PLEIADES High Resolution optical sensors radiometric and spatial calibration based on stars

The accurate on-orbit radiometric calibration of optical sensors has become a challenge for both ensuring the consistency of space measurements and reaching the absolute accuracy required by increasingly demanding scientific requirements. Different targets are traditionally used for calibration depending on the sensor or spacecraft specificities: from on-board calibration systems to ground targets, they all take advantage of our capacity to characterize and model them. Thanks to their agility, some satellites have the capability to view extra-terrestrial targets such as the moon or stars taking advantage of the absence of atmosphere. The moon is widely used for calibration and its albedo is known through ROLO (RObotic Lunar Observatory) USGS model but its limited accuracy constrains its use to sensor drift monitoring or cross-calibration. The spectral irradiance of some stars is known with a very high accuracy, providing an absolute reference for remote sensors calibration. But the low irradiance of stars requires an instrument with a small Instantaneous Field Of View to observe them. A good knowledge of the instrument's Modulation Transfer Function (MTF) is also necessary to perform an accurate radiometric calibration. This paper describes a method based on stars for simultaneously computing the radiometric calibration of PLEIADES 1B' high resolution optical sensor and its MTF. The radiometric model is solved in Fourier space for point sources whose irradiance is controlled. Results are compared to the official MTF and radiometric calibration based on Pseudo Invariant Calibration Sites (PICS) and the moon. The quality of long time series of measurements is discussed as well as their accuracy.