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PLEIADES is a dual Earth observation system composed of two satellites, PLEIADES 1A and PLEIADES 1B, respectively launched at the end of 2011 and 2012. This imagery system, led by the French National Space Agency, CNES, is derived from the instrument developed for the SPOT programs. It is composed of four spectral bands, blue, green, red and near infrared, with a resolution of 2.8 m in vertical viewing and a panchromatic band with a resolution of 0.7 m in vertical viewing. Its swath is about 20 km. It is characterized by a very high level of agility which allows it to acquire extraterrestrial objects as stars and the Moon. This presentation focuses on the activities conducted during the commissioning phase of PLEIADES 1A and PLEIADES 1B about the absolute radiometric calibration of the sensors and especially about the analysis of the Moon acquisitions. The moon is one of the natural sites used for the PLEIADES radiometric calibration. Indeed, the Moon is without any disturbing meteorological atmosphere such as aerosols which make it an ideal site for absolute calibration. In a nominal calibration mode, the moon is observed once a month during the descending phase in order to follow the stability of the absolute calibration of the different spectral bands. The lunar calibration method is based on the ROLO model (RObotic Lunar Observatory), developed by the USGS (United States Geological Survey). This semi-empirical model takes into account the librations of the Moon and is able to simulate the global reflectance of the Moon at any date. For each lunar acquisition, the calibration method consists in calculating a standardized lunar irradiance integrated on the spectral range of the sensor. This value is in theory invariant and only depends on the sensitivity of the instrument. Taking advantage of the high level of agility of PLEIADES, we performed intensive characterization of the Moon to better understand the sensitivity of the calibration method. Many questions could be addressed by this type of characterization. What is the impact of the dark signal (background noise) on the integrated signal of the moon? How to integrate it precisely? What is the impact of the geometrical sampling of the sensor to the calibration results? What is the sensitivity of the method to the moon phase angles? What is the sensitivity of the method regarding the position of the sensor on its orbit? To be able to address all these points, several hundred of moon acquisitions were acquired during the commissioning phases of the two PLEIADES satellites. These evaluations of the impacts of each contribution will be precious inputs in order to derive recommendations for a better use of the lunar acquisitions. In a second time, knowing the sensitivity of the method, we performed different kind of calibration methods on the PLEIADES data: multi-temporal calibration of the different spectral bands to follow the temporal changes of the instrument, as well as inter-band calibration. The strong potentiality for inter-calibration will also be highlighted based on the experience between PLEIADES 1A and PLEIADES 1B. |
