=
2
=
0
2
2
0
2
0
where C is the radar constant and ro is a reference range which we later set to 1 km. This is
identical to the first three terms of the dB version of the equation with the definition that:
0
=
0
2
=
0
2
0
ℎ
0
=
where:
Z
0
Calibration reflectivity factor. It is the equivalent radar reflectivity factor at the
reference range when the return signal power is equal to the noise power (SNR=0 dB).
It is sometimes called the minimum detectable dBZ at 1 km, though it is more correct
to call it the 0dB SNR detection level.
I
o
Measured noise power at IF with appropriate calibration for the system gain.
The measurement of I
o
is based on the measurement of the system noise at the time of
calibration. However, if the receiver gain changes after calibration, the use of periodic noise
sampling properly corrects for this. For example, if the receiver gain were to change by a
factor k, then we would measure a noise value of kN and an autocorrelation value of kT
o
,
that is:
=
2
=
0
2
2
0
2
0
Thus the k's cancel to give us the same result for Z. This makes the approach robust to
system gain fluctuations. As long as the system sensitivity (noise figure) does not change,
then the system does not require re-calibration.
Calibrating RVP900 involves
defining the radar constant C and measuring the value of I
o
.
See 7.6 Reflectivity Calibration (page 214).
The following table shows the individual terms in the dB form of the equation.
Table 52 Terms in the dB Equation Format
Term Term Name Description
1
st
Term
10log
0
The eect of this term is to subtract the
measured noise and then divide by that
noise. The result is a Signal-to-Noise ratio.
2
nd
Term
dBZ
o
: Calibration Reflectivity (see
discussion above)
dBZ
o
is the minimum detectable dBZ at a
reference range r
o
=1 km
Chapter 7 – Processing Algorithms
201
To Next Page
Loading...