1.1.1 Geophysical Measurements
1.1.1.1 Primary Geophysical Measurement
The primary mission objective of the AATSR instrument is to
extend the long-term consistent set of
global SST measurements started with ATSR-1 and continued
with ATSR-2.
Sea Surface Temperature (SST)
AATSR has thermal infrared channels measuring
upwelling radiance from the sea surface and
the atmosphere at 3.7, 11 and 12 µm.
From the calibrated top of the
atmosphere Brightness Temperatures (BT)
available from these bands, SST is
calculated using the 11 and 12 µm
channels during the day and the 11, 12 and
3.7 µm channels at night.
Very high levels of accuracy and precision
are required to support global climate
monitoring and change detection. Therefore,
the AATSR instrument and ground
processing system are required to produce
SST retrievals routinely from the
corresponding BTs with an absolute
accuracy of better than 0.3K, globally, both
for a single sample and when averaged over
areas of 0.5° longitude by 0.5°
latitude, under certain cloud free
conditions (i.e. >20% cloud free samples
within each area). The AATSR programme also
aims to maintain an instrument stability
of 0.1K during the mission lifetime.
It is important to note that the (A)ATSR
instruments return SST measurements for the
ocean's 'skin' and that the
temperature of the sea skin surface is
typically a few tenths of a degree cooler
than the temperature a few centimetres
below. Due to the limited penetration of
thermal infrared radiation through the water
column, the infrared radiometric temperature
will be that of only the top few tens of
micrometres, whereas the oceanographically
understood SST is a measure of the
temperature in the top 10 cm.
1.1.1.2 Additional Capabilities
Land Remote Sensing
AATSR also includes three visible/near
infrared channels centred at
0.55, 0.67 and 0.87µm. These were first
introduced on ATSR-2 and have extended
the instrument's
capabilities over land, particularly for
the study of vegetation.
Much of the time, the basic calibrated top of
the atmosphere reflectance
measurements from the AATSR visible
and near infrared channels are
used for studies of vegetation
quantity and quality. In order to
cope with all possible normal
variations in brightness over the
Earth's surface without saturation
whilst maximising the precision
of the measurements, the gain and offset of
the visible channels is selectable in
flight. These channels have a
signal to noise ratio of 20:1 at 0.5%
spectral albedo and measure top of the
atmosphere radiances to an
absolute accuracy of 5% over its entire range.
Two specific geophysical parameters are also
available in the AATSR products
over land:
NDVI
The standard AATSR Level 2 product includes
the Normalised Difference
Vegetation Index (NDVI) using the 0.67 and
0.87 µm channels.
Land Surface Temperature
The AATSR thermal bands used for SST
retrieval are also available
over land and are used to retrieve Land
Surface Temperature (LST) in the
AATSR gridded (1 km) product.
Algorithms for deriving LST using
split-window radiances are sufficiently
advanced that accuracies of
1-3 K are possible. Better
accuracies (about 1 K) are
obtained at night when
differential surface heating is absent.
Measurements of radiance over exceptionally
hot targets such as deserts,
burning vegetation and volcanic eruptions
are also possible using
AATSR's 'low-gain'
mode. This prevents
saturation of the thermal
channels and applies to targets
with an upper limit of 500°C.
1.1.1.3 Cloud and Atmospheric Measurements
Currently AATSR does not retrieve cloud and
atmospheric parameters on a routine basis.
However, the different AATSR channels can be
used directly to provide information on
the basic location, extent and structure of
clouds. Two fields in the Level 2 product
have also been reserved for Cloud Top
Temperature (CTT) and Cloud Top Height
(CTH), should suitable retrieval algorithms
become available in the future.
Work with ATSR-2 data has also demonstrated
that the visible and thermal channels can be
combined to estimate a number of specific
properties within the cloud field. These include:
- Optical Depth, broadly related to the
vertical dimension of the cloud;
- Phase, which determines whether the
cloud contains ice or water;
- Particle Size, the effective radiative
dimension of the cloud particles; and,
- Pressure, reflecting cloud top pressure
or altitude.
The dual views provided by AATSR also offer a
stereo viewing capability that can be used
to discriminate between the different layers
and structures within the cloud and also to
estimate cloud top height.
1.1.1.4 Cryosphere
AATSR visible and thermal channel data can be
used for studies of the cryosphere. Used on
their own, or more often synergistically
with other sensors, potential
applications include snow cover monitoring,
monitoring variations in annual sea ice
growth and decay, location of boundaries
and features of the Antarctic ice sheet, and
feature mapping for radar altimetry over
land and ice. The discriminatory power of
the 1.6 µm channel for cloud and
ice/snow is a topic of particular research interest.
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