Four-year result of external calibration for Precipitation Radar (PR) of the Tropical Rainfall Measuring Mission (TRMM) satellite

External calibration experiments using active radar calibrator (ARC) were conducted for the calibration of the Precipitation Radar (PR) of the Tropical Rainfall Measuring Mission (TRMM) satellite from 1998 to 2001. Three modes of ARC operation are used for the experiment: the ARC transmitter mode for the receiver system of PR, the ARC receiver mode for the transmitter of PR, and the transponder mode for the overall system of PR. The experiments were conducted several times a year. The results of the experiments show that the performance of the PR is close to the prelaunch performance and that PR has been working stably for four years.