Minimize Design

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:

  • use of low-noise infrared detectors, cooled to near-optimum temperatures (i.e., less than 95 K) by a Stirling cycle mechanical cooler;
  • continuous on-board radiometric calibration of the infrared channels against two stable, high-accuracy blackbody calibration targets and, in the case of ATSR-2 and AATSR, calibration of the visible and near infrared channels with an on-board visible calibration system;
  • use of the multichannel approach to SST retrieval previously demonstrated by the AVHRR instruments;
  • use of the "along-track scanning" technique to provide two views of the surface and thus an improved correction for atmospheric effects.


ATSR's field of view comprises two 500 km-wide curved swaths, with 555 pixels across the nadir swath and 371 pixels across the forward swath. The nominal instantaneous field of view (IFOV) pixel size is 1 km2 at the centre of the nadir swath and 1.5µm processing.

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

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.

Feature

Wavelength

Bandwidth

ATSR-1

ATSR-2


AATSR

Detector type

Chlorophyll

0.55µm

20µm

N

Y

Si

Vegetation Index

0.67µm

20µm

N

Y

Si

Vegetation Index

0.87µm

20µm

N

Y

Si

Cloud Clearing

1.6µm

0.3µm

Y

Y

PV InSb

SST retrieval

3.7µm

0.3µm

Y

Y

PV InSb

SST retrieval

10.8µm

1.0µm

Y

Y

PC CMT

SST retrieval

12.0µm

1.0µm

Y

Y

PC CMT

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 inclusion of 3 extra spectral bands in the visible, mainly for vegetation monitoring
  • an on-board visible calibration system

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.