Each ATSR instrument has been designed for exceptional sensitivity and stability of calibration which are achieved through the incorporation of several innovative features in the instrument design:
Along Track Scanning
Application of the along track scanning technique is the ATSR instrument's most innovative development. This works by making two observations of the same point on the Earth's surface through differing amounts of atmosphere; the "along track" view passes through a longer atmospheric path and so is more affected by the atmosphere than the nadir view.
First, the ATSR views the surface along the direction of the orbit track at an incidence angle of 55× as it flies toward the scene. Then, some 150s later, ATSR records a second observation of the scene at an angle close to the nadir.
By combining the data from these two views a direct measurement of the effect of the atmosphere is obtained, which yields an atmospheric correction for the surface data set which is an improvement on that obtained from a single measurement.
ATSR-1 was launched as part of the payload of ESA's ERS-1 satellite on 17th July 1991, and was the test-bed for the along track scanning concept. It carries infrared channels at 1.6µm, 3.7µm, 10.8µm and 12.0µm, and has no visible channels. Routine ATSR-1 operations stopped when ERS-1 was put into hibernation in June 1996, but the instrument is still capable of operation as, even after nearly 7 years of use, the signal to noise performance of the detectors is higher than for a typical AVHRR at launch.
ATSR-2 and AATSR
The ATSR-2 and Advanced ATSR (AATSR) instruments are developments from the original experimental ATSR-1 instrument which, in addition to the ATSR-1's infrared channels, carry extra visible channels at 0.55µm, 0.67µm and 0.87µm for vegetation remote sensing. The evolution of ATSR-2 was constrained by the requirement to maintain the ATSR-1 precision measurement of global SST.
The ATSR-2 instrument, launched in April 1995, is currently flying as part of the payload of the ESA ERS-2 satellite, and AATSR will be launched early next century on ESA's Envisat platform.The AATSR instrument represents an orderly development of the ATSR series of instruments.
TABLE 1. ATSR-1, ATSR-2 and AATSR Spectral Channels
The ATSR-2 instrument for ERS-2 is largely the same as ATSR-1 except for:
The AATSR instrument is functionally the same as the ATSR-2, but the structure and some of the other components have been re-worked to match the environment of the Envisat platform, which is somewhat different to the ERS satellites.
The major purpose of AATSR is to provide continuity of the crucial sea surface temperature data sets which have been produced by ATSR-1 and ATSR-2. Therefore, the key scientific parameters which were optimised for ATSR, are retained for AATSR. Thus details of the scan, the optical system, the basic spectral bands, thermal calibration system, spatial resolution and swath have been kept as close as possible to those of the original instrument to ensure continuity.
The major advantage AATSR has over ATSR-2 is in the telemetry bandwidth available on Envisat. For ATSR-2, the limited telemetry available on ERS-2 imposed severe limitations on the collection of visible channel data; on Envisat there are no such restrictions, so AATSR can telemeter all the visible channel data it can collect. This significantly simplifies the ground processing required for AATSR data, as the processor does not need to cope with the wide range of data formats that are possible from ATSR-2.