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<!-- The Advanced Along Track Scanning Radiometer (AATSR) is one of the Announcement of Opportunity (AO) instruments on-board the European Space Agency(ESA)'s Envisat platform. This instrument is a multi-channel imaging radiometer with the principal objective of providing data concerning global Sea Surface Temperature (SST) to the high levels of accuracy and stability required for monitoring and carrying out research into the behaviour of the Earth’s climate. The required accuracies are better than 0.3 ºC with a stability approaching 0.1 ºC /decade.

In order to achieve this accuracy while viewing the Earth's surface through the atmosphere, AATSR views the surface at two angles, one close to the nadir (immediately below the satellite) and the other along the satellite track at approximately 50º to the nadir, thereby providing two views of each point on the Earth's surface, each with a different effective atmospheric thickness. This enables a particularly accurate estimate to be made of the signal degradation due to atmospheric absorption and scattering.

AATSR also embodies an exceptionally precise and stable on-board calibration system, comprising two reference targets specially designed for high uniformity and stability. These two targets, known as ‘black bodies’ are maintained at temperatures near to the extremes of the Earth temperatures as measured by AATSR and they are both viewed during each scan cycle of the instrument. This calibration system ensures that measurements of thermal radiation from the Earth’s surface are properly calibrated and do not rely on ground-based measurements, although such measurements are continually used to evaluate AATSR’s performance.

AATSR has three channels at thermal infrared wavelengths, from which surface temperatures are derived over both sea and land surfaces. In addition, AATSR has four visible and near-infrared wavelength channels which are used to identify cloudy areas and to measure solar radiation that is scattered and reflected from the Earth’s surface and atmosphere. These channels provide measurements from which land-cover properties, for example, NDVI, and atmospheric particulate matter, or aerosols ), can be studied. In the case of vegetation properties the along-track scanning technique is able to provide improvements in accuracy, especially in cases where the measurements are affected by the presence of excessive aerosol matter in the atmosphere.

AATSR is the third in a series of instruments started by ATSR-1, which was launched in 1991 on the European Remote-Sensing Satellite, ERS-1. This was followed by ATSR-2 on ERS-2 in 1995 and by AATSR on the Envisat satellite in 2002. This has led to a near-continuous SST dataset extending from 1991 to the present day. Looking to the future, a successor instrument is being developed to fly on ESA’s Sentinel 3 satellite, which is part of the space segment of the European Global Monitoring for Environment and Security (GMES) programme. This will ensure the continuity of AATSR-standard SST data into the foreseeable future.

One of the most important aspects of the ATSR series of space instruments is that it has, over a period of just over 15 years, undergone the transition from experimental sensor on the ERS satellites, developing the technique and demonstrating the accuracy that can be achieved with along-track scanning, to that of an operational system, within Europe’s Envisat and future GMES Programmes.

The AATSR is a nationally-provided instrument, funded by the UK’s Department for Environment, Food and Rural Affairs (Defra) in order to support their programme of climate prediction and research. It was developed and is operated in collaboration with ESA. There is also a significant Australian contribution to the AATSR programme, now managed by the Commonwealth Scientific and Industrial Research Organisation. -->