1.1.5 Special Features of AATSR
AATSR has a number of special features including:
18.104.22.168 Onboard Calibration The AATSR instrument has proven
onboard calibration systems for the
thermal and visible channels, to
ensure that the science goals of AATSR are met.
Thermal Calibration System
Two black body reference
targets are viewed on each
scan, with one at roughly at 265K and the
other at 305K, as this is expected to
encompass the full global range
of SSTs. They use the design
concept which has been proven on
ATSR-1 and ATSR-2.
The calibration sources are designed such
that uncertainties in the
radiance from them will not exceed an
equivalent temperature error
of more than 100mK throughout the
mission (see section ).
The visible calibration needs two sources for
gain and offset measurements. By
using one of the thermal calibration
sources as a zero radiance calibration
point, only one source of higher
radiance is necessary. The calibration system for the
(VISCAL) provides a stable source for
calibration once per orbit, using sunlight
to illuminate a diffusing plate.
The system has been proven on ATSR-2.
22.214.171.124 Dual View
As described already in section 1.1.3. , AATSR employs a
dual view. This can improve the
atmospheric correction and thus enable more
precise sea surface temperature
measurements. The (A)ATSR instruments have a
viewing geometry where each terrestrial
scene is viewed at two angles, at nadir and
at a forward angle of 55°. From two
views with different atmospheric path
lengths, it is possible to obtain
independent information about the
atmospheric contributions to the signal, and
perform an accurate atmospheric
correction. A dual view can also be used in
land remote sensing to give information on
the bi-directional reflectance
distribution function of different surfaces.
Another unique feature of the (A)ATSR design
is the use of closed-cycle mechanical
coolers to maintain the thermal environment
necessary for optimal operation of the
infrared detectors. The FPA for the thermal
infrared wavelength region is cooled to
about 80 K whilst the other is
maintained at ambient temperature. ATSR-1
was the first environmental sensor to carry
such a cooler into space.
126.96.36.199 Full 12 bit digitisation
AATSR offers the same combination of visible,
near infrared and thermal channels as ATSR-2
but has the added advantage that the
improved data rates available on ENVISAT
will provide global coverage at the highest
(12 bit) digital resolution over the whole
swath throughout the orbit.
Details of the differences between the ATSR-2
and AATSR source packets are discussed in section 2.5.2. of the Reference Guide.