Multiple-Direction/ Polarisation
Instruments
Description
Advances in satellite instrumentation have
resulted in a general trend towards
multifunctional capabilities in many types
of sensors, resulting in instruments with
the capability to operate using different
viewing modes and angles, as well as
multiple polarisations. The latest SAR
instruments demonstrate this trend. The
category of ‘multiple direction/polarisation
instruments’ is used here, however, to
describe instruments that are custom-built
for observing the directional or
polarisational characteristics of the
target’s signature (either visible/IR or
microwave), as a means of deriving
geophysical information.
Multi-directional radiometers can make
observations from more than one incidence
angle of the transmitted or diffused
radiation emitted by a particular element of
Earth’s surface or clouds. In this way,
information on anisotropies in the radiation
may be identified. The emphasis in these
instruments is on spectral (rather than
spatial) information, with the result that
the detection channels, which typically span
the visible to the IR, are precisely
calibrated and the spatial resolution is
usually about 1 km.
Polarimetric radiometers are used for
applications in which radiative information
is embedded in the polarisation state of the
transmitted, reflected or scattered wave.
Some polarimetric radiometers also have a
multi-directional capability, so that
directional information can be determined or
used during retrievals of geophysical
parameters.
Applications
Using IR channels, multiple-angle viewing
capabilities are used to achieve accurate
corrections for the effects of (variable)
atmospheric absorption, making it possible
to infer precise temperature values, for
example, of sea and land surfaces.
Multi-directional radiometers are also
capable of measuring cloud-cover and
cloud-top temperatures, together with
atmospheric water vapour and liquid water
content.
In the visible and near-IR spectrum, these
instruments allow for improved measurements
of the scattering properties of particles
such as aerosols, as well as measurement of
the angular characteristics of the various
contributions to Earth’s radiation budget,
including surface albedo.
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Current & planned
instruments
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MIRAS (SMOS)
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Next Gen APS (ACE)
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MISR
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POLDER-P
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MWR
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They also enable accurate measurement of
parameters such as Normalised Difference
Vegetation Indices, which are used to assess
vegetation state and crop yield at regional
and global scales. MISR, currently flying on
NASA’s Terra mission, is providing new types
of information for scientists studying Earth’s
climate, such as the partitioning of energy
and carbon between the land surface and the
atmosphere, and the regional and global
impacts of different types of atmospheric
particles and clouds.
Polarisation information is used to infer a
variety of parameters, including the size and
scattering properties of liquid water, cloud
particles and aerosols, while providing
additional information on the optical
thickness and phase of clouds. Polarimetric
radiometers also provide information on the
polarisation state of the radiation
backscattered from Earth’s surface,
supplementing measurements obtained from other
land- and sea-imaging instruments. Such
measurements are of interest in a range of
applications, from investigations of albedo
and reflectance to agriculture and the
classification of vegetation. ESA’s SMOS
mission, launched in late 2009, uses an L-band
(1.4 GHz) microwave interferometer to measure
estimates of soil moisture (a key variable for
numerical weather and climate models) and
ocean surface salinity (important for ocean
circulation and sea level models).
Seasonal changes in Earth’s surface albedo
as measured by the MISR instrument on the
Terra mission.
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