Notching. Which is more effective on older pulse doppler. Modern AESA not so much.Most radars have frequency filters to assist them in discriminating likely targets out of busy backgrounds. One of those filters is speed relative to the emitter and other background.
I believe one of the tactics in trying to counter radar during air engagements is to have yourself moving at a direction roughly perpendicular to a radar source when they are trying to discriminate and get a target quality lock. This can put your plane in their "slot" where their filters tune you out because you have no approach or departure vector in relation to the emitter and receiver. Exploiting this software filter has been very successful in BVR maneuvering against an opponent.
No expert on any of the above.
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If a lab radar can pick up a bullet with battery power, think of what a multiple kilowatt array can detect. To change the filtering to look at small slow moving targets would definitely mean you are tracking birds.
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