Minimize Design

MIPAS is a high-resolution Fourier Transform Infrared spectrometer which is designed to measure concentration profiles of various atmospheric constituents on a global scale. It will observe the atmospheric emissions from the Earth horizon (limb) in the mid infrared region (4.15 µm - 14.6 µm) providing global observations of photochemically interrelated trace gases in the middle atmosphere, in the tropopause and in the upper troposphere.

These data will contribute to the development of a better understanding in the following research areas:

  • Stratospheric Chemistry: Global ozone problem, polar
    stratospheric chemistry,
  •  Global Climatology: Global distribution of climate
    relevant constituents,
  • Atmospheric Dynamics: transport exchange between
    troposphere and stratosphere,
  • Upper Tropospheric Chemistry: Correlation of gas
    distribution with human activities.

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The instrument is designed to allow the simultaneous measurement of more than 20 relevant trace gases, including the complete NOy family and several CFCs. The atmospheric temperature as well as the distribution of aerosol particles, tropospheric cirrus clouds and stratospheric ice clouds (including Polar Stratospheric Clouds) are further important parameters which can be
derived from MIPAS observations.

The data are obtained with complete global coverage, for all seasons and independent on illumination conditions, allowing measurement of the diurnal variation of trace species.

The atmospheric emissions will be measured at the horizon of the Earth (limb) over a height range of 5 to 150 km. This observation geometry allows the maximum measurement sensitivity and a good profiling capability to be achieved.

The MIPAS data products are calibrated high-resolution spectra which are derived on ground from the transmitted interferograms. From these spectra, geophysical parameters such as trace gas concentrations, temperature profiles, mixing ratios, are retrieved permitted to establish global maps of atmospheric
constituents in geophysical coordinates.

MIPAS will perform measurements in either of two pointing regimes: rearwards within a 35° wide viewing range in the anti-flight direction and sideways within a
30° wide range on the anti-sun side. The rearward viewing range will be used for most measurements, since it provides a good Earth coverage including the polar
regions. 

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The sideways range is important for observations of special events, like volcano eruptions, trace gas concentrations above major traffic routes or concentration gradients across the dawn/dusk border. In nominal measurement mode, MIPAS will perform series of measurements at different tangent heights by performing elevation scan sequences with a duration of 75s in the rearward viewing range. Such an elevation scan sequence comprises typically 16 interferometers
sweeps.

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For special event observations, viewing elevation scans sequences in the sideways range can be commanded. Radiometric calibration will be performed using two measurements:

  • gain calibration approximately once per week, applying
    a two-point calibration method, where radiances from
    deep space and an internal blackbody are recorded in
    sequence,
  • offset calibration, prior to every elevation scan sequence,
    in order to correct for the instrument self-emission.

Another calibration mode will be required for the inflight determination of the line of-sight pointing direction, which is based on the observation of stars crossing the instrument field of view and subsequent correlation of the actual with the predicted time of star crossing.

The MIPAS blockdiagram depicts the functional elements and their relationship. The radiation emitted from the observed scene will enter MIPAS through the Front-End Optics comprising an azimuth scan mirror, an elevation scan mirror and a telescope. The radiation propagates through a dual slide, dual port Michelson-type interferometer, which is designed to provide an unapodized spectral resolution of better than 0.035 cm-1 throughout the observed spectral range.

The input signals are divided at the beamsplitter inside the interferometer and directed to cube corners moving at a constant velocity of 25 mm/s along a path of 100 mm. Hence one spectrum is typically recorded within 4 s. From the cube corners the IR beam is reflected to the beam recombiner and then directed to
the output ports.

Depending on the optical path difference in the two interferometer arms, the recombined signal is an intensity modulated interferogram. The optical path difference signal for the interferogram sampling is derived from a reference laser the interferogram. The output signal enters the Focal Plane Subsystem where beam size matching, beam splitting and optical filtering is performed.

After optical filtering the input spectrum is separated into eight narrow spectral bands for detection by eight Hg:Cd:Te-detectors operating at about 70 K.

After pre-amplification, the eight signals are amplified, lowpass filtered and digitized by the analogue processing part of the Signal Processing Electronics. Its digital part separates the spectral range of interest by complex filtering, undersamples the input sequences individually and equalizes channels to be combined.

Word length truncation is then performed to reduce the data rate for
downlink budget reasons. In a final processing step, measurement data is multiplexed and formatted to source packets. On ground, the incoming source packets are sorted according to measurement/calibration data
and the interferograms are converted into calibrated spectra.

MIPAS is developed under leadership of Dornier Satellitensysteme GmbH.

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