The dBZ0 fed into RVP900 is the basic
0
2
, while the processor modifies the value by
the ratio Nr/Nc. Values read out by the GPARM command, and so on are the
modified value.
This is the only place where the calibration-time noise level Nc is used in the processing.
It is possible for this value to be unknown, in which case it is set to nan internally. In this
case, if you request the corrected values, the correction is not applied, and you get the
message "Cannot enable Z0 noise correction because calibration is
missing" in the rvp log file.
7.4.4 Velocity
For a Doppler power spectrum that is symmetric about its mean velocity, the velocity is
obtained directly from the argument of the autocorrelation at the first lag, that is,
=
4
1
ℎ
1
= arg
1
ʎ is the radar wavelength, Ʈ
s
is the sampling time (1/PRF). θ
1
is constrained to be on the
interval [-π, π]. When θ
1
= ± π, then V =± V
u
where the unambiguous velocity is ,
=
4
If the absolute value of the true velocity of the scatterers is greater than V
u
, then the velocity
calculated by 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, RVP900
calculates the mean velocity for the normalized Nyquist interval [-1,1], that is, the output
values are,
′ =
1
For example, an output value of -0.5 corresponds to a mean velocity of - V
u
/2. The
normalized velocity V' is more
ecient use of the limited number of bits.
Chapter 7 – Processing Algorithms
203
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