Tropical Rainfall Measuring Mission (TRMM) is a joint space program between USA and Japan to measure rainfall of tropics where about 60% of global rainfall is concentrated. TRMM is the first space mission dedicated to measurements of tropical rainfall with the first precipitation radar in space. Communications Research Laboratory has made the system study of the precipitation radar for the TRMM in the joint feasibility study of TRMM between USA and Japan and moreover in the follow-on studies. The basic system parameters of TRMM precipitation radar, such as frequency, antenna beam width, transmitting peak power, scan angle, horizontal and vertical resolutions, pulse repetition frequencies and so on are determined in order to satisfy the mission requirement shown by NASA. The precipitation radar system must adopt the high speed electric scanning in order to observe a raining area without any gaps between scanning lines that are perpendicular to the moving direction of the satellite. The frequency agility technique, which uses dual frequency separated 6 MHz, is adopted to attain the required independent sample number because the dwell time of the antenna beam in one angle-bin direction is limited. In the trade-off studies, three major competitive items were discussed for the selection of a preferred candidate for TRMM radar. These competitive items are (1) a passive array radar with a TWTA (Traveling Wave Tube Amplifier) versus active array radar with SSPAs (Solid State Power Amplifiers), (2) a pulse compression radar versus conventional type radar, and (3) antenna type (planar array antenna versus cylindrical parabolic antenna). Trade-off studies of these items show that the non-pulse compression active array radar with planar array antenna is considered as the most suitable candidate for the TRMM precipitation radar at 13.8 GHz.