Study On Method Of External Calibration Of The Microwave Radiometer

The external calibration of the microwave radiometer on-board satellite is the basis of quality and quantitative application of the radiometer data. When microwave radiometer is running in orbit, performance of the instrument is possible to change as a result of impact of adverse environment, such as solar irradiation, cosmic radiation. Because this instrument can not be tested in orbit, external calibration is the primary method for inspecting performance of the microwave radiometer. The principle of external calibration is calculation of brightness temperature on the pupil of antenna based on measurement data, and new input-output relationship could be established. Therefore, Calculation of brightness temperature on the pupil of antenna is the key issue of the external calibration of the microwave radiometer on-board satellite.The main methods including atmospheric model and sea surface model in radiative transmission of external calibration of microwave radiometer at home and abroad have been organized and analyzed by the numbers in this paper. Based on the analyse, the relevant physical parameters of sea surface's and atmospheric models have been studied, the impact of these parameters'measurement errors on the calculation accuracy of brightness temperature has been analyzed. With regard to impact of sea surface's parameters on the model, based on the sea surface reflectivity model studied by Wentz(2000), this paper has studied impact of the parameters, such as sea surface temperature , sea surface salinity , sea surface wind speed and angle of observation , and analyzed the impact of these parameters'measurement errors on the calculation accuracy of brightness temperature. With regard to impact of atmospheric parameters on the model, based on the atmospheric absorption model studied by Liebe(1989), this paper has studied impact of the atmospheric parameters, such as air temperature, air pressure, relative humidity, and emphasized analyzing the impact of relative humidity and liquid's measurement error on the calculation accuracy of brightness temperature. According to experience at home and abroad, the overall technical project of the external calibration's field test has been summed up. Based on the impact of sea surface's and atmospheric parameters on brightness temperature, and another possible factor in field test, the condition of the selected test area should be guaranteed that the average temperature of sea surface is not less than 18 degree in six month and more of the year, and changes within 1 degree; the average wind speed of sea surface is less than 4 m/s, and changes within 1 m/s in this area; air pressure, air temperature and relative humidity respectively changes within 10mb, 1 degree, 5 percent in the test area; meteorologic condition is stable, the average of sunny days is more than 100 in one year. According to the above principles, this paper has analyzed marine and atmospheric data of China's coastal water over the year, and selected the area of field test. Furthermore, the measured parameters and instruments have been analyzed, and the precision of instrument has been made in the paper. Based on the different platforms (ship, oil platform, buoy, aircraft) carrying the different instruments, the corresponding methods of measurement have been summed up. When simultaneous measurement is being operated, the paper has analyzed the navigation lines of ship and aircraft.