General Information

 TRMM Microwave Imager

 Visible Infrared Scanner

 Precipitation Radar

TRMM is joint NASA/NASDA space mission. NASA provided the satellite, TMI, VIRS and the ground system including a science data processing system. Japan's NASDA provided the H-II launch vehicle, launch facility at Tanegashima Space Facility in Japan and the Precipitation Radar instrument. PR was jointed designed by NASDA and Japan's Communications Research Laboratory (CRL). It was launched on November 27, 1997 (November 28, 1997 in Japan).

The TRMM orbit is a circular orbit at an altitude of 350 km. It has an inclination of 35 degrees to the Equator. The coverage is, therefore, 35 degree south latitude to 35 degrees north latitude. This orbit yields extensive coverage in the tropics and allows extraction of rainfall data over the 24 hour day. The low altitude also ensures that TRMM instruments are able to resolve the clouds' radiances over small areas. This permits more accurate translation of those signals to the average rain over the area than if the sensing area were larger.

The TRMM observatory weighs approximately 3620 kg and provides approximately 1100 watts of power from a gallium arsenide solar array/nickel cadmium battery subsystem. The observatory has a three-axis attitude control subsystem that keeps instrument pointer to within 0.2 degrees. Transmission of data and command telemetry is done through NASA's Tracking Data Satellite System (TDRSS). A hydrazine reaction control subsystem maintains the orbit at 350 km ( +- 1.25 km).

TRMM hosts five instruments. The TRMM Microwave Imager (TMI), Visible Infrared Scanner (VIRS) and the Precipitation Radar (PR) comprise the rainfall measurement package that is the primary mission payroll. The remaining two instruments, CERES and LIS, are part of the early Earth Observing System program. Additional information will be provided on this page for the rainfall measurement instruments; no information will be provided on the two EOS instruments. Interested individuals may receive additional information about these two instruments from the EOS program.

The core TRMM science mission objectives are:

 To obtain and study multi-year science data sets of tropical and subtropical rainfall measurements

 To understand how interactions between the sea, air and land masses produce changes in global rainfall and climate

 To help improve modeling of tropical rainfall processes and their influence on global circulation in order to predict rainfall and variability at various time scale intervals

 To test, evaluate, and improve the performance of satellite rainfall estimates measurements and techniques.

Back to top

TMI is similar to the SSM/I instrument flown on the DMSP satellite series. There are some key differences. These are:

The TMI is a 9 channel, 5 frequency, linearly polarized, passive microwave radiometric system. The instrument measures atmospheric and surface brightness temperatures at: 10.7, 19.4, 21.3, 37.0 and 85.5 GHz. Each frequency has one vertically and one horizontally polarized channel except for the 21.3 GHz that has only vertical polarization.

The 10.7, 19.4, 21.3 and 37.0 GHz channels are considered low resolution and the 85.5 GHz channels are considered high resolution. TMI has a conical scanning geometry, rotating continuously about a vertical axis. It receives upwelling radiation from 49 degree off nadir. Scene radiation is recorded from left to right (looking in the +X flight direction) over an annular sector of 130 degrees about the sub-satellite track. The separation between successive scans is 13.85 km along the X which is nearly equal to the resolution of the 85 GHz beams. The swath width is 758.5km. This swath is covered by 104 low resolution pixels or 208 high resolution pixels.

Back to top

The VIRS instrument is similar to the AVHRR instrument flown on the NOAA satellite series. VIRS has a swath width of 720 km and a horizontal resolution of 2 km at nadir. VIRS measures radiance values in the following channels:

All five channels are in operation day and night. VIRS is a cross-track scanning system and records scene radiation from right to left looking in the +X direction over a scan angle of +- 45 degrees from the nadir. The swath width is covered by 261 pixels and every scan line has the same geometry.

Back to top

PR is a Japanese instrument jointly developed by NASDA and CRL. PR is an active 13.8 GHz radar, recording energy reflected from atmospheric and surface targets. The PR electronically scans from right to left looking in the +X direction across the ground track of the satellite every 0.6 secs with a swath width of 215 km.

Each PR scan contains 49 rays sampled over an angular sector of 34 degrees. For any given ray, the instrument begins recording samples at a fixed distance from the satellite and records a certain number of samples every 125 km along the ray. The starting distance and the number of samples are different for each ray. Assuming the satellite altitude is 350 km, the sampling begins about 23 km above mean sea level and extends for a certain distance along the ray. This distance along the ray is 33.5 km at the two rays farthest from nadir, monotonically declining to 30.25 km at the two rays adjacent to nadir and jumping to 34.75 km at the single nadir ray. The extra data in the nadir ray is known as "the mirror" because it records energy reflected not once from a target, but three times (ground to target to ground). Rays other than the nadir ray also sample "below" the surface. The purpose of this extension "to see" below the surface is to detect clearly the location of the surface.

Back to top

Last Revised: April 8, 1999