How is frequency difference and time difference measured in FMCW radar?

Answer 1

Frequency modulated continuous-wave (FM-CW) radar is usually used with a couple of different antennas. One is for the transmitted signal, and another one (or more) is (are) used for the reception of any returns. We usually think of radar as a pulse of electromagnetic energy that is transmitted out, and then a receiver looks for any returning (reflected) signal. In FM-CW radar, the transmitter is "always on" and a continuous signal is being sent. Additionally, the frequency of the transmitted signal is varied (or "swept") above and below a center frequency which the local oscillator runs on and maintains. The new units are largely microprocessor controlled pieces of equipment. They take the now-FM-modulated signal and broadcast it continuously (per the name - CW, which is continuous wave), and the returns are picked up by a receiver. The frequency of the return signal is compared to the instantaneous frequency of the transmitter, and the frequency difference can be looked at by a processor which can derive a time difference between the two signals (based on the differential frequency). This derived time difference will be proportional to the distance to the target that returned the signal. Another bit of processing on the signal returned from that target will produce a rate of change of distance, which can be converted to speed. As the power of the processors that we task with these objectives in mind has increased so dramatically, the processor can plot a course and speed with all appropriate vectors we wish to visualize when it puts up its findings on some kind of display - and in color! Particulars become technical very quickly, but the overview is accurate. A link to the Wikipedia article on FMCW is supplied, but at this writing, the article is still a stub that will supply little more than is here already.