Rosetta Consert Radio Sounding Experiment: A Numerical Method for the Inverse Problem

Abstract:

Rosetta’s module Philae will soon land on 67P CG nucleus, giving unprecedented insight about a comet nucleus, its composition and interior. The CONSERT instrument is one of the 20 scientific instruments of the mission. It’s a bistatic two-modules radar, one on the orbiter, one on the lander. They generate EM waves that are transmitted through the nucleus. The signal is therefore delayed and attenuated by the nucleus materials and possible inhomogeneities. An accurate measurement and processing of these signals, repeated along the orbit, will allow us to perform a tomography, and for the first time, map the dielectric properties of a comet nucleus internal structures .
Our approach for the resolution of this inverse problem is to use a custom built software called SIMSERT, which simulates the end-to-end experiment, using a ray-tracing algorithm. This tool is the key to prepare CONSERT operation and perform signal analysis. Given a comet shape and a landing site, we have conducted simulations to understand, quantify and get rid of the biases due to the discretization of the shape model.
The first inversion using the comet shape model given by OSIRIS and NavCam teams , will assume a propagation in an homogeneous medium. The first goal is to identify and correct artefacts due to the surface interface. The second goal is to evaluate the coherency of the different permittivity estimations given by inverting the latter model on the signal measured at different positions along the orbit. Then it is likely that, based on the first investigations, more sophisticated models (rubble pile, strata) and inversions will be required. A comparative approach between the simulated data and the CONSERT data, will lead to permittivity maps of the nucleus, that are coherent with the observation, with a certain probability. These maps, the first of this type, will provide unprecedented information about the internal structure, the accretion history and the nucleus time evolution.