Recommended Calibration Procedures for GPM Ground Validation radars

Calibration of weather radars has a direct impact on the accuracy of measurements and therefore is critical for most applications. For this reason, calibration has been an active topic of study since the early days of radar meteorology, involving both the research and operational community. More recently the discussion on radar calibration is enriched by new challenges. First, the operational use of dual-polarization radars has created a new set of demands for differential measurements. Secondly, the use of radars in a network has highlighted the importance of best practices and standards to assure that all the radars of the network provide meaningful and comp arable measurements. Moreover, modern technology has provided many avenues for automating the calibration process, thereby minimizing errors. This manual has been developed as for Tier 1 GPM Ground validation Radar, although it aims at a broader audience. It takes a fundamental look at the weather radar calibration process, and illustrates how a collection of consolidated techniques and modern technologies can be used to the purpose of calibrating radar, with special attention to dual polarization systems. This manual is organized as follows: The introduction aims to introduce the problem of calibration using a high level view of the radar divided into subsystems that are useful for calibration. Subsequently procedures to characterize radar subsystems, particularly transmitter, antenna, and receiver are presented. Many of the described techniques make use of external sources, such as the Sun or installations using standard gain horns to characterize performance of two subsystems (e.g. transmitter-antenna or antenna-receiver). Since a radar is a complex system, characterization of each of its many components is not easy and therefore, calibration of radar subsystems must be complemented with methods allowing an end-to-end calibration of the radar. Methods based on the use of artificial targets (metallic spheres or corner reflectors) or natural sources (meteorological scatterers) are illustrated. An example, implemented on CSU-CHILL radar, of how modern technologies can be used to monitor radar calibration parameters is finally provided.