Considerations for the Design of Ground Clutter Cancelers for Weather Radar
Interim rept. Apr 1980-Feb 1981
NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION NORMAN OK NATIONAL SEVERE STORMS LAB
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Study of the ground clutter problem at the Norman site indicates that clutter cross sections of 40 to 50 dB below 1 sq. msq. m are typical. Doppler spectrum width, Tau sub c, of the clutter signal is between 0.1 and 1 ms with the mean value of 0.25 ms at an antenna rotation rate of 10 degs. The design of clutter filters is very much dependent on this width and for proper cancellation the passband cutoff velocity v sub p must be larger or equal to 4.8 Tau sub c. Several canceling schemes are investigated, and the most efficient one is with a third order recursive filter. This filter achieves a 50 dB rejection in the stop band with total annihilation of DC. One dB ripple is in the passband, and the ratio of passband cutoff v sub p to stop band cutoff v sub s is about 3.5. The filter operates best in steady state, but it can also be made to operate in transient by properly initializing its memory elements. Performance on 64 simulated and real time samples show that no more than 10 dB loss in effective canceling over steady state occurs if the filter is initialized. To minimize weather signal loss, canceling is recommended only in regions that contain clutter. Therefore, the range extent over which canceling is performed should vary with elevation. Also the antenna rotation at the two lowest angles should be slower.
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