<title>The technique of satellite cross-calibration for ocean color sensors (Invited Paper)</title>

Abstract

The one of the most important technique of satellite ocean color remote sensing is the radiance measurement accuracy, because the water leaving radiance is only about 5% to 1 0% of the total radiance arrived at sensor at the satellite altitude.It is necessary to guarantee the accuracy of water leaving radiance measurement of about 5% (relative error) to meet the reversed accuracy of ocean color factors ( such as chlorophyll, suspended material and so on) within relative error about 30 % in open sea (Case I water). When sensor has been in the orbit, it is important to take orbit calibration to make up some deficits of the pre-launch calibration in the laboratory. Two kinds of data could be used for orbit calibration, one is in-situ measurement date, so-called in-situ field calibration, and another is other satellite data with higher radiance measurement accuracy, so called crossing-calibration. China has launched the third spaceship SZ-3 in March, 2002. The main remote sensing sensor is the Chinese Moderate Imaging Spectroradiometer (CMODIS) which has total 34 channels (30 channels of 2Onm interval in the spectral range of 0.403 -1 .O43im, and four infrared channels with 2.15 -2.25um, 8.4 -8.5um, 10.3 -11.3um and 11.5-12.5um Following SZ-3, China had the first ocean color satellite, HY-1A, in May ofthe same year, whose main sensor is for ocean color measurement, called as Chinese OceanColor and Temperature Scanner, COCTS, providing 8 visible and near-infrared channels similar to SeaWiFS21. In this paper, first, the methodology and procedure of satellite cross-calibration are discussed in detail, with taking an example of CMODIS. Then the results of this orbit cross-calibrated by American ocean sensor, Sea Wide Field-of-View Sensor (SeaWiFS), for CMODIS and COCTS are presented with comparing the normarization from pre-lunched Lab-calibration and in-situ measurement.Here, for easy understanding the methodology and procedure of satellite cross-calibration, the CMODIS will be taken as an example to discuss. In principle, if two sensors measure the same target at the same observation condition, the measured radiance ofthe target by two sensors should be the same. So, one sensor which is unknown its accuracy in the orbit could be calibrated by another sensor with the higher accuracy one by the comparison of the radiance measured by two sensors respectively. The SeaWiFS accuracy ofradiance measurement is higher than 5 % (relative error) [3,4,5] so CMODIS is possible to be calibrated by SeaWiFS, even though these two sensors have their own observation geometric parameters which depend on the zenith of the sun and sensor, and azimuth between sensor and sun, supposed (e0,o,Aç) and (e() ,e, Aco)CMOI)IS for SeaWiFS and CMODIS observation geometries, respectively.First, the normalized water leaving radiance is possible to be calculated by SeaWiFS, and the type of aerosol can also be