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 gathers 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.

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 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 GPM Microwave Imager being constructed in a lab by Ball Aerospace & Technology Corporation

Components of the GMI

1. Instrument Spacecraft Structure - the base structure for GMI.
2. Spin Mechanism Assembly - the DC motor that spins GMI.
3. Slip Ring Assembly - passes receiver subsystem data from the instrument’s spinning portion to its non-spinning portion (see #15, the ICA), which then sends data to the spacecraft and then to Earth.
4. Instrument Base Structure (IBS) - holds all the receiver components.
5. Hot Load Tray – keeps the sun out of the hot load (#7) and helps maintain its thermal environment.
6. Feed Horns - focus the reflected RF energy and send it down the wave-guides (#12) to the RF receivers (#11).
7. Hot Load - provides warm temperature reference for the receivers. GMI self-calibrates by using the constant temperature references of the hot load, the cold sky reflector (#9), and the noise diodes (#12).
8. Despin Assembly - holds the cold sky reflector and the hot load tray in place.
9. Cold Sky Reflector - looks at deep space to provide a cold temperature reference for the receivers.
10. Launch Locks - hold the IBS to the deck during launch.
11. RF Receivers (attached to the IBS’s sides) - detect/measure the RF energy associated with the Earth’s precipitation (moisture from humidity, dew, snow, rain, and so on). 
12. Wave-guides, Noise Diodes, Mixer-Preamplifiers, RF Coaxial Cables - the wave-guides and RF coaxial cables are conduits from the feed horns (#6) to the receiver subsystem. The noise diodes provide mid-range temperature references to the three low-frequency receivers for calibration purposes. The mixer-preamplifiers are the front ends for the three high-frequency receivers.
13. Sunshade and Struts - the sunshade exposes one side of the instrument to the Sun and blocks the other side from the Sun; the struts hold the main reflector.
14. Main Reflector (and its support structure) - reflects the RF energy emitted from the Earth’s atmosphere into the feed horns; its coating of aluminum and then silicon oxide diffuses sunlight, which could damage the feed horns. The feed horns are located near the focal point to achieve the best combination of efficiency and beam-width performance across the various frequencies.
15. Instrument Control Assembly, or ICA (attached below the Instrument Spacecraft Structure) - controls the power from the spacecraft to the instrument; controls the temperature of the instrument; controls the transmittal of science data from the receiver system; and maintains the spinning of the system through the spin mechanism assembly (#2).