Chapter 6 ______________________________________________________ Processing Algorithms
VAISALA______________________________________________________________________ 221
If the absolute value of the true velocity of the scatterers is greater than V
u
,
then the velocity calculated by the RVP900 is folded into the interval [- V
u
,
V
u
] , which is called the Nyquist interval. Folding is usually easily
recognized on a color display by a discontinuous jump in velocities. For
example, if the true velocity is V
u
+ ΔV , then the velocity calculated by the
RVP900 is - V
u
+ ΔV , which is 2V
u
away from the true mean velocity.
For 8-bit outputs, rather than calculating the absolute velocity in scientific
units, the RVP900 calculates the mean velocity for the normalized Nyquist
interval [-1,1], that is, the output values are,
For example, an output value of -0.5 corresponds to a mean velocity of -
V
u
/2. The normalized velocity V' is more efficient use of the limited
number of bits.
6.3.5 Spectrum Width Algorithms
The spectrum width is a measure of the combined effects of shear and
turbulence. To a lesser extent, the antenna rotation rate can also effect the
spectrum width. At high elevation angles, the fall speed dispersion of the
scatterers also effects spectrum width.
There are two choices for the spectrum width algorithm used in the
RVP900, depending on the speed and accuracy that are required for the
application:
R
0
, R
1
"fast" algorithm valid when SNR >> 10 dB
R
0
, R
1
, R
2
"accurate" algorithm for SNR >> 0 to 5 dB
The approach used is selected in the SOPRM command.
R0, R1 Width Algorithm
Given samples of the Doppler autocorrelation function, numerous
estimates of spectral variance can be computed (Passarelli & Siggia, 1983).
The particular estimator used by the RVP900 employs the magnitudes of
R
0
and R
1
and assumes that the Doppler spectrum is Gaussian (usually an
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