After the launching of FY-3A in May

2008

calibrating the payload in orbit turned important. The first post launch site calibration experiment of FY-3A visible and infrared radiometer (VIRR) was held in 2008’s site calibration experiment in Dunhuang remote satellite radiometric calibration site

Gansu province

China. Because of the bidirectional reflectance anisotropies of the land surface

the precision of the visible and near-infrared channels’ post launch site calibration was disturbed obviously. Getting a correct directional reflectance through the observing view angle and at the right time payload passing over is an important work. The vertical reflectance of 11 synchronization points in Dunhuang site was measured by ASD FR spectral meter and standard reference board during the period 1 hour fore-and-aft the satellite passing over. With a new reflectance correction method the calibration precision was improved: the directional reflectance at the moment the platform passing by was calculated by the algorithm for modeling bidirectional reflectance anisotropies of the land surface (AMBRALS) invented by TERRA/MODIS land group and multiplied by the correction coefficients. The correction coefficients were gotten by dividing each vertical reflectance measured by ASD of 11 synchronization points by the one calculated by AMBRALS and averaging them. Results showed that in some bands

the difference between measured one and calculated one reached about 10%. It showed the efficiency and importance of the new correction method. The aerosol optical depth (AOD) was measured by 10 bands sun photometer CE318 placed near the synchronization region. The AOD of 550nm is calculated according to Junge distribution. The layer water content was calculated by the sounding data measured by Dunhuang national climatic station (No. 52418). The vapor pressure was calculated by the Wexler’s formulas. The apparent reflectance was calculated by the use of 6S model and was corrected by the sun zenith cosine and sun-earth distance. Totally 3 days’ (2008/09/06

2008/09/10

2008/09/11) calibration slopes and intercepts of FY-3A/VIRR were gotten. The relative standard deviation(RSD) of them was below 2.6%. In order to validating the post launch calibration’s veracity

23 targets’ apparent reflectances were compared by FY-3A/VIRR and TERRA/MODIS

they matched well. The results showed the stability and veracity of the calibration arithmetic except for the water vapor absorption bands. In vapor absorption bands

the 3 days’ slopes varied obviously. Compared to the pre launch calibration

the post launch calibration changed about 7.73%~21.52% in different bands

showed the imperative of post launch calibration right after the satellite launching. Now the pre launch calibration slopes and intercepts had been substituted by the post launch ones.