d. Atmospheric Correction
The impact of atmospheric effects on AVHRR composite imagery of Channels 1 and 2 still is significant, although the compositing has been used to remove cloud contamination and atmospheric effects. An atmospheric correction algorithm for AVHRR imagery in the operational mode is used (Lei and Yokoyama, 1997). The algorithm works with look-up tables (LUTs) of the atmospheric corrective parameters derived from 6S and maintained in a production environment. However this algorithm is validly when solar zenith angle and sensor zenith angle is less than 60 and 50 degree respectively in the input geometrical parameters. For atmospheric profile models, a standard climatology with latitudinal, seasonal dependence are used, and the aerosol model was taken to be continental. The elevation data corresponding to imagery area was taken from GTOPO30. The output of atmospheric correction for every pixel is the surface reflectance of Channels 1 and 2. For atmospheric conditions, subarctic summer, Midlatitude summer and Tropical atmospheric profile are used for the pixels located between 80oN and 50oN, 50oN and 23oN, and between 23oN and 20oS respectively. The aerosol concentration is taken in visibility 60km (assumed very clear atmosphere).

4.Prototype Composite Image
The automatic processing system processed NOAA -14 afternoon AVHRR raw data for August 1-10, 1998 and produced prototype 10-day composite image for Asian region (Figure 1). 82 scenes of raw AVHRR data were used for the prototype composite image.


Figure 1: NDVI composite image of Asian region for August 1-10, 1998
Conclusions
The first prototype of Asian 1-km composite imagery have been created from the raw 1-km AVHRR data received from ground receiving stations of Tokyo University, Kuroshima (Japan), AIT (Thailand) and Ulaanbaatar (Mongolia). The composite images will contribute to research activity in environmental analysis of Asian region. During the development of AVHRR composite data set, it has been showed difficulties in the data processing because of frequent cloud coverage over large areas and geometric distortion at large scan angle associated with the edges of the across-track scan. Despite the number of difficulties, the improvement of the data processing method should lead to increase a quality of the composite images.

Acknowledgement
The research is partially supported by Research and Development applying Advanced Computational Science and Technology, Japan Science and Technology Corporation and Telecommunications Advanced organization of Japan.

References

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