Calibration of the Dual-Frequency Precipitation Radar Onboard the Global Precipitation Measurement Core Observatory

Abstract

This article describes four-year calibration results of the dual-frequency precipitation radar (DPR) onboard the Global Precipitation Measurement (GPM) Core Observatory. The calibration method basically follows the method that was used to calibrate the precipitation radar (PR) onboard the Tropical Rainfall Measuring Mission (TRMM) satellite. However, both the hardware and data processing method for calibration are improved by taking advantage of the lessons learned from the PR's calibration. Since the response of the radar receivers was found to depend on the waveform, the active calibrator was improved in such a way that the external calibration can be performed with both continuous and pulse waves. The methods for evaluating the calibration data were also improved. Instead of assuming a Gaussian antenna pattern, the effective beamwidths were determined by assuming an antenna pattern created by the Taylor distribution that was used to design the antennas. The results of the calibration including these improvements provide the new precise parameters of DPR's calibration. The new parameters increased the Ku-band precipitation radar's (KuPR's) radar reflectivity factor (Z) by about 1.3 dB and that of the Ka-band precipitation radar (KaPR) by about 1.2 dB from the precalibrated Z values, and the minimum detectable radar reflectivities were 15.46, 19.18, and 13.71 dBZ for KuPR, matched beam of KaPR, and high-sensitivity beam of KaPR, respectively. After applying the new calibration methods to both DPR and PR, normalized radar cross sections (σ0) from the DPR and PR agree with each other.