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.
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.
The 140 degree GMI swath represents a swath of 904 km on the Earth's 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.
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The GPM
radars will be used to accurately measure, via reflectivity and
estimates of attenuation, the vertical profiles of the clouds and
precipitation, including the drop size distribution.
A primary application for the short latency
GMI data is the integration into a near-real-time global rainfall
map created from measurements by all the GPM constellation
radiometric sensors and with overall rain map data latency less
than 3 hours.
Conical Scanning Geometry (view large)
Partners
In March 2005, NASA chose Ball Aerospace
& Technology Corporation to provide the GMI instrument(s).
Related Links
+ DPR Instrument
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