PRFs to shift both the doppler and range blind zones. Also, if the radar only detects the target with one PRF, its limited to its maximum unambiguous range (MUR), which could be very short.

I cannot speak to how the Hornet radar specifically works. That being said, the most well known technique is the "3 of 8" PRF logic. In this, the radar must detect a target with at least 3 separate PRFs in order to resolve target range (removes the limit of MUR). A total of 8 different PRFs are used in order to ensure at least 3 of these PRFs will see a target at any given speed or range.

ie you shift the harmonics around at 8 different PRFs to ensure at least 3 PRFs will detect the target.

EDIT: This BTW is one reason MPRF has lower detection range than HPRF. Because you need to use 8 different PRF integrations(each PRF used fires a long train of pulses which are summed in the range bins), you can only integrate (sum) 1/8 of the pulses that reflected off the target. HPRF gets to integrate a lot more pulses as its uses only 1-3 PRFs(usually), and it transmits more pulses overall. Though FMR used with HPRF will hinder the pulse integration processes, thus reducing the detection range. Hence why on the AWG-9, PD search (no FMR) has greater range than RWS (also a HPRF mode, but with FMR)

And I thought I knew stuff about radar technology :book:

Because we are talking about the radar. Does ED said anything about updating it in the near future? I mean it's still missing alot like for example the effects of jamming.

Hi,

 

I was flying to a tanker. To find the tanker more easily I used the radar and locked it with STT.

At one point the tanker was flying 90 degrees to me, then I lost the lock. But the tanker was only 9nm far away from me and I was on the same FL. I wonder if this is the real performance of the radar from the fa18. What do you guys think about that?

Also, in defense of the OP, most radars can turn off their main lobe notch filter in look up conditions. Even the DCS AWG-9 does this.

So coming back to the initial point:

Shouldn't the radar stay locked on that target?

So coming back to the initial point:

Shouldn't the radar stay locked on that target?

If the tanker fell into the Doppler notch...no it should break lock.

If the tanker fell into the Doppler notch...no it should break lock.

Someones not listening..

The notch filter can be turned off in some radars during look up conditions (weak sidelobe reflections when tilting antenna upward).

Sometimes its just the mainlobe ground filter, other times its both the altitude line and main lobe ground filter. It all depends on how they designed the radar to work.

If you guys think the best radar engineers in the world are going to let a simple fix get overlooked on a multi-million dollar radar (each) for thousands of aircraft, you're sorely mistaken.

So coming back to the initial point:

Shouldn't the radar stay locked on that target?

Depends on the geometry, and also things we wont ever know about the APG-73.. If its a look up condition probably not. There's so much more to it than even what I've explained.

For instance, the "doppler notch" in digital radars can just be a higher detection threshold instead of a black and white /yes or no filter. In that case if the target is very large and close enough it can still detect it even if its "in the notch".

Also, some radars can change waveforms and processing techniques to meet tracking requirments. A radar could hypothetically go into a low PRF mode and continue tracking the target in range but not doppler (just like some of you are doing with the DCS AWG-9, but the radar may do this automatically).

Guys, also take into account one thing that is not realistic and perhaps should have been addressed by now: The fact that when you go into STT the PRF stays in whatever it was at the time of commanding the lock. This often makes a lock to be lost for instance due to being very far (like 75nm) and stuck in MED prf. If you change the PRF to HI during the lock and before it is lost then the lock is retained.

As far as I understand and was taught, any modern radar selects the best PRF for retaining a lock when commanding STT.