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The principle of operation of pulse radar is based on time separation between the transmitted and reflected signal and measurement of the time interval between transmitted and received pulses. In FM radar, because continuous radiation is used, separation of transmitted and reflected signals in time is impossible. Hence, reception of information about the range to a target is possible only when the transmission is modulated in amplitude or phase.

Amplitude modulation is not used because it is practically impossible to select the reflected signal against the interfering background of the transmission, the reflected signal from targets even a few meters from the radar being some tens of decibels less than the transmitted signal. Thus the only means to determine the reflected signal delay relative to the transmission is on the basis of the phase difference of these angular modulated signals. This operation can be easily carried out by multiplication of transmitted and reflected signals. After multiplication, two signals are formed, one with a phase equal to the difference of phases of the multiplied signals, and the other equal to the sum of these phases. The later signal is easily filtered out, as its frequency is twice that of the radiated signal.

Reference:

Komarov, I. V. & Smolskiy, S. M. Fundamentals of short-range FM radar Artech House radar library, Artech House, 2003, -