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.