A Pipeline for In-Flight Geometric Distortion Correction of HERA’s Asteroid Framing Cameras

HERA is ESA’s contribution to the international AIDA collaboration, aimed at characterizing the effects of DART kinetic impact on Didymos binary system. During March 2025 Mars flyby, HERA acquired Earth-based radiometric data and optical images of Mars and its satellites, Phobos and Deimos, providing an in‑flight testbed for the mission’s optical navigation (OPNAV) processing chain and orbit determination tools.

The OPNAV image‑processing pipeline developed for HERA and adapted to the flyby dataset is described. Starting from raw images, the workflow begins with geometric calibration of the Asteroid Framing Cameras (AFCs) using star‑field images to estimate the pointing of the camera and the optical distortion parameters. In-flight calibrated parameters are exploited to extract limb and center‑of‑figure measurements from the flyby images, that yield angular constraints on the spacecraft–body geometry for navigation. Those measurements are used as inputs in an Orbit Determination pipeline for the trajectory estimation of the spacecraft, as well as to derive astrometric or normal points useful for future ephemeris improvements.

For star detection and centroiding ARAGO software, developed by the Observatoire de Paris based on Caviar, was used to locate stellar and satellites centroids. Within some images of the flyby, also a subset of Mars’ craters have been included as fixed points to further constrain the spacecraft pointing. The resulting coordinates are then included in the orbit determination filter, which uses the JPL-NASA MONTE python library.

In this work, we present the AFC calibration and orbit determination activities performed for the Mars flyby campaign. The image analysis enabled the identification of systematic pointing effects and the estimation of camera distortion parameters, which were incorporated into the navigation pipeline and orbit determination process.

The flyby also provides an opportunity to validate and refine the processing pipeline in preparation for the arrival of Hera at the Didymos system in November 2026. The developed methodology will support future optical navigation analyses at the Didymos system and the extraction of high-precision measurements from imaging data for the characterization of the post-impact dynamics of the Didymos–Dimorphos system.