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Chapter 3
The MIPAS Instrument
3.1 Instrument description
3.1.1 Overview
MIPAS is a Fourier transform
spectrometer for the detection of limb emission
spectra in the middle and upper atmosphere. It
observes a wide spectral interval throughout the mid
infrared with high spectral resolution.
Operating in a wavelength range from 4.15 microns to
14.6 microns, MIPAS detects and
spectrally resolves a large number of emission
features of atmospheric minor constituents playing a
major role in atmospheric chemistry. Due to its
spectral resolution capabilities and low-noise
performance, the detected features can be
spectroscopically identified and used as input
to suitable algorithms for extracting atmospheric
concentration profiles of a number of target
species.
MIPAS is a complex
instrument made of several sub-systems.
The figure below is a schematic view of MIPAS. It shows the major subsystems.
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| Figure 3.1 Schematic view of MIPAS |
Radiation from the Earth limb enters the
instrument (at the bottom of the figure). The
exact location of the target is selected by
rotating two mirrors in the front-end optics
subsystem. The elevation scan mirror is used to
select an altitude and the azimuth scan mirror
is used to select an orientation with respect to
the platform. The radiation is then directed to
the input telescope and collimator. The
collimator produces a collimated beam of
radiation that is redirected to the
interferometer. The interferometer is the heart
of MIPAS. In the interferometer,
the radiation beam is split in two parts by a beamsplitter. The two beam
fractions are reflected back together to the
beamsplitter by two retro-reflectors. When they
return to the beamsplitter, the two beam are
recombined and they interfere. By moving the retro-reflectors, the
interference pattern is modified. The motion of
the reflectors is controlled with a laser. The
beamsplitter separates the recombined beam in
two beams and each is directed to one output
port. Behind the interferometer, the output
beams are collected by a series of eight
detectors (four per output port). The detectors
are cooled by a pair of synchronized
Stirling cycle coolers. The detectors record the
varying interference pattern. By post-processing
of the recorded interferograms, the spectral
distribution of the radiation can be
obtained.
During the calibration sequence, the
azimuth mirror is pointed to an internal
calibration blackbody. This blackbody is a
reference source that can be used to calibrate
the measured signal.
The instrument
control unit contains all the electronics to
drive the front end optics mirrors (ESU and ASU) and the interferometer
subsystems such as the laser diode and the
cube corners actuators. It
also contains modules used to perform basic
processing of the acquired data before
transferring it to the ENVISAT platform.
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Keywords: ESA European
Space Agency - Agence spatiale europeenne,
observation de la terre, earth observation,
satellite remote sensing,
teledetection, geophysique, altimetrie, radar,
chimique atmospherique, geophysics, altimetry, radar,
atmospheric chemistry
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