On-Orbit Star-Based Calibration and Modulation Transfer Function Measurements for PLEIADES High-Resolution Optical Sensors

Abstract

The accurate on-orbit radiometric calibration of optical sensors has become a challenge for space agencies who combine their efforts through international working groups such as CEOS/WGCV or WMO/GSICS with the objective of ensuring the consistency of space measurements and to reach the absolute accuracy required by increasingly demanding scientific requirements. Different targets are generally used for calibration, depending on the specific features of the sensor or spacecraft: from on-board calibration systems to ground targets, they can all profitably be characterized and modeled. Thanks to their agility, some satellites have the capability to view extra-terrestrial targets such as stars. Although the spectral irradiance of some stars is known with very high accuracy, very little work has been done on their use as an absolute reference for the calibration of remote sensors. The reason is their low irradiance, requiring a small instantaneous field of view to observe them. Moreover, their use as a calibration reference also requires a good knowledge of the instrument’s modulation transfer function (MTF). The MTF which drives the spatial quality is a major parameter for high-resolution (HR) optical remote sensors as it indicates how spatial frequencies are transmitted and weakened by the imaging system. On the other hand, stars provide ideal targets for MTF measurement as they can be assimilated to point sources. This paper proposes a method for simultaneously computing the absolute radiometric calibration of PLEIADES’ HR optical sensors and their MTF, using stars. The radiometric model is solved in Fourier space for point sources whose irradiance can be easily controlled. Results are provided for both PLEIADES satellites and compared to the official radiometric calibration and MTF. The quality of the results and the main error contributors are discussed.