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Vehicle collision warning / avoidance features are based on FM-CW Doppler or pulse Doppler radars operating at typical frequencies of 24, 60, or 77 GHz. 10.5 GHz forward looking, side looking, and / or rear looking heterodyne radars have also been fielded as a more economical solution than the millimeter wave systems
The simplest radar sensor approach uses a homodyne front end in which a portion of the transmitter power is used for the receiver LO. This zero frequency IF design results in a direct output of the Doppler frequency shift.
Use of a heterodyne (non-zero IF) improves S/N ratio by reducing 1/f noise, thus reducing transmitter output power requirements. Typical IFs for heterodyne systems are in the 1 to 2 GHz range.
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