prf radar modes

Understood everything but the last paragraph.

 

Would you be so kind to tech me more about this? I have been reading a lot of stuff online and Im beginning to understand a lot as well. Just need to polish some things like these.

 

 

Thanks in advance.

Pulse compression is a term used to describe any form of intra-pulse modulation(modulation that occurs during the pulse, rather than between.. ie frequency or phase modulation) that enables the use of specialized signal processing that improves detection range, reduces noise (from environment and jammers), and improves range resolution.

example

As you may know, long pulse durations are good for increased detection ranges (more energy on target). But having a long pulse reduces your radars range resolution. If two targets are separated by 100 ft in range, and your pulse is 400 ft in length the return from the two targets will look like one long return (one large target) rather than two. Pulse durations need to be short enough to see the 'nothing' between the two targets.

Given the same scenario

'two targets on the same azimuth separated by 100 ft in range'

a pulse of less than than 200 ft would be needed to see two separate returns from the two targets. 200 ft because even though the targets are separated by 100 ft, the pulses of light have to travel out an extra 100 ft to the second target and than an extra 100 ft back. thus the second reflection trails the first reflection by 200 ft.

Anyway, back to pulse compression.

One form of pulse compression is linear frequency modulation (commonly referred to as a CHIRP) on the pulse. This means that the carrier frequency of the pulse gradually slides to a higher or lower frequency.

Radar engineers want long pulses for improved detection ranges but ALSO fine range resolution that you'd get from a short pulse. By using a CHIRP, they can emit a single long pulse, say 500 ft long. Upon receiving the reflections from the target, the receiver will filter the long pulse duration by frequency into maybe 100 ft sections. Then insert a delay and sum the outputs together.

The result is a short pulse with the amplitude of the long pulse. If two returns exist, then the output would have two separate pulses.

Use this visual reference.

http://www.radartutorial.eu/08.transmitters/pic/pulscompression.big.gif

The visual reference above has a pulse compression ratio of 5:1

If the pulse duration was 5 microseconds long than the 'effective' pulse duration is 1 microsecond.

But Im now interested in that PRF modulation. What is it exactly?

PRF/PRI modulation is a form of inter-pulse modulation (the altering the time between pulses). I can explain it later when im not busy.

One form of pulse compression is linear frequency modulation (commonly referred to as a CHIRP) on the pulse. This means that the carrier frequency of the pulse gradually slides to a higher or lower frequency.

 

Radar engineers want long pulses for improved detection ranges but ALSO fine range resolution that you'd get from a short pulse. By using a CHIRP, they can emit a single long pulse, say 500 ft long. Upon receiving the reflections from the target, the receiver will filter the long pulse duration by frequency into maybe 100 ft sections. Then insert a delay and sum the outputs together.

So the modulation occurs during the output (pulse) or when the echoes are recieved?

Thanks for your reply.

So the modulation occurs during the output (pulse) or when the echoes are recieved?

 

Thanks for your reply.

Modulation occurs during the transmission of the pulse. Imagine having a flashlight that starts it's 'flash' as red light but gradually slides to green and then to blue... Every time you hit the flashlight button it starts red and slides through spectrum to blue.

Excellent.

Also, why is this technique only applied to high prf and not low prf? Low prf is meant to have strong and long pulses, so why doppler effect is not present there? Frequency shift will always be possible.

Bushmani said above that low prf has small pulse width. I think thats quite incorrect and in fact the opposite. Right?

Modulation occurs during the transmission of the pulse. Imagine having a flashlight that starts it's 'flash' as red light but gradually slides to green and then to blue... Every time you hit the flashlight button it starts red and slides through spectrum to blue.

I think I have a good question:

If the modulation is in frequency, then the doppler effect will tamper with the original frequency when the reciever gets the echo. How does it know what frequency portion is which?

ok

I think I have a good question:

 

If the modulation is in frequency, then the doppler effect will tamper with the original frequency when the reciever gets the echo. How does it know what frequency portion is which?

Short version: compensating for doppler distortion in FM coded pulses is done using complicated magic, ehm, digital signal processing.

Long version: here's an IEEE paper on the topic: http://ecs.syr.edu/faculty/sarkar/pdf/2007_IEEE_Radar_297.pdf

For the mathematically inclined, Bassem Mahafza's books ("Radar Systems Analysis and Design Using Matlab" and "Matlab Simulations for Radar Systems Design") do a quite good job explaining these phenomena.

Excellent.

Also, why is this technique only applied to high prf and not low prf? Low prf is meant to have strong and long pulses, so why doppler effect is not present there? Frequency shift will always be possible.

1. LPRF systems can use CHIRP. (linear RF modulation during the pulse) Perhaps you are confusing CHIRP with Frequency Modulated ranging (FMR)? FMR offers poor range accuracy and degrades coherent pulse integration. FMR is mainly used with HPRF since range is ambiguous. It may also be used with any waveform (LPRF, MPRF) for electronic protection/anti-jamming purposes.

2. Doppler is ambiguous with low prf. Doppler processing cannot be achieved by an airborne system with a low PRF. However, if they have a coherent waveform they can perform MTI. Which is to say they can filter out the ground clutter but they cannot determine a targets velocity. If you'd like to know more about doppler ambiguity read the links below

http://www.radartutorial.eu/01.basics/Doppler%20Dilemma.en.html

https://en.wikipedia.org/wiki/Frequency_ambiguity_resolution

Bushmani said above that low prf has small pulse width. I think thats quite incorrect and in fact the opposite. Right?

LPRF waveforms are not restricted to small or large pulse widths. older LPRF tracking radars had small pulse widths for range resolution. Modern LPRF radars likely use longer pulse durations with pulse compression because it gives them good range resolution AND more energy on target.

I think I have a good question:

 

If the modulation is in frequency, then the doppler effect will tamper with the original frequency when the reciever gets the echo. How does it know what frequency portion is which?

1. if you are talking about a CHIRP, then to answer your question doppler isn't derived from a single pulse. Doppler is derived from a series of pulses. The shorter the FFT the more ambiguous the frequency spectrum. Remember, frequency is the measure of cycles per second. Running a FFT on a single pulse would smear the doppler spectrum. Also, there would be aliasing due to the de-synchronization of the PRF to the Doppler frequency of the target. Also, there are a large number of doppler bins if you're worried about a doppler shift affecting the pulse compression

This again may help

https://en.wikipedia.org/wiki/Frequency_ambiguity_resolution

2. if you are again referring to FMR, then yes range errors could occur from the targets doppler IF you were using a saw-tooth waveform. However, a triangular waveform allows you to determine the range and doppler of a target by sweeping the frequency (pulse to pulse, not within one pulse) both directions.

Read this page throughly

http://www.radartutorial.eu/02.basics/Frequency%20Modulated%20Continuous%20Wave%20Radar.en.html

1. LPRF systems can use CHIRP. (linear RF modulation during the pulse) Perhaps you are confusing CHIRP with Frequency Modulated ranging (FMR)? FMR offers poor range accuracy and degrades coherent pulse integration. FMR is mainly used with HPRF since range is ambiguous. It may also be used with any waveform (LPRF, MPRF) for electronic protection/anti-jamming purposes.

 

2. Doppler is ambiguous with low prf. Doppler processing cannot be achieved by an airborne system with a low PRF. However, if they have a coherent waveform they can perform MTI. Which is to say they can filter out the ground clutter but they cannot determine a targets velocity. If you'd like to know more about doppler ambiguity read the links below

 

http://www.radartutorial.eu/01.basics/Doppler%20Dilemma.en.html

 

https://en.wikipedia.org/wiki/Frequency_ambiguity_resolution

 

 

 

 

LPRF waveforms are not restricted to small or large pulse widths. older LPRF tracking radars had small pulse widths for range resolution. Modern LPRF radars likely use longer pulse durations with pulse compression because it gives them good range resolution AND more energy on target.

 

 

 

 

1. if you are talking about a CHIRP, then to answer your question doppler isn't derived from a single pulse. Doppler is derived from a series of pulses. The shorter the FFT the more ambiguous the frequency spectrum. Remember, frequency is the measure of cycles per second. Running a FFT on a single pulse would smear the doppler spectrum. Also, there would be aliasing due to the de-synchronization of the PRF to the Doppler frequency of the target. Also, there are a large number of doppler bins if you're worried about a doppler shift affecting the pulse compression

 

This again may help

https://en.wikipedia.org/wiki/Frequency_ambiguity_resolution

 

2. if you are again referring to FMR, then yes range errors could occur from the targets doppler IF you were using a saw-tooth waveform. However, a triangular waveform allows you to determine the range and doppler of a target by sweeping the frequency (pulse to pulse, not within one pulse) both directions.

 

Read this page throughly

http://www.radartutorial.eu/02.basics/Frequency%20Modulated%20Continuous%20Wave%20Radar.en.html

Ok, now Im more confused than ever. You brought up several concepts to the table but explained none. Those articles are meant to be read by more knowleded people-

If you want to understand all those concepts, maybe a forum is not the best way to satisfy your curiosity (I'm not criticizing Beamscanner, he is doing a great job explaining all those concepts, however I fear simultaneously trying to explain multiple radar technics to different people will result in a bit of confusion) try to read the beginning of the Introduction to Radar system by Mr. Skolnik or, if you understand French, the books by Mr. Darricau (they are free and legal to download except for the book by Mr. Skolnik). They provide a good base (even though I found both of them to be a bit lacking behind in terms of technology).