The ATSR instrument measures the earth radiance. They are supplied as well as geophysical quantities in the ATSR products. Below is a list of the geophysical information as given in Zavody et al., 1994.

An essential part of the ATSR data processing is the cloud filtering as ground surface parameters can be retrieved only over cloud free pixels, due to the interfering effect of clouds, in the ATSR spectral bands.

Brightness Temperature (BT). It is derived from the 3.7 micron spectral channel (only at night), and the 10.8 and 12 micron spectral channels. The radiance is converted to BT using calibration coefficients derived from the two instrument internal reference blackbodies (hot and cold).

Reflectance. It is derived from the 1.6 micron (near infrared) and from the visible (ATSR-2) spectral channels. The calibration of the visible spectral channels (ATSR-2) is based on the sun radiation spectrum. Stable targets at the ground surface can also be considered for calibration purposes (in cloud free conditions).

Sea Surface Temperature (SST) (skin temperature). It is derived for cloud free pixels by means of a split-window algorithm, using a radiative transfer model for the derivation of the coefficients. The ATSR SST product pixels are given at a reference grid having 10 arcmin lat/long spatial resolution. Pixels are classified as land or sea and those cloudy are flagged; the views used for SST retrieval are given as well.

Spatially Averaged Sea Surface Temperature (ASST). It is derived relatively to a standard ground surface reference grid, having a spatial resolution of 10 or 30 arcmin latitude/longitude. SST standard deviations are also supplied along with confidence flags and with the difference between the SSTs values derived by using single or dual view data.

Cloud Top Temperature (CTT). It is derived from the nadir view of the pixels classified as cloudy, independently of the terrain below.

Cloud classification. It is derived from CTT values over 10 arcmin lat./long. cells. Four cloud categories are identified: fog/low-level, medium-level, high-level, mixed type; the cloud cover percentage is also given for each type.

A cloud filtering procedure is applied in the processing of ATSR SST products. (Zavody et al., 1994) The procedure is based on six cloud detection tests derived from Saunders and Kriebel (1988). Recently some improvements have been presented at the ERS Symposium (Florence, March 1997) by Zavody et al.; a complete description of the cloud filtering scheme is in publication (J. Geophys. Res.).

The cloud detection tests are:

1- Near infrared channel test

2- Spatial coherence test

3- Gross cloud test

4- Thin cirrus test (infrared histogram test)

5- Fog/low stratus test

6- Medium/high level cloud test

Zavody, A.M., Gorman, M.R., Lee, D.J., D. Eccles, D., Mutlow, C.T., and Llewellyn-Jones, D.T., 1994, The ATSR data processing scheme developed for the EODC, Int. J. Remote Sensing, 15, 4, 827-843.

Zavody, A.M., Mutlow, C.T., and Llewellyn-Jones D.T., Proceedings 3rd ERS Symposium, Florence, 17-21 March 1997.

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry