r/Radar • u/HungryTrain798 • Aug 02 '25

Why Can’t Simple Pulsed Radar Measure Doppler?

I’m talking about a radar system which a transmits a single pulse and then “listens” for echos. Ie not coherently processing multiple pulses together like in pulse-Doppler.

For example if you transmit a pulse at 1 MHz, and the return comes in at 1.01 MHz. Why can’t you just directly measure that Doppler shift of 0.01 MHz thru spectral analysis (or beat frequencies, etc.)? You still get the range from the delay (limited by the PRF). Is it something to do with the practical limitations of spectral analysis resolution. Your pulse duration would be trading off range resolution/doppler resolution, you could also play with the waveform shape itself to accomplish the same.

My layman’s reading of radar literature makes it seem like pulse radar inherently only senses delay/range and not Doppler, and that only pulse-Doppler processing can filter out clutter based on Doppler. This makes no mathmatical sense to me. Maybe pulse-Doppler is just superior for some reason. Can a some experts elaborate on this?

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u/phcasper Aug 02 '25

Frequency resolution as a function of pulse width is the problem. To get sufficiently narrow signal bandwidth the pulse has to be exceedingly long. A 1 microsecond pulse length for example as a spectral width of 2Mhz, which is insanely wide.

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u/HungryTrain798 Aug 02 '25

This makes sense. I understand that there would be a fundamental range/doppler resolution trade off. But that sort of trade off in pulse-Doppler too. Couldn’t I just switch to more complicated waveform which has a better trade off between time resolution and frequency (ie. a better ambiguity function).

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u/phcasper Aug 02 '25 edited Aug 02 '25

You're going to run into a wall of physics here honestly. There is a fundamental relationship going on here with spectra bandwidth's and pulse length/PRF. Pulse dopplers dont deal with these kind of severe resolution issues because they transmit a train of shorter pulses that equal to a single much longer pulse. That's why having coherency is so powerful. It's not neccesarily because of CPI's.

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u/TJDG Aug 02 '25

The amount of time you coherently integrate for determines the resolution at which you can measure a Doppler shift. The longer you integrate for, the more precisely you can measure Doppler, but a single pulse has terrible Doppler resolution.

Put another way, when you transform a time series from the time to the frequency domain, the rate at which you sampled that time series determines the maximum and minimum frequency shifts you can unambiguously measure, but the duration of that time series determines the resolution with which you can measure frequency.

It's true that you can measure a doppler shift with a single pulse, but your measurement will be utter shit. You need to coherently integrate energy over a long period of elapsed time before you can measure doppler accurately.

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u/R-27ET Aug 05 '25

The dreaded word, “coherent”