1. A scan-to-scan integrator for use in radar apparatus, comprising: window defining means for defining a search window for use in verifying a candidate detection, the window being bounded by an intersection of a first zone and a second zone so as to be smaller than the first and the second zone, the first zone comprising a rectangular detection window relative to a Cartesian reference frame, and the second zone comprising a range estimate tolerance region defined by a range estimate for the candidate detection and predetermined tolerance limits, this second zone being bounded by arcuate bounds centred on an antenna of radar apparatus with which said integrator is employed.