Vaisala RVP900, User Manual

Page: 221 / 484

If the test signal generator output is -50 dBm, the injected power is
���
��������
= − 50 − 30 + 2 = − 82 ���
The equivalent power at the feed is then 3 dB more than this:
���
����
= − 82 + 3 = − 79 ���
During the calibration, there are several ways to handle the losses using these equations. For
example:
• Correct each signal generator value for losses so that the calibration plot shows IFDR
measured power against the received power at the feed.
This is recommended for manual calibration.
• Plot the signal generator values directly and correct the intercept power I
0
for losses
so that it is properly referenced to power at the feed.
This is the approach used by the IRIS
Zauto utility. See IRIS and RDA Utilities Guide .
7.6.4 Determining dBZo
The calibration reflectivity is determined from the radar equation as follows:
���
0
= 10log ��
0
2
�
0
where I
o
is in mW (corrected for receive losses), the reference range r
o
is 1 km, and the
radar constant C is:
� = 2.69 × 10
16
�
2
�
�
����
2
�
�
where:
�
Radar wavelength in cm.
�
�
Transmitted peak power in kW.
�
�
Transmit loss (for example, 3 dB corresponds to �
�
= 2 )
�
Pulse width in microseconds.
�
Horizontal half-power full beamwidth.
�
Vertical half-power full beamwidth.
�
Antenna gain (dimensionless) on beam axis.
The radar constant is determined from the characteristics of your radar (check with the
manufacturer if you are unsure of the values). Note that transmit losses are accounted for in
the radar constant, while receiver loss is usually included in the calculation of
I
0
.
Chapter 7 – Processing Algorithms
219