threshold towards the new clutter signal level when the clutter transition is within a few range cells of cell under test 82 the alarm threshold is being calculated almost exclusively using range data units from second window 84 thus allowing greatly increased sensitivity however as the clutter transition approaches even closer it is necessary to quickly alter the alarm threshold to reflect the new clutter signal level to accomplish this transition detection circuit 112 closes all of the switches in first group of switches 86 and opens the switches in second group 88 immediately prior to the entry of the clutter transition into cell under test 82 consequently the new alarm threshold is set exclusively in response to range data units following the clutter transition as the clutter transition is shifted through second range window 84 transition detection circuit 112 closes switches in the second group 88 beginning with the first switch in group 88 so as to allow more and more of the range data units following the clutter transition to effect the alarm threshold calculation as is apparent the number of range cells coupled to the threshold calculation apparatus varies from 2n to n and back to 2n as the clutter transition passes through the apparatus this requires variation of the threshold constant used in the threshold calculation to reflect the.