The launch of the first spaceborne precipitation radar (PR) aboard the Tropical Rainfall Measuring Mission (TRMM) satellite provides a unique opportunity to address the ground-based weather radar system's calibration problem. The PR measures the same variable as ground-based systems—radar reflectivity— and is proven to be consistent with respect to calibration accuracy. PR calibration tests, conducted by the Space Agency of Japan (NASDA) using an active radar calibration system [Kumagai et al, 1995] located in Japan, have shown calibration stability within ±0.8 dBZ [Kozu et al, in press, 2000].
A fundamental error source in the measurement of precipitation by radar is the inaccurate definition of the radar calibration constant, which obviously affects the interpretation of radar measurements. This has become an emerging issue, as weather radar is used worldwide for applications ranging from hydrological to climatic. Typically radar systems use internally generated test signals to monitor variations of the radar system's parameters associated with temperature changes in the radar electronics. However, calibration drifts may occur due to gradual degradation of the system's performance. During these times, external calibration using either metal spheres or active sources of radiation required.