Originally Posted by a64pilot
I don't think the 4G radar is chirp, I think it is a continuous transmitted constant power, but with frequencies that sweep up and down, the return signal based on its frequency will determine the time since that frequency was transmitted and based on time distance can be computed as speed of light is a constant.
However if it does work like that, how does it handle Doppler shift?
I've not seen a decent operating principle of the thing in print myself.
FMCW (aka 4G) is a chirp. But it does not use pulse compression
at the receive end. Instead it multiplies the received signal by the transmitted signal. The output of the multiplier produces the sum and difference frequency between the transmitted and received signals. The sum is filtered out leaving only the difference. The received frequency will a little different than the transmit frequency owing to the round trip speed of light time to the target (the transmit frequency has increased by the time the reflected signal returns). The exact difference in frequency being a linear function of the out and back distance to the reflecting target. There is a "magic" box (circulator) in the antenna
that can separate the transmitted from the received energy so the transmitter can be always on without interfering with the receiver. The chirp can either be a linear ramp
up or down in frequency with the results being the same in either case.
A target moving away from the transmitter will be shown ever so slightly further away than it was when illuminated due to doppler shift but at boat
speeds this error is insignificant. It's actually a interesting case study in the uncertainty principle (can't know both position and speed simultaneously).
make both FMCW and pulse compression