Clutter folding is a term used in describing "clutter

Clutter (radar)

Clutter is a term used for unwanted echoes in electronic systems, particularly in reference to radars. Such echoes are typically returned from ground, sea, rain, animals/insects, chaff and atmospheric turbulences, and can cause serious performance issues with radar systems.- Backscatter coefficient...

" seen by 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. Clutter folding becomes a problem when the range extent of the clutter (seen by the radar) exceeds the pulse repetition frequency

Pulse repetition frequency

Pulse repetition frequency or Pulse repetition rate is the number of pulses per time unit . It is a measure or specification mostly used within various technical disciplines Pulse repetition frequency (PRF) or Pulse repetition rate (PRR) is the number of pulses per time unit (e.g. Seconds). It...

 interval of the radar, and it no longer provides adequate clutter suppression, and the clutter "folds" back in range. The solution to this problem is usually to add fill pulse

Fill pulse

Fill pulse is a term used in radar 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...

s to each coherent dwell of the radar, increasing the range over which clutter suppression is applied by the system.

The tradeoff for doing this is that adding fill pulses will degrade the performance, due to wasted transmitter power and a longer dwell time.