Pseudorange measurement and Sun phase angle estimation using CNN-based Image Processing algorithm for Hera mission

Hera is the European Space Agency (ESA)’s Space Situational Awareness contribution to the international collaboration Asteroid Impact Deflection Assessment, the first planetary defense mission. The Early Characterization Phase (ECP) of the Hera mission is a proximity operation that has the objective of conducting physical and dynamical characterizations of binary asteroid system (65803) Didymos. In this phase, an Image Processing (IP) algorithm is designed to estimate the position of the Center of Mass (COM) of the primary to enable Line of Sight navigation. To measure the range with Didymos, Hera uses the Planet ALTimeter (PALT), a lidar experiment that is not operating during the ECP as it requires closer distances with Didymos. However, standard IP algorithms need to introduce correction terms depending on the Sun phase angle (Sun-asteroid-spacecraft) to increase the accuracy of their estimation. Within this context, this paper develops a methodology to measure the pseudorange with the primary and to estimate the Sun phase angle by exploiting a Convolutional Neural Networks (CNN)-based IP algorithm. The training, validation and testing datasets are generated with the software Planet and Asteroid Natural scene Generation Utility (PANGU). For the first aim, the proposed methodology regresses a set of keypoints on the visible border of Didymos and evaluates its apparent radius, which is used to measure the pseudorange with the spacecraft. For the second aim, the methodology measures the Sun phase angle using the pixel position of the subsolar point of the primary. The High-Resolution Network (HRNet) is used as CNN architecture as it represents the state-of-the-art technology in keypoint detection. The HRNet-based IP algorithm measures the pseudorange and estimates the position of the subsolar point with high accuracy. The Sun phase angle estimation is accurate with limited dependence on the shape of the asteroid