PRF

Been messing around with the F14 trying to grok the pro's and con's of pulse doppler radar vs plain old pulse (in search mode, not STT). In a nutshell, if I'm understanding correctly, the advantage of doppler radar is it has at least some look-down capability.

I've heard PD in the Tomcat referred to as high-PRF and PS called med-PRF.

Is that true for other modules, like the F15 and the F18? In both cases, med-PRF helps you find a flanking target, but will it have trouble picking something out of ground clutter?

Pulse Repetition Frequency has to do with the ranging algorithm. In order for a radar to accurately measure contact range, each pulse's echo has to return to the transceiver during its own bespoke interval; that is to say, it has to come back before the next pulse is sent, otherwise the radar could interpret the first echo as a response to the second pulse which would ruin the range estimate.

High PRF is useful for tracking contacts that are either already inside visual range, or are in front aspect. The shortening range means that the echos are returning quickly, so the interval required for a pulse to traverse the distance between transceiver, contact, and back in time for the next pulse is shorter. Lower PRF is useful for tracking rear aspect contacts, for the opposite reason.

It does "work" or is simulated in most of the modules. Except things like helicopters whose blades should stick out loud on a doppler return, are subject to being very quiet on the filters.

Pulse Repetition Frequency has to do with the ranging algorithm. In order for a radar to accurately measure contact range, each pulse's echo has to return to the transceiver during its own bespoke interval; that is to say, it has to come back before the next pulse is sent, otherwise the radar could interpret the first echo as a response to the second pulse which would ruin the range estimate.

More or less, you can use FM ranging

High PRF is useful for tracking contacts that are either already inside visual range, or are in front aspect. The shortening range means that the echos are returning quickly

That can't be right, EM radiation always travels at the same speed

OK I found this https://www.eetimes.com/document.asp?doc_id=1278808#

I think the bottom line is, high PRF is better at resolving a wide range of closure speeds. Which makes it better at rejecting clutter, and better at picking up a fast target from far away. Medium PRF is better at ranging, giving you a chance to pick up somebody who is trying to notch you, but isn't as good at resolving targets down in ground clutter.

I get the feeling modern a/c are always using pulse doppler. Not totally clear to me that you can adjust the PRF in pulse doppler mode in the F14

If I'm right, that explains a lot...

More or less, you can use FM ranging .

 

That can't be right, EM radiation always travels at the same speed

Actually , as a former USMC radar tech (5941) , i found his reply to be spot-on .

To amplify your post, which is totally correct but misses the LPRF piece of the picture:

HPRF is better for resolving targets based on their closure (Doppler effect,) thereby reducing susceptibility to ground clutter. Targets at high Vc can be detected at higher range. However, range ambiguity is high and so an HPRF mode will give poor range information. (Think velocity search modes.) (Good Doppler resolution, poor range resolution.)

LPRF gets a lot of sidelobe clutter and has poor Doppler resolution, but provides very accurate range resolution. LPRF modes need to be air to ground radar or look-up only, hence why they are rarely seen in fighter applications. (Poor Doppler resolution, good range resolution.)

MPRF is a balance. It can handle ground clutter better than LPRF, and resolve range better than HPRF. Because there are ambiguities in both range and Doppler, it requires more advanced signal processing and automation. This is why you see more challenging manual functionality in older radar sets like the AWG-9 showing various depictions of velocity and range, whereas the Hornet's APG-65 is all an automated B-scope.

The F-35 and other modern fighters, for further contrast, have a true "god's eye view" PPI display, without the distortion of the B-scope. Progress...

EM radiation always travels at the same speed

Yes, you're right. The speed of the energy wave doesn't change enough to be noticed, but because it travels the same speed it takes longer to travel 40 miles than it does to travel 20 miles. Its constant speed is what makes it usful for range determination. The point is, range data (without FM ranging) requires that the interval between pulses take longer than the pulse's time of flight. The purpose of changing PRF rates is to resolve range ambiguity.

The main difference between Pulse-search and PD-search is not PRF, but rather the method for resolving background clutter. Picking a target out of ground clutter requires that either the contact be moving towards or away from you fast enough to trigger a doppler shift (in PD-search mode), or that the target be physically close enough to you that the contact's return is coming back significantly faster than the ground return (in Pulse-search mode). In Pulse-search mode, you can adjust the PRF and range gate until the range bins containing the ground return are filtered out, and thus not displayed. In PD-search mode, the notch filter removes everything without a doppler shift.

Oh, OK i get it. Thanks! That's explaining a lot actually.

I can be hard-headed sometimes but I really appreciate you guys teaching me this stuff, I could just play the game I guess but I find this interesting

I guess but I find this interesting

I completely understand! :thumbup:

I read whole textbooks on this kind of stuff because i think the subject is fascinating

There's a little more that's going on there but that website gets the gist of it (it doesn't get into blind speeds). Another method of resolving the range ambiguity is changing the pulse interval. The one return that comes back at the same time across different intervals is the "real" one.