The Along Track Scanning Radiometers (ATSR) are multi-channel imaging radiometers 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 ATSRs are a series of 'Announcement of Opportunity' (AO) instruments, jointly funded by the UK and Australia and flying on board the European Space Agency's ERS-1, ERS-2 and Envisat satellites. The first in the series was designated ATSR-1, the second ATSR-2 and the currently operational instrument is the third in the series, the Advanced ATSR (AATSR). The fourth ATSR, called the Sea and Land Surface Temperature Radiometer (SLSTR), is being funded and developed directly by ESA and will be flown on ESA's Sentinel-3 satellite.
ATSR-1 was initially funded by the (then) Science and Engineering Research Council (SERC) and following a re-organistion of UK research organisations, by the Natural Environment Research Council (NERC), who continued and enhanced the programme with ATSR-2. The first two instruments were designed by the Rutherford Appleton Laboratory (RAL) as experimental scientific instruments. The current AATSR programme, funded by the Department of Energy and Climate Change (DECC) was procured from EADS Astrium and has seen the ATSR series progress to operational status. The first three ATSR instruments were also supported by the (then) Australian Space Office and the Commonwealth Scientific and Industrial Research Organisation (CSIRO). The fourth sensor, currently designated SLSTR, has full operational status within the EU's Global Monitoring for Environment and Security (GMES) programme.
The required accuracies for ATSR-derived SST are better than 0.3 °C with a stability approaching 0.1 °C /decade. In order to achieve this accuracy consistently under a range of typical atmospheric conditions, ATSR views the surface at two angles, one close to the nadir (immediately below the satellite) and the other along the satellite track at close to 55° from the nadir, thereby providing two views of each point on the Earth's surface, each with a different effective atmospheric thickness. In difficult atmospheric conditions, especially in the presence of atmospheric aerosols, this enables a particularly accurate estimate to be made of the signal degradation due to atmospheric absorption and scattering.
ATSR 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 'blackbodies' are maintained at temperatures near to the extremes of the Earth temperatures as measured by ATSR 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 ATSR's performance.
ATSR has three channels at thermal infrared wavelengths at 3.7, 11 and 12 µm, from which surface temperatures are derived over both sea and land surfaces. A reflected infrared channel at 1.6 µm is used to improve the detection of clouds by day. In addition, from the second instrument onwards, ATSR has three more visible and near-infrared channels at 0.55, 0.67 and 0.87 µm 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, as well as clouds 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.