GPM Microwave Imager (GMI)

The Global Precipitation Measurement (GPM) Microwave Imager (GMI) instrument is a multi-channel, conical- scanning, microwave radiometer serving an essential role in the near-global-coverage and frequent-revisit-time requirements of GPM.

Illustration of the GMI

The instrumentation enables the Core spacecraft to serve as both a precipitation standard and as a radiometric standard for the other GPM constellation members.

The GMI is characterized by thirteen microwave channels ranging in frequency from 10 GHz to 183 GHz. In addition to carrying channels similar to those on the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI), the GMI carries four high frequency, millimeter-wave, channels about 166 GHz and 183 GHz.

With a 1.2 m diameter antenna, the GMI will provide significantly improved spatial resolution over TMI.

Scan Geometry

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. To maintain similar geometry with the predecessor TMI instrument, the-earth-incidence angle of GMI was chosen identical to that of the TMI. 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.

Illustration of the conical scanning geometry.

Diagram of the conical scanning
geometry.

The 140 degree GMI swath represents a swath of 904 km (562 miles) on the Earth's surface. For comparison, the DPR instrument is characterized by cross-track swath widths of 245 km (152 miles) and 120 km (75 miles), 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.