TRMM Observatory and Instruments
GSFC designed, built and tested the observatory "in house" at its Greenbelt, Md., facility. At launch, the observatory weighed 7,920 lbs. (3,600 kg). It is about 17 feet tall (approximately 5 meters) and 12 feet (3.6 meters) in diameter. A gallium arsenide solar array/nickel cadmium battery power subsystem provides 1,100 watts of load power to the satellite.
A three-axis attitude control subsystem stabilizes the observatory and keeps the instruments pointing toward Earth to within 0.2 degrees. A command and data handling subsystem provides onboard commanding, data collection, processing and storage. This subsystem uses state-of-the-art technology employing a fiber optic data bus and solid state recorders.
A reaction control subsystem maintains the orbit at approximately 217 miles (350 km). Data for each orbit is stored on board and transmitted to the ground by the communication subsystem through TDRSS once per orbit.
The observatory instruments for primary rainfall measurements are a precipitation radar, a multi-frequency microwave radiometer and a visible/infrared radiometer. For observations related to precipitation, NASA added a Lightning Imaging Sensor (LIS) and a Clouds and the Earth's Radiant Energy System (CERES). A brief description of the five instruments follows:
The PR determines the vertical distribution of precipitation by measuring the "radar reflectivity" of the cloud systems and the weakening of a signal as it passes through the precipitation. A unique feature of the PR is the measurement of rain over land, where passive microwave channels have more difficulty.
The TRMM Microwave Imager (TMI) is a multi-channel radiometer, whose signals in combination can measure rainfall quite accurately over oceans and somewhat less accurately over the land. The TMI and PR data, will yield the primary precipitation data sets.
The VIRS measures radiance in five bandwidths from the visible through the infrared spectral regions. Scientists use Infrared (IR) data to make rough estimates of tropical precipitation. The VIRS, PR and TMI data will help improve the techniques by which scientists use IR data from other satellites to calculate rainfall. This is the third component of TRMM's rain package.
The LIS is an optical telescope and filter imaging system that will investigate the distribution and variability of both atmospheric and cloud-to-ground lightning over the Earth. These instruments will contribute to our understanding of storm dynamics and will be correlated to levels of precipitation and the release of latent heat.
The CERES is a visible/infrared sensor designed especially to measure energy rising from the surface of the Earth and the atmosphere including its constituents (e.g., clouds and aerosols). This energy, when balanced against the energy received by the Earth from the Sun, constitutes the Earth's radiation budget. Understanding the radiation budget, from the top of the atmosphere to the Earth's surface, is important to understanding climate and its variability.