Range folding is a radar sampling ambiguity that occurs when an echo returning from a target beyond the maximum unambiguous range arrives after the next pulse has already been transmitted, so the display either places that echo at the wrong distance or removes the velocity data entirely. It is one flavor of a broader problem called aliasing, which also affects Doppler velocity when winds exceed the radar's maximum unambiguous velocity.
The physics of the ambiguity
A pulsed radar measures range by timing how long an echo takes to return, and it assumes the returned pulse is the most recent one it sent. The maximum distance a pulse can travel out and back before the next pulse is transmitted defines the maximum unambiguous range.
If a target sits farther out than that, its echo comes home after the next pulse has already gone, and the radar mistakenly assigns it to the newer pulse. A storm 225 kilometers from the radar can appear at 25 kilometers instead when the unambiguous range is only 200 kilometers.
The Doppler dilemma behind it
Range and velocity ambiguity are linked by the pulse repetition frequency, or PRF, which is how often the radar sends pulses. A low PRF gives long unambiguous range but a small unambiguous velocity, so fast winds alias. A high PRF gives a wider velocity window but shortens the unambiguous range, so distant echoes fold.
This tradeoff is called the Doppler dilemma. Modern WSR-88D scan strategies use dual-PRF techniques and multiple elevation passes to keep both problems manageable, but neither can be eliminated at every angle.
How forecasters handle it
In AWIPS, range-folded velocity pixels typically display as a distinct color, often labeled 'RF', so operators can see where the algorithm gave up. When a suspected circulation sits inside a folded patch, forecasters check a lower or higher elevation tilt where the geometry may be cleaner, or pull data from an adjacent radar that sees the same storm from a different angle.
Second-trip echoes from range folding often have a stretched or grainy appearance and do not line up with returns on nearby radars. That mismatch is a useful clue that a feature is a ghost of a storm farther out rather than something real at close range.