Phase noise, coherency, and clutter suppression

A Doppler radar measures the radial velocity of a return by measuring the phase shift of the received signal with respect to an appropriately delayed sample of the transmitted signal. As the measurement is one of a shift in phase, any noise in the phase component of the signal (phase noise) will limit the velocity detection capability and any processes that rely on this ability, such as clutter suppression. The more noise existing in a system, the less effective the filters will be at suppressing clutter.

Phase noise is an important performance characteristic, yet there remains confusion in specifications of a radar systems concerning phase noise characteristics and its relationship to clutter rejection. The operator is primarily concerned with the latter. The radar purist is concerned with the former. This paper draws upon our experience with the National Weather Service, Evansville radar program, the associated analyses and testing performed to achieve active clutter suppression greater than 65 dB. From this analysis, misconceptions concerning phase noise and clutter suppression are identified and mitigated. Finally a mathematical model to estimate the clutter suppression available to a radar system is presented and evaluated.