Full-Waveform Inversion Method for Data Measured by the CONSERT Instrument aboard Rosetta

Abstract:

The primary scientific objective of the Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT) aboard Rosetta is to perform a dielectric characterization of comet 67P/Chuyurmov-Gerasimenko’s nucleus by means of a bi-static sounding between the lander Philae launched onto the comet’s surface and the orbiter Rosetta.

For the sounding the lander part of CONSERT will receive and process the radio signal emitted by the orbiter part of the instrument and transmit a signal to the orbiter to be received by CONSERT.

With data measured during the first science phase, a three-dimensional model of the material distribution with regard to the complex dielectric permittivity of the comet’s nucleus is to be reconstructed.

In order to perform the 3D characterization of the nucleus we employ a full-waveform least-squares based inversion in time-domain. The reconstruction is performed on the envelope of the received signal. The direct problem of simulating the wave-propagation inside the comet's nucleus is modelled using a wideband nonstandard finite-differences in time-domain approach and a compensation method to account for the differences in free-space path-loss due to the removal of the carrier in the simulation. This approach will yield an approximation of the permittivity distribution including features large compared to the bandwith of the sounding signal.

In order to account for restrictions on the measurement positions by the orbitography and limitations on the instrument dynamic range we employ a regularization technique where the permittivity distribution and the gradient with regard to the permittivity is projected in a domain defined by a viable model of the spatial material distribution. The least-squares optimization step of the reconstruction is performed in such domain on a reduced set of parameters.

To demonstrate the viability of the proposed approaches we provide reconstruction results based on simulation data and scale-model laboratory measurements.