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Instrument: AMMS : Advanced Microwave Moisture Sounder
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Description
The Advanced Microwave Moisture Sounder (AMMS) instrument ,
which was mounted on NASA's DC-8 aircraft for the TOGA COARE
Field Experiment, is a scanning radiometer that measures
brightness temperatures in degrees Kelvin. It was operational
during 16 mission flights of the DC-8 between January 5 and
February 23, 1993 under the direction of Principal Investigator
Jim Wang of NASA/Goddard Space Flight Center.

The AMMS was designed to profile atmospheric water vapor and was
mainly used for this purpose in the past. It is also sensitive
to cloud cover and precipitation. Because the weighting
functions of its four frequency channels peak at different
altitudes, depending on water vapor density and profile, AMMS
has the potential of estimating the height of frozen
hydrometeors associated with a convective storm. For TOGA COARE
the sensor was combined with other radiometers in the same
aircraft to measure the radiometric response of convective
rainfall systems in the frequency range of 10-183 GHz. AMMS is
a 4-channel, mechanically scanned, imaging microwave radiometer
operating at 92, 174, 178 and 181 GHz. It has a 15-cm aperture
giving an angular resolution of about 2 degrees at 92 GHz and 1
degree at 183 GHz. After every 6 scans, the beam is directed to
view heated (330 K) and cooled (250 K) external calibration
targets for 2 seconds each, resulting in a total frame time of
~30 secon ds (including slewing time). The radiometric signals
and the measured physical temperatures from these calibration
targets form the basis for the derivation of the scene
brightness temperatures. The calibration accuracy is on the
order of 1 K in the 250 - 300 K brightness temperature
range. The temperature sensitivity (delta T) of the sensor has
gradually deteriorated over the past 10 years. For water vapor
profiling, averaging of up to 50 radiometric samples is needed
to meet the requirement of delta T of <= 1 K. The microwave
signatures from precipitation are much stronger than water
vapor at the AMMS frequencies, and data from this sensor
averaged over a few samples will be sufficient to derive
important information about the hydrometeors.

The beam is scanned in 50 steps of 1.8 degrees from nominally
45 degrees to the right through nadir, and to 45 degrees to the
left with a total scan time of ~4 seconds.

Additional information available at
http://daac.gsfc.nasa.gov/CAMPAIGN_DOCS/TOGA/amms.html

[Summary provided by NASA]