With processed primary radars (including Watchman) its the groundspeed of the target that decides whether it will be displayed on a radar screen; if you're flying below the threshold speed you'll be automatically deleted 'cos the stupid processor doesn't know you're an aircraft, and assumes the controller doesn't want to see you. With Watchman, the controller can select 'background video' which allows view of non-processed radar, but the newer ASR10 does not have this facility.
Positioning of an SSR aerial on an aircraft CAN affect the response characteristics, for instance, it the aerial is close to the fin, then in 'tail on' aspect to the radar head, the reply can be masked leading to loss of the SSR label from the display. Chinooks seem to be particularly prone to this phenomenon.
Last edited by chevvron; 19th Feb 2007 at 08:25.