velocity folding.) A basic sampling problem that arises when the unambiguous velocity
sampling interval is less than the full range
of naturally occurring velocities, causing the erroneous appearance of higher velocities within the sampling interval.
This phenomenon occurs in Doppler velocity
measurements when the maximum unambiguous velocity
) is less than the full range of velocities being measured. Any true velocity, V
, appears within the interval from -Vmax
, with the value V
′, which is related to the true velocity by V
′ ± 2n'Vmax where n is an integer. Therefore a given measured velocity V′ may be caused by many values of the true velocity V. For example, suppose Vmax = 25 m s-1 and the measured velocity V′ = -15 m s-1. Then the values of true velocity that could account for this measurement are the following: -15 m s-1 (for n = 0); +35 or -65 m s-1 (for n = 1); +85 or -115 m s-1 (for n = 2); etc. In some instances the erroneous velocities can be recognized and ambiguities resolved by additional considerations, such as the requirement of spatial continuity of the velocity field.
See also aliasing, Nyquist frequency.
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