prf radar modes

Can someone explain to me the prf modes please. I'm not sure when use the different modes and what they do differently.

 

Low PRF radar have reduced sensitivity in the presence of low-velocity clutter that interfere with aircraft detection near terrain. Moving target indicator is generally required for acceptable performance near terrain, but this introduces radar scalloping issues that complicate the receiver. Low PRF radar intended for aircraft and spacecraft detection are heavily degraded by weather phenomenon, which cannot be compensated using moving target indicator.

 

 

Medium PRF is used with Pulse-Doppler radar, which is required for look-down/shoot-down capability in military systems. Doppler radar return is generally not ambiguous until velocity exceeds the speed of sound.

A technique called ambiguity resolution is required to identify true range and speed. Doppler signals fall between 1.5 kHz, and 15 kHz, which is audible, so audio signals from medium-PRF radar systems can be used for passive target classification.

 

 

High PRF is limited to systems that require close-in performance, like proximity fuses and law enforcement radar.

For example, if 30 samples are taken during the quiescent phase between transmit pulses using a 30 kHz PRF, then true range can be determined to a maximum of 150 km using 1 microsecond samples (30 x C / 30,000 km/s). Reflectors beyond this range might be detectable, but the true range cannot be identified.

 

Sumary:

Low PRF : Mostly used as weather radar

Med PRF : Good for long range detection, but not very precise

High PRF : Best precision, but limited to medium/short range.

So is it best to leave it in medium until you get into a fight then switch to high? Is it even working properly yet?

In game:

You have reduced radar range in ENT compared to HFR.

But HFR is more susceptible to beaming, and isn't great on tail aspect target.

BFR works great on tail aspect and beaming target. Buy range is limited to 40Nm and you can't lock target.

For range BFR< ENT< HFR.

For beaming resistance and tail aspect target detection HFR< ENT< BFR

You can lock target in TWS in ENT but you can shoot (need HFR to guide missile).

You can't lock at all in BFR.

In game:

You have reduced radar range in ENT compared to HFR.

But HFR is more susceptible to beaming, and isn't great on tail aspect target.

BFR works great on tail aspect and beaming target. Buy range is limited to 40Nm and you can't lock target.

For range BFR< ENT< HFR.

For beaming resistance and tail aspect target detection HFR< ENT< BFR

You can lock target in TWS in ENT but you can shoot (need HFR to guide missile).

You can't lock at all in BFR.

But wouldnt technically be the doppler filter the only thing limiting detection of a low relative velocity target ?

PRF does only influence max non ambigious range and speed by doppler right ?

So is it best to leave it in medium until you get into a fight then switch to high? Is it even working properly yet?

No, read it more carefully and keep in mind that max detection range is below the high prf range determination drop off.

At least in the F-15 (numbers may vary with mirage, but general usage criteria is the same), high prf is optimal for detecting targets heading towards you, medium prf is optimal for targets heading away from you or flanking.

Additionally, high prf has a longer detection range and will start picking up targets around 50-60nm away, whereas medium prf will start picking up targets around 25-30nm.

Thus, it's generally best to start out in high prf if you expect your enemies to be over 30nm away when you turn your radar on and then switch to medium prf once you've closed distance with your enemies to within 30nm.

If you're hanging out in the mountains at bullseye on the 104th server for example, you would be in medium prf because you expect there to be a few enemies within 0-20nm of you and there is a great possibility that they're not flying head on.

Also, I am not sure about the exact pattern and frequency switching for the mirage's interleaved mode, but in general it's better not to use it as it's a compromise between the two modes, possibly reducing overall detection range, or increasing the time it takes to scan the same section of sky in both high and medium prf.

One final thing, I don't have the manual handy, but I think your lock and fire capabilities may be different in different prf modes, so you should take that into consideration as well.

Someone feel free to chime in if I am wrong/missing anything here.

But wouldnt technically be the doppler filter the only thing limiting detection of a low relative velocity target ?

PRF does only influence max non ambigious range and speed by doppler right ?

Not only.

IRL, in look down, PRF also influence the amount of ground clutter.

In high PRF = low ground clutter, and with Doppler filter you have look down/ shoot down capacity.

low PRF = a lot of ground clutter.

So AG mode = low PRF.

But in AA low PRF isn't good in look down.

All that being said, some more modern radar make better use of low/ med PRF (like RDY radar). But some of RDI limitations in low PRF may lie in computing power*.

MED PRF isn't really acknowledged in public documentation about RDI.

Anyway, at the end, you need High PRF to guide Super 530D, like most AA Fox 1 (APG-63 also switch to HPRF to guide AIM-7M).

And HPRF gives good head on range, both look up and look down.

* computing power was the reason in delay for RDI radar development, and this is why the first 37 FAF Mirage 2000C were equipped with RDM radar.

All that is pretty confusing...

What i understand is that PRF determine the time between each pulse, so to say, the time dedicated to recieve the echos of the previous pulse...

This ca influence target detection in two manner:

- Target distance: If target is too far, the echo is recieved after the next pulse was emitted, so the radar is confused.

- Target closing speed: If closing speed is too low or negative, the recieved echo wavelenght can be to long for the intervale to determine the proper speed of the echo, or even to be detected...

Not only.

 

IRL, in look down, PRF also influence the amount of ground clutter.

In high PRF = low ground clutter, and with Doppler filter you have look down/ shoot down capacity.

low PRF = a lot of ground clutter.

 

So AG mode = low PRF.

But in AA low PRF isn't good in look down.

 

All that being said, some more modern radar make better use of low/ med PRF (like RDY radar). But some of RDI limitations in low PRF may lie in computing power*.

MED PRF isn't really acknowledged in public documentation about RDI.

 

Anyway, at the end, you need High PRF to guide Super 530D, like most AA Fox 1 (APG-63 also switch to HPRF to guide AIM-7M).

And HPRF gives good head on range, both look up and look down.

 

* computing power was the reason in delay for RDI radar development, and this is why the first 37 FAF Mirage 2000C were equipped with RDM radar.

I mean there is no difference between ground clutter and a low relative velocity target. I mean ground clutter is the definition of a low relative velocity target.

PRF is only the time interval between radar pulses, and if you get one pulse with the entire ground reflecting the pulse or 10 in the same time doesnt make you get more or less ground return.

The setting of the doppler filter is influencing how many ground returns get discarded and which dont. I know that is that way in game and I think there is probably something in the bush in terms of signal processing back then.

I can imagine that high PRF with no or limited doppler filter simply overloads the onboard computer with returns to evaluate. But that is a wild guess, but from the signal itself PRF really only determines the max speed and max range you can get with a single pulse. If you have staggered PRF different frequencies etc you can work around all that but I guess that is pretty advanced stuff.

Minimum speed isnt a problem for the speed detection. The only problem emerges if the doppler return frequency is too close to the PRF, because then you cant determine if the doppler frequency is the PRF or double or three times that.

But technically PRF doesnt limit the lower end of speed detection, neither the low end range.

I guess the medium PRF is just coupled to a less aggressive doppler filter, but due to the larger amount of returns to be processed it switches to a lower PRF. That is my wild guess at it, trying to find an explanation for roughly a week now. No luck so far.

ENT (~MPRF) is a mix of HPRF and LPRF.

In real aircraft, ENT isn't MED PRF.

It's alternating HFR and BFR scan.

(ENT > Entrelacé > Interleaved... i say it since nobody say it... )

ENT (~MPRF) is a mix of HPRF and LPRF.

Yep, but why do Radars even use lower then PRF for low relative velocity targets inside high PRF range ?

The technical reasons are mysterious to me. :D

What does HFR and BFR mean ?

 

What does HFR and BFR mean ?

I guess they mean,

Basse fréquence de repetition (BFR) = Low repetition frequency (LPRF)

Haute fréquence de repetition (HFR) = High repetition frequency (HPRF)

Yep, but why do Radars even use lower then PRF for low relative velocity targets inside high PRF range ?

 

The technical reasons are mysterious to me. :D

 

What does HFR and BFR mean ?

I guess they mean,

 

Basse fréquence de repetition (BFR) = Low repetition frequency (LPRF)

 

Haute fréquence de repetition (HFR) = High repetition frequency (HPRF)

That's it.

H = Haut/Haute = High

B = Bas/Basse = Low

HFR = Haute Fréquence de Répétition

BFR = Basse Fréquence de Répétition

And ENT = ENTrelacé = Interleaved

Wikipedia precisely say : "Low PRF radar have reduced sensitivity in the presence of low-velocity clutter that interfere with aircraft detection near terrain."

Technically, i don't know why, but as i understand, low PRF is only good for weather or terrain.

Wikipedia article on PRF is just confusing. First of all you need to know that high PRF mode is different from low PRF (and med PRF) in many other ways than just the PRF. Most important secondary difference is pulse width and duty cycle which is the ratio of pulse width to pulse repetition interval.

Since high PRF has small unambiguous range detection capability it has to use different method for ranging than direct pulse flight time measurement. Essentially high PRF mode encodes each pulse so you can later identify the original pulse from the reflection and then calculate the pulse flight time. This method reduces range resolution. As you now use a different method for ranging echoes, you can ramp up duty cycle as much as you want (all the way to 50%, ie. the radar spends equal time sending and listening) to output as much energy as possible. This gives the high PRF the long detection range it's famous for. You also get the best frequency resolution due to high PRF and therefore good clutter rejection.

Low PRF has small pulse width and large pulse interval hence low duty cycle. Low duty cycle is essential for large unambiguous range measurement capability. Low PRF mode can't use doppler frequency detection so it's essentially a pulse radar mode (like the radar in MiG-21). Hence it can't filter out ground returns by doppler shift either. You can still detect targets if you set up the geometry so that ground clutter is minimized (target above horizon and radar sufficiently high above ground) or target is so close that the target has stronger echo than the ground. You can also see targets below horizon if the "ground" doesn't produce much echoes like over calm sea. What you see displayed is something like a weather radar or marine radar. Because of small pulse width LPRF has superior range resolution. On the other hand because of small duty cycle it has low average power and hence low detection range.

Range resolution plays a role when trying to detect two targets flying close to each other. When you have good range resolution, the targets need to fly much closer to be detected as single target compared to if you had bad range resolution. DCS doesn't model resolution cells so all radars have infinitely small resolution cells and therefore you cant hide your presence by flying close to another plane.

I don't understand all, but i understood the essential i think...

High PRF:

- Long range detection (high power, why ?).

- Good velocity resolution (best for ground clutter rejection)

- Bad range resolution (computed on PRF cycle basis)

Low PRF:

- Short range detection (limited by power, why ?).

- Bad velocity resolution (even non existant, ground clutter rejection almost impossible)

- Good range resolution (high precision in return time computing)

This is a detail (in our context) but what i don't understand is why Low PRF can't use doppler shift to evaluate velocity ?

I don't understand all, but i understood the essential i think...

 

High PRF:

- Long range detection (high power, why ?).

- Good velocity resolution (best for ground clutter rejection)

- Bad range resolution (computed on PRF cycle basis)

 

Low PRF:

- Short range detection (limited by power, why ?).

- Bad velocity resolution (even non existant, ground clutter rejection almost impossible)

- Good range resolution (high precision in return time computing)

 

This is a detail (in our context) but what i don't understand is why Low PRF can't use doppler shift to evaluate velocity ?

High power because you are transmitting with full radar power for a longer time, as Bushmanni said, up to 50% duty cycle. More emitted power, more reflected power, easier detection.

Vice versa low duty cycle low amount of transmited power low amount of reflected power low range.

Low PRF cant use doppler filter [forgot that one myself] because the maximum doppler frequency you could technically identify is the PRF, and that often doesnt bring you further then a few 100kph. So completly unusable for a lookdown radar mounted on a supersonic fighter.

Thank you for the explanations Bushmanni ! Really brought some light in the hole stuff.

So, Am I correct, assuming that for removing the problem of detecting a nearly 0 relative velocity target against the sky you simply switch the doppler filter off, as soon as that is implemented, and you only switch to low prf if its below the horizon and hope the target reflection is stronger then the ground returns you are getting ?

That would actually make sense to me :D

How about you guys stop guessing and read the manual? It`s explained there very well what the difference is as well as when to use what PRF setting.

Additionally, really, don`t ever quote wiki as a source. Just don`t. :doh:

How about you guys stop guessing and read the manual? It`s explained there very well what the difference is as well as when to use what PRF setting.

 

Additionally, really, don`t ever quote wiki as a source. Just don`t. :doh:

Its not about when to use what, that is easy ans well understood. :D

The question is what happens on a technical level in the real thing in the real plane.

How does the signal look like, what does the doppler filter reject in what mode and what does the computer have to deal with afterwards. :D

Last time I read the manual there was no big theoretical part about pulse doppler radar functionality. :D

EDIT: just checked the manual again, no unexpected theoretical part about pulse doppler radar signal processing ! :D

High power because you are transmitting with full radar power for a longer time, as Bushmanni said, up to 50% duty cycle. More emitted power, more reflected power, easier detection.

Vice versa low duty cycle low amount of transmited power low amount of reflected power low range.

I guess it is related to an hardware limitation that cannnot emit at full power instantaneously... or i don't understand why "duty time" could affect power...

Low PRF cant use doppler filter [forgot that one myself] because the maximum doppler frequency you could technically identify is the PRF, and that often doesnt bring you further then a few 100kph. So completly unusable for a lookdown radar mounted on a supersonic fighter.

Here i don't understand what you mean by "doppler frequency" in relation with the PRF... there obviously something i don't understand correctly about Pulse/Listen logic... in my mind the pulse emits a wave with a given frequency (which is not related to PRF), then the radar recieve the echo with a shifted frequency... PRF should not affect this shift detection, except if this detection is not made as i believe.

Its not about when to use what, that is easy ans well understood. :D

 

The question is what happens on a technical level in the real thing in the real plane.

How does the signal look like, what does the doppler filter reject in what mode and what does the computer have to deal with afterwards. :D

 

Last time I read the manual there was no big theoretical part about pulse doppler radar functionality. :D

 

EDIT: just checked the manual again, no unexpected theoretical part about pulse doppler radar signal processing ! :D

You do see what question the thread started with, right? Check first post. Unless you guys de-railed, the answer he seeks lies in the manual. Also, the stuff you all are discussing is what it does. There is no talk on a technical level about how each of these PRF settings work.

If you are interested in this however, there are plenty of books to read about this (cannot remember any titles atm). No problem with that, or even getting your hands on them. I would only guess that you`d be yawning before you even get through the first page, as you need to be interested in this subject to bother with it.

my 5 cents ;)

EDIT: For pure basics, you can search on google for a book called "Radar handbook" by MerRill Skolnik, you should find a pdf of it among first links. Dude worked 30 years in US Naval Research Lab, he`s got skill and knowledge. Again, this is nothing more than basic understanding, because if you want to get further into this, then you gotta be an engineer and truly study this subject. This if you want to talk about technical level for radars and radar waves, etc... Believe me when I tell you that what is being discussed here is well, not to be offensive or anything but, well, kindergarten. There is nothing technical in this thread.

I guess it is related to an hardware limitation that cannnot emit at full power instantaneously... or i don't understand why "duty time" could affect power...

 

light bulb, emiting 60W of power.

If you shine that bulb at a ideal reflector reflecting 100% of the energy after one hour it has reflected 60Wh. An radar doesnt simply show a contact as soon as there is an return. There is a need for consecutive returns with matching cirterias. If you have an long pulse you get a longer echo => higher total pulse energy, "better" signal, longer range acquisition.

There is emiting power and power over time. And power times pulse length give you the whole pulse energy. :)

 

Here i don't understand what you mean by "doppler frequency" in relation with the PRF... there obviously something i don't understand correctly about Pulse/Listen logic... in my mind the pulse emits a wave with a given frequency (which is not related to PRF), then the radar recieve the echo with a shifted frequency... PRF should not affect this shift detection, except if this detection is not made as i believe.

To keep it simple, if the doppler frequency, the shift as you call it, exceeds the PRF, you cant tell anymore if its under the PRF or a doppler frequency multiplied by a factor you are looking at. Same as with the range. If the echo takes longer then the listening time you cant tell if you are listening to an echo from the cycle which is running at that moment or one of the impulses before.

If you want to get your brain even more confused here: https://en.wikipedia.org/wiki/Frequency_ambiguity_resolution :D have fun :D

Yes we are derailing it, a bit at least. Or lets say evolving the original theme. :D

The initial question was already answered wasnt it ?

EDIT: Thx for the info will have a look.

If you want to get your brain even more confused here: https://en.wikipedia.org/wiki/Frequency_ambiguity_resolution :D have fun :D

In fact that made my brain less confused. If i well understand in fact the recieved frequency is sampled through PRF cycles (one sample by "listening" phase, causing aliasing problem), and i thought this frequency was sampled directly during one "listening" phase.

In fact that made my brain less confused. If i well understand in fact the recieved frequency is sampled through PRF cycles (one sample by "listening" phase, causing aliasing problem), and i thought this frequency was sampled directly during one "listening" phase.

That is good and correct ! :)