User's Manual Part 2

Processing Algorithms (draft)
RVP8 User’s Manual
April 2003
5–62
S Collect ZDR data at vertical incidence while the antenna is rotating in azimuth.
S Use a separate application program to average the ZDR values around a full 360
degrees at each range bin (height). Generate a plot of 360-average ZDR vs height.
S You should observe that the average ZDR values in regions of strong signal (>20
dB SNR) below the bright band are approximately constant with height. This is
the value that should be used in your application software for GDR.
S Enter the value and repeat the calibration to verify that the average ZDR is now 0
dB.
The rationale for this approach is as follows. When viewed at vertical incidence, rain should
have a ZDR of 0 dB since the drops will all appear circular. The reason for averaging over 360
degrees is to cancel-out effects from sidelobe contamination from nearby ground targets and
other artifacts of the antenna/feed/radome system. For example the radome may have an
obstruction light on the top. Some of these artifacts can be minimized by assuring the the
weather targets are strong, i.e., heavy rain is preferred for this calibration.
XDR Calibration for LDR
XDR is the relative gain in dB between the co- and cross-receiver channels for LDR
measurements. Analogous to GDR, it is defined as the dB value of the ratio of the vertical to
horizontal receiver gains, i.e.,
XDRĄ +Ą 10ĄLOGĄ
g
r
v
g
r
h
and xdrĄ +Ą
g
r
v
g
r
h
Three techniques for calibration of XDR are discussed. It is recommended for the transmitter to
be off for all of these methods.
S 1. Solar method
Use the sun to measure LDR. The measured value of LDR is then the XDR offset.
LDR should be measured in fixed mode for both LDRH and LDRV. The values
should be reciprocal (e.g., +1 dB and –1 dB). Use the average of the absolute
value if they are not precisely reciprocal (e.g., for +1.4 and –1.2 use 1.3). Finally
after inputting the XDR value, retest to verify that the sun has been properly
corrected to have zero LDR.
S 2. Signal generator method with connection to waveguide
Connect a signal generator with a splitter to both channels and measure XDR
directly. This does not account for any effects that are before the coupler (e.g.,
waveguide, feed, radome, antenna gain).
S 3. Linear feed horn remote radiator method
Use a calibrated linear feed horn with an RF source located several hundred
meters from the radar. Maximize the H channel return and measure the response
using the RVP8 pr command “Filtered” power in the “Primary Channel”. Now
rotate the feed horn to vertical and maximize the power in the “Secondary
Channel”. The difference in dB is XDR. Note that signal multi–path effects could
bias the results from this technique.