h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Hi All,

Can anyone explain the following:

1) What's the difference between Up Chirp and Down Chirp?

2) Any advantages between them. Why some radar uses Up and some Down Chirp and some even both?

3) How wide should the Chirp range be in order to be effective? Any practical value?

4) For radar that uses both Up and Down Chirp doesn't it cause interference to the radar itself? What I meant is can a radar uses a PW eg, 50 µs for both Up and Down Chirp simulatenously but at different radio frequency?

5) What's the difference between linear and non-linear Chirp? Any advantages between each other?

Thanks for your attention.

Hello R200104 (how about giving yourself a nickname?)

1) What's the difference between Up Chirp and Down Chirp?
I had hopes that this would be clear from the entry and the sketch . An up chirp has the low frequency portion first and would sound like 'ooooeeeiiiip' if it was performed by a singer. A down chirp goes the other way round: 'iiiieeeoooom'.


2) Any advantages between them. Why some radar uses Up and some Down Chirp and some even both?
AFAIK, there's no difference as far as the properties are concerned. Otherwise, it depends on which type of matched filter can be manufactured easier. Splitting an inventory of radars into some that use up-chirp and others using down-chirp also helps to avoid mutual interference: if a down-chirp radar were to receive a non-modulated pulse then this pulse doesn't get compressed and thus only contributes to the noise floor. If the same radar were to receive an up-chirped pulse then this pulse gets *spread out* rather than compressed, so its effect is even less.


3) How wide should the Chirp range be in order to be effective? Any practical value?
That's completely up to you Long range HF radars are limited to chirp bandwidths of a few kHz unless the differences in propagation delay among the frequency components are taken care of. For the others, this is a question of how to keep all the filters linear across the chirp bandwidth. The upper limit is given by the antenna, because the operating frequency enters into the equation for its beamwidth. Chirp and 'frescan' antennas (frequency scanning antennas, like the one of the AN/SPS-52C) are hard to combine, if not completely incompatible.


4) For radar that uses both Up and Down Chirp doesn't it cause interference to the radar itself? What I meant is can a radar uses a PW eg, 50 µs for both Up and Down Chirp simulatenously but at different radio frequency?
Of course, if the signal from subsystem #1 enters subsystem #2 and is doppler-shifted such that it's within the bandwidth then you're adding to the noise floor.

5) What's the difference between linear and non-linear Chirp? Any advantages between each other?
I tried to explain that by means of the sketch... linear chirp has a constant rate of frequency change, non-linear chirp hasn't. The rest is all to do with Woodward's ambiguity function, thumbtacks and weird maths For more details, I'd like to recommend 'Radar Design Principles' by Nathanson et al.


Bossel

Bossel, thanks for your explanation.

RADAR ENTHUSIAST