Provision of a rainfall measuring mode in a satellite-borne radar altimeter

The World Climate Research Programme announced the Global Energy Water Cycle Experiment (GEWEX), with the objective of achieving a quantitative understanding of the Earth´s energy cycle and water balance. A cornerstone of this experiment is the ability to make global measurement of precipitation. Satellite-borne instruments provide probably the only way of making global, consistent measurements. Radar methods have the significant advantage that they can discriminate in range, and that the measured parameters are directly related to rainfall rate. It has been suggested that, with relatively minor modifications, a satellite-borne radar altimeter could provide both useful data and valuable experience to aid in the design of future instruments, in a relatively short timescale. With this in mind, the European Space Agency have commissioned a short study to establish the feasibility of a rainfall-measuring mode for the RA-2 radar altimeter, due to fly on the ENVISAT mission. A key point is that the addition of the rainfall mode should involve as little modification to the basic altimeter design as possible. The present paper presents the results that have been achieved in that study