End-to-end Calibration of NEXRAD Differential Reflectivity with Metal Spheres

The desired accuracy for the measurement of differential reflectivity (ZDR) with the newly upgraded polarimetric NEXRAD radars is 0.1 dB, a challenging goal. The end-to-end calibration (through transmit and receive paths) is the optimal method to measure accuracy. That method of choice on a volume target involves the observation of raindrops (with circular cross section) at vertical incidence. Unfortunately, NEXRAD radars were not engineered to point vertically. Accordingly, end-to-end calibration of one NEXRAD radar (KOUN in Norman, OK) has been undertaken with high quality metal spheres, stably tethered in altitude (120-160 meters) in virtually calm wind conditions with neoprene balloons. This approach provides simultaneous absolute calibration of radar reflectivity as well. The engineering specification for sphericity of the 6” and 12” diameter spheres enables ZDR bounds of <0.007 dB and <0.043 dB, respectively. A theodolite was used to locate the suspended sphere to guide the pointing of the radar beam. The sphere target returns at ~3400 meter range were typically 20-25 dB stronger than the surrounding ground clutter. The pendulum motion of the suspended sphere was readily discernible in the Doppler spectral observations. Best results for ZDR measurements on the 6” and 12” spheres were -0.56 dB and -0.52 dB, showing excellent agreement. The evidence for negative bias in ZDR in these measurements for one NEXRAD radar has motivated comparisons with other upgraded radars in the network. Results will be presented as they are available, but a general tendency for negative bias is already evident.

This work was sponsored by the Federal Aviation Administration under Air Force Contract No. FA8721-05-C-0002. Opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endorsed by the United States Government.