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The primary scientific objective of the AATSR is to establish continuity of the ATSR-1 and ATSR-2 data sets of precise Sea Surface Temperature (SST), thereby ensuring the production of a unique 10 year near-continuous data set at the levels of accuracy required (0.5 K or better) for climate research and for the community of operational as well as scientific users developed through the ERS-1 and ERS-2 missions.

The second objective is to perform quantitative measurements over land surfaces. The land and cloud measurement objectives will be met through the use of an additional visible focal plane assembly, which will lead to indications of

  • vegetation biomass,
  • vegetation moisture,
  • vegetation health and growth stage

The visible channels will also contribute to the measurement of cloud parameters, like water/ice discrimination and particle size distribution.

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The SST is one of the most stable of several key geophysical variables which, when determined globally, contribute to the characterisation of the state of the Earth’s climate system.

The precise measurement of small changes in SST provides an indication of quite significant changes in ocean/atmosphere heat transfer rates, especially in the tropics; also it is known that small amplitude anomalies occurring in specific areas are sometimes associated with massive atmospheric perturbations, leading to widespread and damaging changes in the global weather system. For example, the ‘El Nino’ anomaly in the tropical East Pacific is associated with a reversal of the atmospheric ‘Walker Circulation’. This in turn creates widespread perturbations to the global weather system.

The exact causal relationships between such phenomena are not fully understood but a significant ‘El Nino’-event can evolve from an SST anomaly of 2-3 K, and therefore the ability to detect, for example, a 10% change in the anomaly field will require measurements of the accuracy provided by the series of (A)ATSR instruments.

The principle of removing atmospheric effects in measurements by viewing the sea surface from two angles is the basis of the family of (A)ATSR instruments. The SST objectives will be met through the use of thermal infrared channels (centred on 1.6 µm, 3.7 µm, 10.85 µm and 12 µm), identical to those on ATSR-1&2.Atmospheric modelling for ERS-1 has shown that ATSR, with its thermal IR channels and two-angle viewing geometry, can achieve a global accuracy in SST of better than 0.5 K.

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As with the AATSR thermal infrared channels, the measurement philosophy with respect to the visible channels is to develop and exploit a capability for making accurate quantitative measurements of radiation from the Earth surface, using an on-board calibration system for radiometric accuracy, also using a two-angle
viewing technique to obtain accurate atmospheric corrections.

The most important two visible channels are centered on 0.865 µm and 0.67 µm respectively and will provide measurements of Vegetation Index in the same way as AVHRR. The AATSR will have the capability for making global measurements with 1 km x 1 km resolution at nadir. An additional visible channel at 0.55 µm is also incorporated, to indicate, from chlorophyll content, the growth stage and health of vegetation.

 

The AATSR functional principles are shown in the block diagram. The infrared-visible radiometer includes an inclined plane scan mirror which is rotated continuously in front of a reflecting telescope to provide a conical scan. The cone axis projects downwards and ahead in the along track direction of the satellite, achieving scanning across the satellite track in two regions.

 

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The Focal Plane Assembly (FPA) is equipped with a number of oversized detectors, one per channel. A pair of Stirling Cycle Coolers, controlled by the Cooler Control Unit are used to cool the detectors to 80 K. Two blackbody infrared calibration targets controlled by the Black Body Electronics Unit and a diffuse reflector visible calibration target are viewed between the nadir and along-track scans.

 

The electrical signals from the FPA are conditioned by preamplifiers which deliver the signals to the Instrument Electronics Unit for further processing. The Digital Electronics Unit (DEU) performs the data formatting and command & control functions. The DEU also provides the interfaces to the PPF spacecraft.


The AATSR, developed under leadership of MMS-B, is a British/Australian contribution to the ENVISAT-1 Mission.