Fill pulse is a term used in radar

Radar

Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

 systems, to indicate one or more pulses transmitted by the radar, for the express purpose of "filling in range". It serves no other purpose than to push out the range at which folded clutter is seen by the radar. More fill pulses means a longer range before clutter begins "folding" in to shorter ranges, causing clutter-related degradation of the target signal.