The Global Precipitation Measurement (GPM) mission will initiate a new era in precipitation measurement in terms of its global extent and frequency of sampling. Plans call for a GPM constellation consisting of eight spacecraft, with each spacecraft carrying a conical-scanning, microwave radiometer among its instrument complement.

Figure 1. GMI scan geometry.

NASA is acquiring the GMI instruments through a commercial procurement. The contract for GMI was awarded in March 2005 to the Ball Aerospace & Technology Corp. of Boulder, CO. Launch of the Core observatory is planned for December 2010.

Conical Scanning Technology

The conical-scanning geometry of the GMI is illustrated in Figure 1. The off-nadir angle defining the cone swept out by the GMI is set at 48.5 degrees, which represents an earth-incidence angle of 52.8 degrees. Rotating at 32 rotations per minute, the GMI will gather microwave radiometric brightness measurements over a 140-degree sector centered about the spacecraft ground track vector. The remaining angular sector is used for performing calibration; i.e. observation of cold space as well as observation of a hot calibration target. The 140-degree GMI swath represents an arc of 1,170 km on the Earth surface. For comparison, the DPR instrument is characterized by cross-track swath widths of 245 km and 120 km, for the Ku and Ka-band radars, respectively. Only the central portions of the GMI swath will overlap the radar swaths (and with approximately 67-second duration between measurements due to the geometry and spacecraft motion). These measurements within the overlapped swaths are important for improving precipitation retrievals, and in particular, the radiometer-based retrievals.

The GMI is equipped with nine microwave channels. GMI channels lie within protected bands. GMI beam efficiencies for all channels will exceed 90% where beam efficiency is defined as the percentage of energy collected from an isotropic scene within the solid angle defined by 2.5 times the channel half-power beam widths and approximating the antenna main lobe between first nulls.