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Instrument: ATSR : Along Track Scanning Radiometer and Microwave Sounder
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The Along Track Scanning Radiometer and Microwave Sounder (ATSR) is
one of the instruments carried on-board of the European Remote Sensing
Satellites (ERS-1 and ERS-2) launched by the European Space Agency on
17 July 1991 and 20 April 1995.

The ATSR incorporates two separate instruments: an advanced four-
channel infrared radiometer (the ATSR-IRR, commonly called the ATSR)
used for measuring sea surface temperature and cloud top temperatures,
and a two-channel Microwave Sounder (the ATSR-MWR, commonly called the
MWR) designed to measure total precipitable water vapour and the total
liquid water content of the atmosphere.

The ATSR-IRR images are in four co-registered infra-red channels in a
500 km wide swath. The viewing geometry involves a unique scanning
technique where the surface is viewed at two angles, one close to the
nadir (0 degree, nadir swath) and the other at 47 degrees (forward
swath), within the space of two minutes (the time it takes the
sub-satellite point to reach the along track point which is separated
by approximately 800 km). These two swaths are produced by a scanning
mirror with a rotation axis inclined 23.45 degrees from the vertical,
resulting in a near-elliptical path on the Earth's surface.

On-board calibration is carried out by incorporating two 'black
bodies' into the instrument scan pattern. The IR sensors are kept to
80 K by an active cooling system to minimize the thermal noise
component in the sensor signals.

ATSR-IRR characteristics:

Spectral channels: 1.6, 3.7, 11 and 12 microns
Spatial resolution: 1 km x 1 km (nadir); 1.5 km x 2 km
Swath width: 500 km
Detector: single element InSb and HgCdTe
Cooler: Stirling Cycle Cooler
Radiometric precision: 0.1 K
SST predicted accuracy: 0.5 K over a 50 kmx 50 km square
with 80% cloud cover

The ATSR was designed to provide the following:

- global sea surface temperature, accurate to better than
0.5K (absolute) with a spatial resolution of 50 Km in
conditions of up to 80% cloud cover
- images of surface temperature, accurate to 0.1K
(relative) with 1 km resolution and 500 km swath width
- total water vapour content of the atmosphere
- observations of clouds, aerosols, haze, land-ice and
sea-ice surface emissivity
- tropospheric range correction of the Radar Altimeter
measurements to better than 5 cm

The ATSR provides important information in scientific disciplines such
as oceanography, climatology and meteorology. When combined with
observations of cloud top temperatures, cloud cover, haze, aerosol and
total water vapour content of the atmosphere, significant improvements
may be expected in the accuracy of medium range weather
forecasting. Also accurate sea surface temperatures can be of use to a
number of commercial users, particularly those involved in fishing and
the management of fishing areas. In the field of research, potential
applications of the ATSR include distinguishing thin new ice from open
water, identifying surface type, and the accumulation rate of land

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