Range resolution enhancement of WISDOM/ExoMars radar soundings by the Bandwidth Extrapolation technique: Validation and application to field campaign measurements Nicolas Oudart, Valérie Ciarletti, Alice Le Gall, Marco Mastrogiuseppe, Yann Herve, Wolf-Stefan Benedix, Dirk Plettemeier, Vivien Tranier, Rafik Hassen-Khodja, Christoph Statz, et al. To cite this version: Nicolas Oudart, Valérie Ciarletti, Alice Le Gall, Marco Mastrogiuseppe, Yann Herve, et al.. Range resolution enhancement of WISDOM/ExoMars radar soundings by the Bandwidth Extrapolation tech- nique: Validation and application to field campaign measurements. Planetary and Space Science, Elsevier, 2021, 197 (March), pp.105173. 10.1016/j.pss.2021.105173. insu-03114236v2 HAL Id: insu-03114236 https://hal-insu.archives-ouvertes.fr/insu-03114236v2 Submitted on 28 Jan 2021 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Distributed under a Creative Commons Attribution| 4.0 International License Planetary and Space Science 197 (2021) 105173 Contents lists available at ScienceDirect Planetary and Space Science journal homepage: www.elsevier.com/locate/pss Range resolution enhancement of WISDOM/ExoMars radar soundings by the Bandwidth Extrapolation technique: Validation and application to field campaign measurements Nicolas Oudart a,*, Valerie Ciarletti a, Alice Le Gall a,b, Marco Mastrogiuseppe a,c, Yann Herve a, Wolf-Stefan Benedix d, Dirk Plettemeier d, Vivien Tranier a,Rafik Hassen-Khodja a, Christoph Statz d, Yun Lu d a LATMOS/IPSL, UVSQ Universite Paris-Saclay, Sorbonne Universite, CNRS, France b Institut Universitaire de France (IUF), Paris, France c Sapienza Universita’ di Roma, Italy d Tecnische Universitat€ Dresden, Dresden, Germany ARTICLE INFO ABSTRACT Keywords: The ExoMars 2022 rover mission has been designed to search for traces of life, past or present, on Mars and more Mars specifically below its hostile surface. This will be the first mission able to collect samples down to 2 m in the ExoMars Martian subsurface, where organic molecules and bio-signatures may be preserved. The images of the subsurface WISDOM (or radargrams) provided by the ground penetrating radar WISDOM onboard the ExoMars rover will be key to Ground penetrating radar select the safest and scientifically most relevant locations for drilling. However, the vertical resolution of WISDOM Bandwidth extrapolation Resolution radargrams is limited (to ~5.5 cm in a typical soil of dielectric constant 4) by the bandwidth of the instrument when applying classical processing techniques. Here we propose to enhance WISDOM radargrams vertical reso- lution by implementing the Bandwidth Extrapolation (BWE) technique. We demonstrate that this technique yields an improvement of WISDOM vertical resolution by a factor of 3 while well preserving the time of arrival and amplitude of the detected echoes. We validate the BWE technique on synthetic data before applying it to experimental observations acquired both in controlled and natural environments. The resulting super-resolved radargrams greatly improve our understanding of the investigated terrains with a better separation of echoes from underground reflectors, and demonstrate the value of implementing the BWE technique in the WISDOM data processing pipeline which will ultimately provide images of the Martian subsurface down to a few meters depth with a vertical resolution of ~2 cm in typical soils. 1. Introduction if not designed as a subsurface sounder, the radar onboard the Cassini spacecraft (NASA/ESA/ASI) (Elachi et al., 2005) has provided insights In the last decades, radar sounders operated from orbit have opened a into Titan’s subsurface and in particular the first bathymetry profile of an third dimension in the exploration of the solar system by revealing the extra-terrestrial (methane-rich) lake (Mastrogiuseppe et al., 2014). These subsurface of a few planetary bodies bringing new clues on their geology radars are all operated from orbit and therefore bring clues on the sub- and history: the Moon with the Lunar Radar Sounder (LRS) onboard the surface structure and composition at large scale, with horizontal reso- Kaguya mission (JAXA) (Kobayashi et al., 2012), the nucleus of comet lutions typically ranging from a few tens of meters to a few hundred 67/P with the CONSERT radar onboard the Rosetta mission (ESA) meters, depending on their bandwidth and operating frequencies (Ciar- (Kofman et al., 1998), and Mars which has been scrutinized by two letti, 2016 for a review). sounding radars for many years, namely MARSIS onboard Mars Express Sounding the subsurface at a much smaller scale to understand the (ESA) (Picardi et al., 2005) and SHARAD onboard the Mars Reconnais- local geological context is possible with ground penetrating radars (GPR) sance Orbiter (MRO/NASA) missions (Seu et al., 2004). In addition, even operating from the surface. In 2013, the Chang’E 3 mission carried such a * Corresponding author. E-mail address: [email protected] (N. Oudart). https://doi.org/10.1016/j.pss.2021.105173 Received 30 August 2020; Received in revised form 18 November 2020; Accepted 11 January 2021 0032-0633/© 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by- nc-nd/4.0/). N. Oudart et al. Planetary and Space Science 197 (2021) 105173 GPR, the Lunar Penetrating Radar (LPR) (Fang et al., 2014), which pro- present the BWE technique, and its integration to the WISDOM data bed the shallow lunar subsurface (down to almost 100 m) with a vertical processing chain. The method is eventually applied to WISDOM experi- resolution better than 30 cm (Zhan et al., 2014). This pioneer instrument mental data collected both in controlled, semi-controlled and natural was followed by another LPR onboard Chang’E 4 which investigated the environments. subsurface of the far side of the Moon. Mars will be the next target of on-surface GPRs with two of them launched in 2020 and one to be 2. The WISDOM instrument launched in 2022: the RIMFAX (Radar Imager for Mars’ Subsurface Experiment) radar (Hamran et al., 2014) onboard the Mars 2020 (NASA) 2.1. The WISDOM ground penetrating radar Perseverance rover, the Subsurface Penetrating Radar (Zhou et al., 2016) onboard the Tianwen-1 rover (CNSA), and the WISDOM (Water Ice WISDOM is the ground penetrating radar mounted onboard the rover Subsurface Deposits On Mars) radar (Ciarletti et al, 2011, 2017) onboard of the ExoMars mission (Vago et al., 2017). It is a polarimetric Stepped the ExoMars (ESA/ROSCOSMOS) Rosalind Franklin rover. WISDOM has Frequency Continuous Wave (SFCW) radar operating in the UHF (Ultra been designed to investigate the subsurface of Oxia Planum down to High Frequency) frequency domain and designed to investigate the first several meters and with a vertical resolution of a few centimetres, while meters (typically 3–10 m below the surface) of the Martian subsurface RIMFAX should be able to probe greater depths (tens of meters) on the with a centimetric vertical resolution. When performing a sounding, Jezero crater region but with a coarser resolution of a few decimetres. WISDOM transmits and receives a series of N ¼ 1001 harmonic signals of The Subsurface Penetrating Radar of the Tianwen-1 mission will perform frequencies νk between νmin ¼ 0.5 and νmax ¼ 3 GHz separated by a soundings with a low-frequency and a high-frequency channels, the constant frequency step of ν ¼ 2.5 MHz. high-frequency being able to sound the Martian subsurface to a depth of a few meters, with a resolution of a few centimetres. This study is dedi- νk ¼ νmin þðk À 1Þν (1) cated to the WISDOM GPR selected for the ExoMars rover mission. Only the real part of the return signal is measured by the instrument, ExoMars is an ESA (European Space Agency)/ROSCOSMOS (Russian and the complex form is derived via Hilbert transform before applying an Space Agency) mission. It will land in 2023 a rover and a surface platform Inverse Fast Fourier Transform (IFFT) to obtain the response of the in Oxia Planum, a geologically old plain of Mars (18.3N 335.4E), to sounded volume. In the time domain, each reflector generates an echo complement the TGO (Trace Gas Orbiter), in orbit since 2016, which was which is characterized by a propagation delay (that can be then con- the first step of the ExoMars program. The main objective of the Rosalind verted in distance with assumption on the velocity of the wave in the Franklin rover is to seek out traces of past or present life on Mars. Mars sounded material) and an amplitude. Since WISDOM antennas are being subject to strong radiation levels and oxidation, if such traces still looking downwards at the surface and, while echoes can sometime be exist, they would likely be in the subsurface (Hassler et al., 2014). To generated by off-track reflectors, the retrieved distances are commonly reach them, the rover is equipped with a drill able to collect 3 cm-long and improperly interpreted as depths. In the following, for the sake of samples down to 2 m in the subsurface (Magnani et al., 2010). These simplification, we represent radargrams in the terms of depth and ver- samples will be then analysed by a suite of analytical instruments in the tical resolution assuming a value of dielectric constant, rather than range rover body (Vago et al., 2017). The selection of safe and scientifically and range resolution. relevant drilling sites requires information on the structure and proper- During operations on Mars in areas of potential interest, WISDOM ties of the Martian subsurface prior to drilling.