Clutter is everything the radar sees but does not want to see: waves, trees, buildings, rain. Against a busy background a small target is hard to find.
In practiceAnti-ship missiles fly a few metres above the waves (sea-skimming) to hide in the sea clutter and below the radar horizon (Radar horizon). Ship radars use Doppler and rapid updates to find them in time all the same.
Ground clutter is the unwanted echo of the earth's surface: hills, woods, buildings, fields and waves all reflect the radar signal. For a ground radar or a look-down airborne radar this is the biggest detection problem there is — the clutter echo can be 40 to 70 dB (10,000 to 10 million times, Decibels and logarithms) stronger than the echo of a low-flying target.
Characteristics of ground clutter:
- Almost zero Doppler: the ground stands still, so the clutter echo sits around Doppler frequency 0 (Doppler effect). This is the key to suppressing it.
- Distributed: clutter fills many range and angle cells at once, unlike a compact point target.
- Dependent on grazing angle and terrain type: steep incidence and rough/wet terrain give more reflection; a smooth sea at a low grazing angle gives little (until it suddenly gives a lot in higher sea states — "sea spikes").
How radars suppress clutter:
- MTI / pulse-Doppler (Pulse-Doppler radar and Moving Target Indicator): filters out everything around zero Doppler and keeps moving targets — the most important technique.
- Clutter maps: the radar remembers where fixed clutter sits and ignores those cells.
- STC (Sensitivity Time Control): reducing the gain just after the pulse, because nearby clutter is strongest.
- CFAR (Threshold, Constant False Alarm Rate and Automatic Gain Control): moving the threshold with the local clutter level.
Clutter is also a hiding place. In the notch/beaming manoeuvre an aircraft flies across the radar's line of sight so that its Doppler drops to zero — it then disappears into the clutter filter the radar itself uses (Pulse-Doppler radar and Moving Target Indicator). Chaff (Chaff) creates artificial clutter, and indirect terrain masking (Terrain masking) lets a target deliberately blend into the ground clutter.