A collocated nadir observation technique has been used as part of the geostationary meteorological satellite (GMS) pathfinder project and is now employed at the Australian Bureau of Meteorology Research Centre to calibrate the visible infrared spin scan radiometer (VISSR) instrument used on GMS-5. It uses satellite-to-satellite cross calibration to bypass many of the problems inherent in the absolute calibration of satellite instruments. Orbital calculations determine where and when the VISSR and advanced very high resolution radiometer (AVHRR) instruments on the GMS and National Oceanic and Atmospheric Administration (NOAA) satellites, respectively, are observing the same earth-atmosphere scene from the same direction at the same time. Results show that, with careful selection of calibration scenes, the satellites are observing the same radiation field, allowing a fundamental count-to-count calibration of the instrumentation. This calibration can subsequently be related to radiance, It should be possible to cross correlate all geostationary satellites currently employed using a similar method, This paper makes the following three contributions. 1) Using radiative transfer modeling, a sound theoretical basis for the calibration technique is provided. 2) Behavior of the GMS-5 VISSR instrument is tracked for the Pathfinder benchmark period (July 1, 1995-June 30, 1996), 3) Detailed theoretical and experimental comparison of the GMS-4 and GMS-5 VISSR instruments, which takes into account their different spectral response functions, is provided.
