NASA Facts: TRMM Instruments
TRMM Microwave Imager
The Tropical Rainfall Measuring Mission�s (TRMM) Microwave Imager (TMI) is a passive
microwave sensor designed to provide quantitative rainfall information over a wide
swath under the TRMM satellite. By carefully measuring the minute amounts of microwave
energy emitted by the Earth and its atmosphere, TMI is able to quantify the water
vapor, the cloud water, and the rainfall intensity in the atmosphere. It is a relatively
small instrument that consumes little power. This, combined with the wide swath and the
good, quantitative information regarding rainfall make TMI the "workhorse" of the
rain-measuring package on Tropical Rainfall Measuring Mission.
Improving on History
TMI is not a new instrument. It is based on the design of the highly successful Special
Sensor Microwave/Imager (SSM/I) which has been flying continuously on Defense
Meteorological Satellites since 1987. The TMI measures the intensity of radiation at
five separate frequencies: 10.7, 19.4, 21.3, 37, 85.5 GHz. These frequencies are similar
to those of the SSM/I, except that TMI has the additional 10.7 GHz channel designed to
provide a more-linear response for the high rainfall rates common in tropical rainfall.
The other main improvement that is expected from TMI is due to the improved ground
resolution. This improvement, however, is not the result of any instrument improvements,
but rather a function of the lower altitude of TRMM 250 miles (402 kilometers) compared
to 537 miles (860 kilometers) of SSM/I). TMI has a 547 mile (878-kilometer) wide swath on
the surface. The higher resolution of TMI on TRMM, as well as the additional 10.7 GHz
frequency, makes TMI a better instrument than its predecessors. The additional
information supplied by the Precipitation Radar further helps to improve algorithms.
The improved rainfall products over a wide swath will serve both TRMM as well as the
continuing measurements being made by the SSM/I and radiometers flying on
the NASA�s EOS-PM and the Japanese ADEOS-II satellites.
Measuring Rainfall with Microwaves
Calculating rainfall rates from TMI requires some fairly complicated calculations. The basis
of these calculations is in Planck�s radiation law, which describes how much energy a body
radiates given its temperature. Water surfaces such as oceans and lakes have an additional
property which is very important. The surfaces emit only about one half the microwave energy
specified by Planck�s law and therefore appear to have only about half the real temperature of
the surface. Water surfaces therefore look very "cold" to a passive microwave radiometer.
Raindrops on the other hand, appear to have a temperature that equal their real temperature.
They appear warm to a passive microwave radiometer and therefore offer a contrast against
"cold" water surfaces. The more raindrops, the warmer the whole scene appears, and research
over the last three decades now make it possible to obtain fairly accurate rainfall rates based
on the temperature of the microwave scene.
Land is very different from oceans in terms of the emitted microwave radiation, appearing to
have about 90 percent of its real temperature. In this case, there is little contrast to observe
the "warm" raindrops. Certain properties of rainfall, however, still can be inferred. The high
frequency microwaves (85.5 GHz) measured by TMI are strongly scattered by ice present in many
raining clouds. This reduces the microwave signal at the satellite and offers a contrast against
the warm land background.
TRMM is NASA�s first mission dedicated to observing and understanding the tropical rainfall and
how this rainfall affects the global climate. It is a joint mission with the National Space
Development Agency of Japan. The primary instruments for measuring pre-cipitation are the
Precipitation Radar, the TMI, and the Visible and Infrared Scanner. Additionally, TRMM
carries the Lightning Imaging Sensor and the Clouds and the Earth�s Radiant Energy System
Instrument. These instruments can all function individually or in combination with one another.
TRMM is part of NASA�s Mission to Planet Earth, a long-term, coordinated research effort to
study the Earth as a global system.
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