GDR calibration for Zdr

The Z_dr offset is the dB value of the relative gain between the co-polarized channels including both transmitter and receiver gain, that is:

$Z D R O f f s e t = 10 L O G \frac{g_{v}^{r} g_{v}^{t}}{g_{h}^{r} g_{h}^{t}}$ and $g d r = \frac{g_{v}^{r} g_{v}^{t}}{g_{h}^{r} g_{h}^{t}}$

GDR is input into the processor as a dB value. However, for these analyses, the linear gdr value is sometimes more convenient.

In principle, if dBZ_o could be calibrated perfectly in both channels, measurement of GDR would not be required. In practice, this is not possible because dBZ_o cannot be calibrated to an absolute accuracy sufficient for Z_dr, that is, to 1/16th of a dB. Therefore, RVP10 uses the GDR approach.

Since GDR includes both transmitter and receiver differential gains, accurate calibration requires that an actual target be observed.

The following steps show one way to do this is.

Set the GDR to be 0 dB using your application software (for example, for Vaisala IRIS systems in the Setup utility RVP section).
Disable clutter filtering for Z_dr in either your application software (by selecting filter 0) or explicitly in the RVP10 TTY setups mp section.
Place the antenna at 90° elevation (vertical incidence) during moderate to heavy rain.

The melting layer should be at a height that is well above the recovery zone of the T/R and in the antenna "far zone". A melting layer higher than 2 km (1.2 mi) is suggested, but you must consider the specific characteristics of the radar.
Collect Z_dr data at vertical incidence while the antenna is rotating in azimuth.
Use a separate application program to average the Z_dr values around a full 360° at each range bin (height).

Generate a plot of 360-average Z_dr against height.
Check that the average Z_dr values in regions of strong signal (>20 dB SNR) below the bright band are approximately constant with height.

Use this value in your application software for GDR.
Enter the value and repeat the calibration to verify that the average Z_dr is now 0 dB.

The rationale for this approach is that, when viewed at vertical incidence, rain should have a Z_dr of 0 dB since the drops all appear circular. The reason for averaging over 360° 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 ensuring the weather targets are strong, that is, heavy rain is preferred for this calibration.