SCIAMACHY

from Wikipedia, the free encyclopedia

SCIAMACHY ( English Sc anning I maging A bsorption Spectro m eter for A tmospheric Ch Artograph y ; Greek σκιαμαχή , mutatis mutandis, to fight against the shadow ) is one of ten instruments on the European environmental satellite Envisat and created global maps of different atmospheric trace gases (O 3 , NO 2 , O 2 , H 2 O, CH 4 , CO 2 , CO, BrO, OClO, SO 2, IO) and other parameters. The spectral data were evaluated many times using the DOAS method.

SCIAMACHY aboard Envisat was launched on an Ariane 5 rocket on March 1, 2002 from Kourou , French Guiana, into sun-synchronous polar orbit at an altitude of 799.8 kilometers with an inclination angle of 98.6 °. On April 8, 2012, the measurements ended due to the loss of communication with Envisat. The direction of flight on the day side of the earth is from north to south. Due to the solar synchronicity, the local overflight times are at a fixed local time over Central Europe at 10 a.m. In polar latitudes, measurements at other times of the day were also possible. Measurements on the night side of the earth were used, among other things, for calibrations. The satellite needs around 100 minutes to orbit the earth. Just over 14 orbits are flown in the course of a day. This made SCIAMACHY suitable for global earth observation.

Observation geometries

SCIAMACHY, nadir and edge scanning.

SCIAMACHY is operated daily in different observation geometries in a fixed order:

  • Nadir observation: The air column below the satellite is detected here. The field of view of the spectrometer at the height of the earth's surface of approx. 750 km is 25 × 0.6 km², due to the speed of the satellite of 7 km / s and the frequency of the scanning mirror of 240 km / s, the resolution of SCIAMACHY in this observation geometry is about 30 km in the direction of flight and 240 km across the direction of flight. This observation geometry provides a relatively good spatial horizontal resolution. Essentially, trace gas columns are measured in the nadir geometry. With one exception, trace gas profiles can also be measured, but the vertical resolution of the ozone profiles is lower than in the Limb geometry.
  • Limb observation (English for edge ) or edge scanning observation: The field of view in this observation geometry is 110 km across the direction of flight and 2.6 km vertically at a distance of approx. 3000 km that is approx. 4 times greater. In limb scattered light measurements, the earth's atmosphere is scanned tangentially from the earth's surface up to a tangent height of 92 km in steps of 3 km. These measurements take a total of approx. 90 s. During this time the satellite moves forward by approx. 630 km, which results in a relatively poor spatial horizontal resolution of 960 × 630 km 2 , but a relatively good vertical resolution of the measurements of 3 km.
  • Sun and moon occultation observation : A spectrum is recorded with the sun or the moon as a direct background. The field of view is 30 km in azimuthal and 2.5 km in vertical direction. The high intensity of the sun and moonlight enables very short integration times of approx. 62 ms. A complete vertical measurement therefore only takes 1 s and the field of view in this observation mode also corresponds to the resolution of 30 km horizontally × 2.5 km vertically. Occultation measurements are among the most accurate, high quality measurements due to the large intensities recorded. Occultation measurements can only be made at certain times of the day (sunrise and sunset). This means that there is only poor spatial coverage.

The eight-channel spectrometer measures transmitted sunlight that is scattered in the earth's atmosphere and reflected on the earth's surface in the ultraviolet to near-infrared spectral range (see Tab. 1). From the measurements, depending on the observation geometry, total column contents and profiles of trace gases and aerosols as well as cloud parameters can be derived. In addition, the polarization of the light is measured in six channels. A 5 watt tungsten white light lamp, a NePtCr cathode discharge lamp and the recording of solar spectra in various observation geometries are used for calibrations.

Tab. 1: SCIAMACHY channels
channel Wavelength [nm] Resolution [nm] Detector material Temperature [K]
1 214-314 0.21 Si 200
2 309-404 0.22 Si 200
3 392-605 0.47 Si 235
4th 598-790 0.42 Si 235
5 776-1056 0.55 Si 235
6th 991-1750 1.56 InGaAs 200
7th 1940-2040 0.21 InGaAs 150
8th 2260-2384 0.24 InGaAs 150

Web links