Radar percormance

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?

Edited July 6, 2019 by Neor

Tanker flying 90 degrees off of your nose is in the notch

whether at 5 miles or 50, he is in the notch and filtered out with ground returns

This description seems to be accurate to the performance of the radar system.

ground returns on angles 12?

yes because your radar beam is not just stopping at the tanker it continues way past it. So anything that flies perpendicular to the radar beam, gets in the notch. That is why in many of youtube videos (

) , also when you're being engaged with a , you defeat it kinetically by being offset to it 90 degrees so its onboard radar emitter or the launching platform does not track you because you entered the notch.

ground returns on angles 12?

 

The radar filters out the ground returns by ignoring anything that is stationary or moving very slowly.

When a plane is flying at (or very close to) 90 degrees from your flight path, its relative speed is very low so is filtered out and the radar cannot detect it "in the notch".

The radar filters out the ground returns by ignoring anything that is stationary or moving very slowly.

 

When a plane is flying at (or very close to) 90 degrees from your flight path, its relative speed is very low so is filtered out and the radar cannot detect it "in the notch".

Not correct. If you turn back and fly away from the opponent your relative speed would be even slower than if you fly 90 degrees. Yet you still wouldn't break the lock.

 

 

 

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Not correct. If you turn back and fly away from the opponent your relative speed would be even slower than if you fly 90 degrees. Yet you still wouldn't break the lock.

 

 

 

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It's the speed in the direction of the radar beam, relative to ground. It's a Doppler thing.

Not correct. If you turn back and fly away from the opponent your relative speed would be even slower than if you fly 90 degrees. Yet you still wouldn't break the lock.

 

 

 

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Flying 90 means that your relative speed is to zero. Flying away still yields a measurable relative speed

Flying 90 means that your relative speed is to zero. Flying away still yields a measurable relative speed

No. I can be lying away from him at his same speed. That's when my relative speed to him is 0. If I fly 90 degrees to him and he is getting closer to me (I am also getting closer to him) it can't be that my speed relative to him is 0.

 

 

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No. I can be lying away from him at his same speed. That's when my relative speed to him is 0. If I fly 90 degrees to him and he is getting closer to me (I am also getting closer to him) it can't be that my speed relative to him is 0.

 

 

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You need to think not in terms of your aircraft, but in terms of the radar wave and the Doppler effect. Something flying 90 degrees relative to your radar neither comes closer nor moves away in relation to the wave. It's perpendicular to it.

An aircraft that flies away at the same speed as you, still moves in relation to the wave.

You need to think not in terms of your aircraft, but in terms of the radar wave and the Doppler effect. Something flying 90 degrees relative to your radar neither comes closer nor moves away in relation to the wave. It's perpendicular to it.

An aircraft that flies away at the same speed as you, still moves in relation to the wave.

But you said "your relative speed is 0" that is wrong.

 

 

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You need to think not in terms of your aircraft, but in terms of the radar wave and the Doppler effect. Something flying 90 degrees relative to your radar neither comes closer nor moves away in relation to the wave. It's perpendicular to it.

An aircraft that flies away at the same speed as you, still moves in relation to the wave.

 

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.

Edited July 6, 2019 by 92nd-MajorBug

But you said "your relative speed is 0" that is wrong.

 

 

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Yeah, my bad on that, bad wording. The target's relative speed, as seen by the radar.

Pulse Doppler (PD) radars rely on wavelength shift, which occurs when the object reflecting the wave is moving towards (shorter returning wavelength or blue shift) or away from the radar (longer returning wavelength or red shift).

In addition to that, you're moving, so the PD radar has to adjust for that and filter out everything that's coming towards you, with your speed, plus a little above and below, to account for cars etc. So everything within that red shift range is left out.

An aircraft perpendicular to you has the same closing speed as the ground, the clouds etc. So it'll be filtered out. An aircraft flying away from you, has a different speed than the ground, so it's not filtered out. A helicopter flying very slowly, its speed comparable to the speed of a car, can often avoid detection.

The above settings can be partially controlled by the pilot, but I don't know to what extent.

A helicopter flying very slowly, its speed comparable to the speed of a car, can often avoid detection.

 

The whirling rotor disc moving just below the speed of sound doesn't show up on radar?

It will show up quite well indeed, but it has a rather small cross section compared to the whole body of the chopper (which is filtered out if stationnary)

Yeah, my bad on that, bad wording. The target's relative speed, as seen by the radar.

Pulse Doppler (PD) radars rely on wavelength shift, which occurs when the object reflecting the wave is moving towards (shorter returning wavelength or blue shift) or away from the radar (longer returning wavelength or red shift).

In addition to that, you're moving, so the PD radar has to adjust for that and filter out everything that's coming towards you, with your speed, plus a little above and below, to account for cars etc. So everything within that red shift range is left out.

An aircraft perpendicular to you has the same closing speed as the ground, the clouds etc. So it'll be filtered out. An aircraft flying away from you, has a different speed than the ground, so it's not filtered out. A helicopter flying very slowly, its speed comparable to the speed of a car, can often avoid detection.

The above settings can be partially controlled by the pilot, but I don't know to what extent.

That's better. However helicopters are usually detected cause of their blades moving in all directions including towards and away from you.

 

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That's better. However helicopters are usually detected cause of their blades moving in all directions including towards and away from you.

 

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Yeah, that's true, forgot that part. Same reason why airplane stealth technology focuses so much on keeping the turbine blades hidden from the front aspect.

The whirling rotor disc moving just below the speed of sound doesn't show up on radar?

It will, I forgot about that part. But anyway, a slow helicopter is more difficult to detect than a fast helicopter, even if it doesn't technically matter. I'm also thinking that the radar-rotor angle may play a role, but I'm just guessing at this point.

Yeah, my bad on that, bad wording. The target's relative speed, as seen by the radar.

Pulse Doppler (PD) radars rely on wavelength shift, which occurs when the object reflecting the wave is moving towards (shorter returning wavelength or blue shift) or away from the radar (longer returning wavelength or red shift).

In addition to that, you're moving, so the PD radar has to adjust for that and filter out everything that's coming towards you, with your speed, plus a little above and below, to account for cars etc. So everything within that red shift range is left out.

An aircraft perpendicular to you has the same closing speed as the ground, the clouds etc. So it'll be filtered out. An aircraft flying away from you, has a different speed than the ground, so it's not filtered out. A helicopter flying very slowly, its speed comparable to the speed of a car, can often avoid detection.

The above settings can be partially controlled by the pilot, but I don't know to what extent.

We're getting out of scope but a pulse doppler radar does not rely on wavelength shift but rather on change in timings within a series of pulses. This is implied in the name; "pulse doppler". But the principle is more or less the same.

It will, I forgot about that part. But anyway, a slow helicopter is more difficult to detect than a fast helicopter, even if it doesn't technically matter. I'm also thinking that the radar-rotor angle may play a role, but I'm just guessing at this point.

 

Given that the rah66 project was canclled because they couldnt reduce the rotor disc RCS I think its much larger than the body rcs, which they could reduce.

We're getting out of scope but a pulse doppler radar does not rely on wavelength shift but rather on change in timings within a series of pulses. This is implied in the name; "pulse doppler". But the principle is more or less the same.

A wavelength shift equals a frequency shift and thus a change in timings, since the wave's speed (light speed) is constant no matter what. Isn't it the same thing, in the end? Or do you mean differences between different pulses?

Given that the rah66 project was canclled because they couldnt reduce the rotor disc RCS I think its much larger than the body rcs, which they could reduce.

Seems logical. The rotor disk is practically screaming "hey, I'm here!", especially if viewed at a good angle, which is true for most cases of fixed wing aircraft detecting helicopters.

I meant the speed of the helicopter doesn't matter as much in the end, since the rotor is more visible anyway, but in any event, a slow heli is technically harder to detect than a fast heli, even if it doesn't matter in the end.

A wavelength shift equals a frequency shift and thus a change in timings, since the wave's speed (light speed) is constant no matter what. Isn't it the same thing, in the end? Or do you mean differences between different pulses?

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)

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)

 

 

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.

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)

 

You are correct. Another point is that the ADC in most radars wont sample the signal anywhere close to the carrier frequency.

 

It usually only needs to sample 2 or 3 times in a given duration equal to the pulse duration (effective Pulse Duration).

 

This means that the radar has to measure doppler after a series of pulses have been accumulated. It looks at the relative phase shift over these pulse returns.

 

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.