NASA Facts: TRMM Instruments
Precipitation Radar
The Precipitation Radar was the first spaceborne instrument designed to provide
three-dimensional maps of storm structure. These measurements yield invaluable
information on the intensity and distribution of the rain, on the rain type, on the
storm depth and on the height at which the snow melts into rain. The estimates of the
heat released into the atmosphere at different heights based on these measurements can
be used to improve models of the global atmospheric circulation.
The Precipitation Radar has a horizontal resolution at the ground of about 3.1 miles
(five kilometers) and a swath width of 154 miles (247 kilometers). One of its most
important features is its ability to provide vertical profiles of the rain and
snow from the surface up to a height of about 12 miles (20 kilometers). The Precipitation
Radar is able to detect fairly light rain rates down to about .027 inches
(0.7 millimeters) per hour. At intense rain rates, where the attenuation effects can be
strong, new methods of data processing have been developed that help correct for this
effect. The Precipitation Radar is able to separate out rain echoes for vertical sample
sizes of about 820 feet (250 meters) when looking straight down. It carries out all
these measurements while using only 224 watts of electric power—the power of just a few
household light bulbs. The Precipitation Radar was built by the National Space
Development Agency (JAXA) of Japan as part of its contribution to the joint
US/Japan Tropical Rainfall Measuring Mission (TRMM)
Technology Challenges and Advances
Among the three primary instruments on TRMM, the most innovative is the Precipitation
Radar. Other instruments similar to the TRMM Microwave Imager (TMI) and the Visible
and Infrared Scanner (VIS) VIRS have operated in space before, but to date there has
not been any radar in space for the purpose of measuring rainfall.
Although weather radars on the ground have been used ever since World War II to estimate
rainfall, there were many technical challenges that had to be overcome before an instrument
of this kind could be used from space.
Power
A fundamental requirement is ensuring that the spaceborne radar has enough power to
detect the weak return echo from the rain drops when seen from TRMM’s orbital height
of 250 miles (402 kilometers) above the Earth.
3-D Resolution
Another challenge is to gather high resolution three-dimensional maps of the rain during
the brief time that the satellite overflies local storms. To meet these demands, Japan’s
Communications Research Laboratory elected a radar frequency about three times higher
than that of a typical ground-based radar. Despite the restrictions on the size of the
antennas in space, the use of a higher frequency provides good resolution and high
quality images of storms. An active-phased array antenna, and sophisticated signal-
processing techniques are used to scan the antenna beam electronically and rapidly,
while ensuring that the transmitted and received radar pulses are synchronized.
Solid state power amplifiers (128) are used both to conserve power and to provide
a design that is robust.
Focusing the Radar Beam
Another problem to be overcome for the spaceborne radar was the need to produce a narrow
radiating beam so that the target area would be small enough to bring out the features
of interest on the ground— that is, good ground resolution. And still another problem
was to be able to make the beam sweep out a path on the ground that would be wide enough
to give good coverage as the satellite moves along its orbit. All of these design
considerations were met through developments at Japan’s Communications Research
Laboratory. Both the narrow beam size and the ability to steer the beam are achieved
through the design of a “phased array” antenna, which uses electronic steering of the beam.
The Tropical Rainfall Measuring Mission 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 JAXA of Japan. The primary instruments for measuring
precipitation are the Precipitation Radar, the TRMM Microwave Imager, 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 all can function
individually or in combination with one another.
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