Difference between revisions of "Velocity aliasing"

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<div class="definition"><div class="short_definition">(''Also called'' 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.</div><br/> <div class="paragraph">This phenomenon occurs in [[Doppler velocity]] measurements when the [[maximum unambiguous  velocity]] interval (&plusmn;''V''<sub>max</sub>) is less than the full range of velocities being measured. Any true  velocity, ''V'', appears within the interval from -''V''<sub>max</sub> to +''V''<sub>max</sub>, with the value ''V''&prime;, which is related  to the true velocity by ''V'' = ''V''&prime; &plusmn; 2''n''''V''<sub>max</sub> where ''n'' is an integer. Therefore a given measured velocity  ''V''&prime; may be caused by many values of the true velocity ''V''. For example, suppose ''V''<sub>max</sub> = 25 m s<sup>-1</sup>  and the measured velocity ''V''&prime; = -15 m s<sup>-1</sup>. Then the values of true velocity that could account  for this measurement are the following: -15 m s<sup>-1</sup> (for ''n'' = 0); +35 or -65 m s<sup>-1</sup> (for ''n'' = 1);  +85 or -115 m s<sup>-1</sup> (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. <br/>''See also'' [[aliasing]], [[Nyquist frequency]].</div><br/> </div>
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<div class="definition"><div class="short_definition">(''Also called'' 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.</div><br/> <div class="paragraph">This phenomenon occurs in [[Doppler velocity]] measurements when the [[maximum unambiguous velocity|maximum unambiguous  velocity]] interval (&plusmn;''V''<sub>max</sub>) is less than the full range of velocities being measured. Any true  velocity, ''V'', appears within the interval from -''V''<sub>max</sub> to +''V''<sub>max</sub>, with the value ''V''&prime;, which is related  to the true velocity by ''V'' = ''V''&prime; &plusmn; 2''n''''V''<sub>max</sub> where ''n'' is an integer. Therefore a given measured velocity  ''V''&prime; may be caused by many values of the true velocity ''V''. For example, suppose ''V''<sub>max</sub> = 25 m s<sup>-1</sup>  and the measured velocity ''V''&prime; = -15 m s<sup>-1</sup>. Then the values of true velocity that could account  for this measurement are the following: -15 m s<sup>-1</sup> (for ''n'' = 0); +35 or -65 m s<sup>-1</sup> (for ''n'' = 1);  +85 or -115 m s<sup>-1</sup> (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. <br/>''See also'' [[aliasing]], [[Nyquist frequency]].</div><br/> </div>
 
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Latest revision as of 17:11, 25 April 2012



velocity aliasing

(Also called 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 interval (±Vmax) is less than the full range of velocities being measured. Any true velocity, V, appears within the interval from -Vmax to +Vmax, with the value V′, which is related to the true velocity by V = 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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