Some improvements on the processes to obtain accurate maps of sea surface temperature from AVHRR raw data transmitted in real time .2. APT images

In this paper we have developed some improvements in order to obtain accurate Sea Surface Temperature (SST) maps with APT (Automatic Picture Transmission) data transmitted in real time from NOAA satellites. The methodology proposed can be profitable for any user interested in accurate retrievals of SST such as meteorologists, climatologists and oceanographers. APT data are converted to 10-bit digital data like in HRPT images in order to perform calibration. Geometric correction is achieved with a subpixel accuracy for APT images. Small clouds are avoided by means of the technique of universal kriging. A single-channel method using radiosounding data has been developed for the atmospheric and emissivity corrections. Atmosphere has been radiometrically characterized for the whole image (3000 km wide), and atmospheric correction can always be achieved with an error better than +/-0.5 deg K when accurate data of coincident atmospheric profiles is used. At the worst, when there are no such radiosoundings, SST can be determined with a standard error of estimate of +/-1.1 deg K (compared with split-window technique) or +/-1.3 deg K (compared with in situ data).