Performance assessment of JuRa internal structure imaging of Didymos using gradient descent algorithms with a linearized forward operator

This study details preliminary work for the data processing of JuRa spaceborne planetary sounding radar which will investigate the interior of the binary S-type asteroid Didymos in 2027 as part of the ESA Hera mission [1]. Spaceborne planetary sounding radars are designed to remotely probe planetary bodies subsurface at decametric to metric resolutions at depths ranging from few hundred meters up to few kilometers depending on the carrier frequency used. These radar characteristics are driven by geophysical (e.g. penetration and spatial resolution) and technical considerations (e.g. power and antenna size). JuRa was designed as a monostatic radar with an antenna composed of two crossed 1.5m dipoles able to emit Binary Phase Shift Keying (BPSK) coded signals in a 20 MHz bandwidth centered around a 60 MHz carrier, and 5 W peak power. Such configuration allows to emit and receive with either dipole antennas allowing full polarization characterization. In order to perform 3D internal structure imaging, a sufficient diversity of geometry of acquisition is required involving multiple orbits and sounding measurements on each orbit. One of the major challenge when exploiting radar data data to reconstruct the internal structure of kilometric-size planetary bodies lies in the relatively large size of the planetary body with regard to the radar carrier signal wavelength. Accordingly, processing JuRa downlinked data using Full Waveform Inversion (FWI) to reconstruct the internal structure of Didymos (800m diameter) and its moon Dimorphos (160m diameter) will prove a computationally challenging task given the relatively short radar carrier signal wavelength (~5m). In order to overcome this limitation, we investigate the possibility to use gradient descent algorithms with a linearized forward operator to process data from spaceborne planetary sounding radar dedicated to asteroid interior imaging. Performances of the proposed internal structure imaging algorithm are evaluated on a previously published asteroid analog anechoic chamber dataset [2] using Discrete Dipole Approximation to compute electric fields. Results showcase the ability to recover main interior structures in the analog case opening promising perspectives for JuRa data processing and for future asteroid interior sounding radars.

[1] P. Michel, M. Küppers, A. C. Bagatin, B. Carry, S. Charnoz, J. De Leon, A. Fitzsimmons, P. Gordo, S. F. Green, A. Hérique, et al., “The esa Hera mission: detailed characterization of the Dart impact outcome and of the binary asteroid (65803) Didymos,” The planetary science journal, vol. 3, no. 7, p. 160, 2022.

[2] A. Dufaure, C. Eyraud, L.-I. Sorsa, Y. Yusuf, S. Pursiainen, and J.-M. Geffrin, “Imaging of the internal structure of an asteroid analogue from quasi-monostatic microwave measurement data – I. the frequency domain approach,” Astronomy & Astrophysics, vol. 674, p. A72, 2023.