AATSR

The required accuracies are better than 0.3 °C with a stability approaching 0.1 °C /decade. Because of its wide angle lens it is possible to make very precise measurements of atmospheric effects on how emissions from the Earth's surface propagate.

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 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. This dual-view system 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, as well as clouds and atmospheric particulate matter (or aerosols) can be studied.

The dual-view measurements provided by the along-track scanning technique is also a key feature of AATSR when used for atmospheric or vegetation measurements. As with the SST measurements, the two measurements of each point on the surface through different atmospheric path lengths greatly simplifies the task of separating the surface and atmospheric contributions to the measured signal. Thus the effects of atmospheric aerosols can be removed from surface products (such as surface reflectance or NDVI).[1] Conversely, a similar approach can be used to account for the surface reflectance in determining aerosol or cloud properties. Additionally, the parallax between the two views can be used to determine cloud top and aerosol plume heights using geometric means.[2]

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.