Geometry and General Principles

The instrument is capable of performing measurements in two pointing regimes

The instrument is capable of performing measurements in two pointing regimes: rearwards within a 35° wide range in the anti-flight direction, and sideways within a 30° wide area in the anti-sun direction. The rearward viewing range is used for most measurements, as it provides a good Earth coverage, including the polar regions. The sideways range is important for observation of special events, like volcano eruptions, trace-gas concentrations above major air traffic routes, or concentration gradients along the dusk/dawn lines.

As a result of the limb viewing geometry, the distance between instrument and tangent point is about 3,300 km. Thus, in order to measure at a predetermined limb height, the pointing of instrument and satellite in elevation direction must be excellent. It is a goal to determine the geometric limb height by pointing information from the spacecraft with a standard deviation below 600 m. Thus, line-of-sight-pointing knowledge with respect to nadir of better than 0.01° (1-sigma) is required. A very high stability of all assemblies affecting the pointing is a design driver for MIPAS as well as for the Envisat satellite.

The spectral coverage is set from 14.6 microns to 4.15 microns (685 cm^-1 to 2410 cm^-1). This range covers almost the complete thermal infrared region, and thus emission lines of most atmospheric species are present. A Fourier transform spectrometer is ideally suited to perform measurements of such wide spectral coverage with highest possible sensitivity.

The high spectral resolution (unapodized) of better than 0.035 cm^-1 (corresponding to 1 GHz resolution or 0.006 nm at a wavelength of 4.15 microns) is necessary to resolve lines in the spectrum and to reduce the interference of overlapping spectral features. With this high spectral resolution, MIPAS provides a total of 50,000 independent spectral samples in each spectrum.

A complete high resolution spectrum is measured within 4 s (plus 0.6 s for the speed reversal of the slides). MIPAS is designed to perform measurements also with a lower spectral resolution in shorter time for special measurement opportunities.

The radiometric requirements on MIPAS are highly demanding. A good radiometric sensitivity is essential to allow detection of weak atmospheric signals without additional averaging. Radiometric sensitivity is expressed here by the noise equivalent spectral radiance NESR, which characterises the instrument noise in terms of incident radiance. The required sensitivity must be better than 50 nW/cm² sr cm^-1 at the long wavelength side, decreasing to 4.2 nW/cm² sr cm^-1 at the short wavelength side.

The requirement on radiometric accuracy is equally stringent: a calibration accuracy in the 1 to 3 % range is difficult to achieve even for ground-based instruments. However, a good absolute knowledge of the received radiance is important to retrieve the atmospheric temperature of the emitting layer, which is a key parameter in the data retrieval.

MIPAS obtains a series of spectra from different limb heights to allow the retrieval of species concentration profiles. One basic elevation scan sequence comprises 16 high-resolution spectra (or up to 75 spectra with reduced spectral resolution) and takes 75 s (corresponding to a forward motion of the spacecraft of 500 km). A typical elevation scan starts at about 50 km limb height and descend in 3 km steps to 8 km, but MIPAS is sufficiently flexible to perform any elevation scan sequence within the range of 5 km to 150 km limb height, even with variable step sizes.

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