Chapter 6 ______________________________________________________ Processing Algorithms
VAISALA______________________________________________________________________ 219
1st Term : Signal to Noise Ratio
The effect of this term is to subtract the measured noise and then divide by
that noise. The result is a Signal-to-Noise ratio.
2rd Term:dBZ
o
: Calibration Reflectivity (see discussion above)
dBZ
o
is the minimum detectable dBZ at a reference range r
o
=1 km,
3th Term: 20 log r : Range Normalization
This term is the range normalization expressed in dB form.
4th Term:ar: Gaseous Attenuation Correction
This term accounts for gaseous attenuation. The constant a is set in the
RVP900 EEROM since it is a function of wavelength. For a C-band system
the default value is 0.016 dB per km (for two-way path attenuation).
5th Term: CCOR: Clutter Correction
This term corrects for the measured ground clutter. It's derivation is
discussed in Section 6.3.7 Clutter Correction (CCOR threshold) on page
221.
6.3.3.1 Noise Correction to Reflectivity Calibration
The dBZ0 number in the above equation is the number which sets the
sensitivity of the radar. Lower numbers mean greater sensitivity. In Figure
45 on page 216 it was assumed that the noise level at calibration time is the
same as the noise level at run time. And that any changes in measured noise
level were due to changes in receiver gain not sensitivity.
Modern digital receivers and low-noise amplifiers are very gain stable, and
this is generally not true. One example of a relatively large noise level
variation is the thermal noise from the relatively warm earth and
atmosphere. So, the bottom degree or so of elevation has a different noise
level, and thus a different sensitivity, and thus a different dBZ0. Normally
we calibrate while aiming the antenna up in the air in a direction away from
the surface, or the sun. For this discussion, let us define two new noise
values:
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