USER’S MANUAL__________________________________________________________________
218 _________________________________________________________________ M211322EN-D
Figure 45 Radar Equation
where C is the radar constant and ro is a reference range which we will later
set to 1 km. This is identical to the first three terms of the dB version of the
equation with the definition that:
Z
o
is called the 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 (See Figure 49 on page 232). The parameter I
o
is the
measured noise power at IF with appropriate calibration for the system
gain. Calibration of the RVP900 involves defining the radar constant C and
measuring the value of I
o
. This is discussed in detail in Section 6.5
Reflectivity Calibration on page 231.
Essentially, the measurement of I
o
is based on the measurement of the
system noise at the time of calibration. However, if the receiver gain were
to change 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
, i.e.,
Thus the k's cancel to give us the same result for Z. This makes the
approach robust to system gain fluctuations. Another way of saying this is
that as long as the system sensitivity (noise figure) does not change, then
the system does not require re-calibration.
The individual terms in the dB form of the equation are summarized below.
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