Imaging Multi–spectral Radiometers (Passive
Microwave)
Description
Operating at microwave wavelengths, these
instruments have the advantage of cloud
penetration and all-weather capability.
Channels within 1 GHz to 40 GHz and 80 GHz
to 100 GHz are used to get day/night
information on Earth’s surface. They have
the advantage over visible/IR radiometers of
being able to probe the dielectric
properties of a surface or penetrate certain
surfaces, a capability that is especially
useful with vegetation, soil, sea ice and
snow. Observations by instruments like
AMSU-A, with channels between 50 GHz and 60
GHz, have been used for deriving atmospheric
parameters, especially atmospheric
temperature.
Like other microwave instruments, these
passive instruments offer accurate spectral
information but their spatial resolution is
poor. At 90 GHz, their spatial resolution is
typically 5 km, and for the lower
frequencies it is of order tens of
kilometres – poorer than that of their
visible or infrared counterparts. As a
consequence, they are most used for global
analysis rather than regional or local,
although some instruments are used to
correct measurements from other sensors,
rather than for imaging applications. These
include the microwave radiometers on the
ERS/Envisat (ended 2012) and
Topex/Poseidon/Jason series of satellites,
which are used to estimate and correct for
atmospheric water vapour content in the
column through which altimetric readings are
being taken.
Applications
Measurements from these instruments may be
used to infer a range of atmospheric and
Earth surface parameters. One of their
primary uses (often in conjunction with
other instruments) is snow and ice mapping,
due in part to their capability for cloud
penetration and their ability to take
measurements during daytime and night time.
Current applications of passive microwave
radiometer data include operational
forecasting and climate analysis and the
prediction of sea-ice concentration, extent
and ice type. Passive microwave radiometers
are also used to provide information on the
liquid water content of clouds (e.g. the GPM
mission).
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Current & planned instruments
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Advanced MTVZA
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L-band Radiometer (SMAP)
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AMR
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MADRAS
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AMSR-2
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MIRAS
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AMSR-E
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MIRAS (SMOS)
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Aquarius L-band radiometer
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MTVZA
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GeoSTAR
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MWI-Cloud
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GMI
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MWI-Precip
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ICI
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MWRI
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IRS (SJ-9B)
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RAD
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JMR
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SSM/I
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K-band radiometers (SCLP)
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TMI
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These instruments can also supply some
information on soil moisture content, which is
a key surface parameter in agriculture,
hydrology and climatology, and provides a
measure of vegetation health. Furthermore,
they are capable of contributing some
information on ocean salinity, which is
important to our understanding of ocean
circulation. Developing these capabilities is
a current research task.
First-ever global map of sea-surface
salinity and soil moisture produced with the
same instrument on ESA’s SMOS satellite.
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