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This research aims at the robust and high-resolution reconstruction of asteroid's interior structure using lander-to-orbiter radar system. In recent years, the full waveform inversion (FWI) has been suggested as a potential method to asteroid tomography. Due to the limitation of computing capacity, FWI is usually performed by local rather than global optimization method, which makes it suffer from local minima problem especially when the signal lacks low-frequency components. To ensure the global convergence, FWI requires the initial model be accurate enough to avoid the local minima. But in practice, the low-frequency components are naturally absent in the remote radar signal as the limitation of bandwidth, and the prior information of asteroid is usually not sufficient to build an accurate initial model, which leads that using conventional FWI directly may not be capable to obtain a robust reconstruction. Considering the above problems, envelope inversion (EI) which works on the baseband signal and therefore, can recover the long-wavelength structure of asteroid is proposed as a supplementary to FWI. Initial model dependence and noise sensitivity of FWI and EI in asteroid tomography are analyzed based on 2-D numerical experiments. The EI + FWI combination constrained by total variation regularization shows the characteristics of good independence on the initial model and high imaging resolution. Based on EI + FWI strategy, a series of 3-D numerical experiments are conducted to test the influence of orbital measurement density and landing site on the tomography.

Low-frequency radar tomography is an important subsurface imaging method for future planetary missions to solar system small bodies. We derive angular and radial sampling criteria for monostatic and bistatic radar tomography algorithms that are based on monochromatic free-space backprojection and spherical apertures. We use the vector Born approximation to highlight the degeneracy of monochromatic bistatic source/receiver direction pair measurements in k-space. Analytical expressions are then derived for the scalar point target response of different spherical sampling geometries. We also derive the angular sampling step and total number of sampling points required to fully reconstruct the point target response for monostatic, bistatic, and non-degenerate k-space spherical apertures. These are evaluated for object sizes and radar operating frequencies expected in small body tomography. We also analyze and derive expressions for the coherence loss of spherical apertures due to random errors in a sensor’s radial position, which provides requirements on the a posteriori ephemeris knowledge. Finally, we derive a vector backprojection algorithm suitable for focusing quad-pol scattering matrix (S-matrix) data that is tested using full-wave S-matrix simulations of dielectric point targets. This work is intended to aid radar instrument performance analysis and inform the design and architecture of future instruments and missions.

The ESA HERA mission approved by the last ESA council Space19+ will be launched in 2024 to deeply investigate the Didymos binary system and especially its moonlet [1]. Onboard the Juventas small platform, The Juventas Radar -JuRa- will fathom Didymoon and provide the first direct observation of an asteroid deep interior. The characterization of the asteroids’ internal structure is crucial for science, planetary defense and exploration [2]. In 2022, DART/NASA will impact the moonlet to quantify the mechanical response of the body, mainly from ground-based observation [3]. Five years later, HERA/ESA is a unique opportunity to observe in detail the bodies, the crater and the ejecta in order to better constrain mechanical models providing a global characterization of the binary system: shape, density, dynamic properties, thermal properties and composition [4]. The Hera mothercraft will carry two CubeSats, Juventas and Milani. The small spacecraft Juventas will investigate the asteroids’ internal structure. Information about the internal structure is crucial for science, planetary defense and exploration since our current knowledge relies entirely on inferences from remote sensing observations of the surface and theoretical modeling [2]. JuRa is a monostatic radar, BPSK coded at 60MHz carrier frequency and 20MHz bandwidth, inherited from CONSERT/Rosetta [5], [6] and redesigned in the frame of the AIDA/AIM phase A/B [4], [7]. The instrument design is under validation for a flight model delivery end of 2022. JuRa maps the backscatter coefficient (sigma zero - s0) of the surface or subsurface, which quantifies the returned power per surface or volume unit. It is related to the degree of heterogeneity at the scale of the wavelength and to the dielectric contrast of heterogeneities, giving access to both, the sub-meter texture of the constituent material and larger scale structures. The first objective of JuRA is to characterize the moonlet’s interior, to identify internal geological structure such as layers, voids and sub-aggregates, to bring out the aggregate structure and to characterize its constituent blocks in terms of size distribution and heterogeneity at from submetric to global scale. The second objective is to estimate the average permittivity and to monitor its spatial variation in order to retrieve information on its composition and porosity. Radar bypasses the near surface alteration by space-weathering and thermal-cycling as observed with optical remote sensing. The observation of the structure and composition of the moonlet will provide constraints on the mechanical model of the impact process. The same characterization applied to the main asteroid of the binary system is among the secondary objectives, to detect differences in texture and composition. When compared to the observation of the moonlet, it will constraint the model of binary system formation to discriminate between progressive versus catastrophic process and more generally on the stability conditions of the system. In this talk, we will review the JuRa science objectives and the instrument development status. We will show the results of the model end-to-end tests and the corresponding instrument performances. Then we will present the proposed operation strategy and the developed approaches for data processing. Acknowledgments Hera is the ESA contribution to the AIDA collaboration. Juventas and JuRa are developed under ESA contract supported by national agencies. JuRa is built by Emtronix (LU), UGA/IPAG (FR), TU Dresden (DE), Astronika (PL) and FZ (CZ). Juventas is built by Gomspace (LU). Juventas navigation plan is developed by GMV (RO) This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 870377 (project NEO-MAPP). References [1] P. Michel et al., « European component of the AIDA mission to a binary asteroid: Characterization and interpretation of the impact of the DART mission », Advances in Space Research, vol. 62, no 8, p. 2261‑2272, oct. 2018, doi: 10.1016/j.asr.2017.12.020. [2] A. Herique et al., « Direct observations of asteroid interior and regolith structure: Science measurement requirements », Advances in Space Research, vol. 62, no 8, p. 2141‑2162, oct. 2018, doi: 10.1016/j.asr.2017.10.020. [3] A. F. Cheng et al., « Asteroid Impact & Deflection Assessment mission: Kinetic impactor », Planetary and Space Science, vol. 121, p. 27‑35, févr. 2016, doi: 10.1016/j.pss.2015.12.004. [4] P. Michel et al., « Science case for the Asteroid Impact Mission (AIM): A component of the Asteroid Impact & Deflection Assessment (AIDA) mission », Advances in Space Research, vol. 57, no 12, p. 2529‑2547, juin 2016, doi: 10.1016/j.asr.2016.03.031. [5] W. Kofman et al., « The Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT): A Short Description of the Instrument and of the Commissioning Stages », Space Science Reviews, vol. 128, no 1‑4, p. 413‑432, mai 2007, doi: 10.1007/s11214-006-9034-9. [6] W. Kofman et al., « Properties of the 67P/Churyumov-Gerasimenko interior revealed by CONSERT radar », Science, vol. 349, no 6247, p. aab0639, juill. 2015, doi: 10.1126/science.aab0639. [7] A. Herique et al., « A radar package for asteroid subsurface investigations: Implications of implementing and integration into the MASCOT nanoscale landing platform from science requirements to baseline design », Acta Astronautica, mars 2018, doi: 10.1016/j.actaastro.2018.03.058.

The SHAllow RADar (SHARAD) is a synthetic aperture radar (SAR) onboard Mars Reconnaissance Orbiter, in Martian orbit since 2006 [1]. With its 20MHz frequency, it can fathom the first hundreds of meters of the subsurface with a range resolution of about 10m in typical Martian materials. In order to detect subsurface interfaces with radars, we need to eliminate the echoes coming from the surface : as the design of most radars in orbit gives them a large antenna lobe, off-nadir echoes could arrive at the same delay as a subsurface nadir reflection. The classical method to remove this so called “clutter” consists in comparing the radar signal to simulations of the surface echoes using Digital Terrain Models (DTMs). Our aim is to study the first tens of meters of the Martian subsurface with SHARAD. To do so, we need high resolution DTMs because in theory, the higher we get in resolution, the more detailed the simulation will be and the clearer the discrimination between surface and subsurface features will be. We will present results on high resolution simulations performed with SPRATS, our coherent simulator. We will show that while being high resolution, models obtained by photogrammetry sometimes contain artifacts that can be misleading for radar data interpretation. Simulations with SPRATS : first study with MOLA DTMs SPRATS is a toolset developed at IPAG that allows to perform both coherent radar simulations of surfaces and 3D SAR processing of them [2], [3]. Those capabilities enable the simulation of the actual signal sensed by SHARAD, with the same processing applied to it, in order to get as close as possible from the instrument results. It allows for a direct power comparison and thus finer analysis. To begin, we performed simulations with DTMs generated with MOLA, a laser altimeter [4]. While having a relatively low resolution, the nature of the acquisition method give those DTMs a high accuracy and precision, resulting in very low artifacts. To study deep reflectors in areas of relatively low rugosity (i.e. the northern plains [5]), those models are sufficient. But the low resolution is a limiting factor when studying close subsurface, or simply to reproduce surface roughness effects on the radar signal. To improve the simulations, higher resolution models are necessary. HRSC models, higher resolution but sometime containing artifacts With a resolution of 50 to 100m, HRSC DTMs [6] yield better results in simulating smaller details. It allows to confirm or discard reflectors identified with MOLA [7]. However, these models are acquired by photogrammetry, a technique that is an estimation of the surface topography, compared to MOLA which is a direct measurement. Photogrammetry introduces artifacts that are not easy to estimate, because they depend on the actual topography. We will show a comparative study of simulations with HRSC and MOLA models on a region of interest located in Terra Cimmeria, following a study made by [8]. The amplitude of the artifacts on the HRSC models is too high to study the first tens of meters of the subsurface with SHARAD. Following the idea of getting as close as possible to the actual SHARAD data, we need models that describe the surface at a resolution better that the radar’s wavelength. Simulations with models at wavelength-scale resolution (CTX) We will present a comparative study of simulations using CTX models — with a resolution of 12m — and HRSC models. We will also show that the scale of the artifacts on these DTMs being below the SHARAD’s wavelength, CTX DTMs yield near perfect surface echoes simulation, allowing for a fine detail comparative analysis of the SHARAD data. However, given their acquisition method [9], CTX DTMs have a relatively poor surface coverage compared to HRSC, so we used photoclinometry with CTX images on lower resolution models [10] to keep the high resolution information. Comparing high resolution simulations using these models to SHARAD data allowed to highlight small-scale artifacts on the CTX DTMs, as they introduce noise in the radargram. Conclusions This study shows that wavelength-scale or smaller artifacts on DTMs are needed to perform shallow subsurface analysis of SHARAD data. It also showed that high resolution models acquired by photogrammetry are prone to artifacts, which can perturb the simulated signal. This artifacts issue can prove to be helpful for DTM quality estimation, especially for missions where no laser altimeter is present to validate the altimetry measurements. References [1] R. Seu et al., « SHARAD sounding radar on the Mars Reconnaissance Orbiter », 2007, doi: 10.1029/2006JE002745. [2] Y. Berquin, A. Herique, W. Kofman, et E. Heggy, « Computing low-frequency radar surface echoes for planetary radar using Huygens-Fresnel’s principle: COMPUTING RADAR SURFACE ECHOES », oct. 2015, doi: 10.1002/2015RS005714. [3] J.-F. Nouvel, A. Herique, W. Kofman, et A. Safaeinili, « Radar signal simulation: Surface modeling with the Facet Method: RADAR SIGNAL SIMULATION » , febr. 2004, doi: 10.1029/2003RS002903. [4] M. T. Zuber et al., « The Mars Observer laser altimeter investigation », 1992, doi: 10.1029/92JE00341. [5] C. M. Stuurman et al., « SHARAD detection and characterization of subsurface water ice deposits in Utopia Planitia, Mars: SHARAD DETECTION OF ICE UTOPIA PLANITIA », sept. 2016, doi: 10.1002/2016GL070138. [6] G. Neukum et R. Jaumann, « HRSC: the High Resolution Stereo Camera of Mars Express », 2004. [7] C. W. Cook et al., « Sparse subsurface radar reflectors in Hellas Planitia, Mars », sept. 2020, doi: 10.1016/j.icarus.2020.113847. [8] S. Adeli, E. Hauber, G. G. Michael, P. Fawdon, I. B. Smith, et R. Jaumann, « Geomorphological Evidence of Localized Stagnant Ice Deposits in Terra Cimmeria, Mars », juin 2019, doi: 10.1029/2018JE005772. [9] M. C. Malin et al., « Context Camera Investigation on board the Mars Reconnaissance Orbiter », 2007, doi: 10.1029/2006JE002808. [10] S. Doute et C. Jiang, « Small-Scale Topographical Characterization of the Martian Surface With In-Orbit Imagery », janv. 2020, doi: 10.1109/TGRS.2019.2937172.

The study of asteroids interior is crucial to the understanding of their accretion in the early Solar System. Furthermore, space-borne sounding radar is the most mature technique to image asteroid’s internal structure. The low-frequency radar is a bistatic radar measuring the radar wave propagation between a Lander and an Orbiter through the asteroid. Time accuracy and frequency stability are fundamental for the success of such an instrument. Furthermore, accuracy in the propagation delay measurement is a way to increase science return. This work studies the impact the drift of the clocks located on the two platforms has on the accuracy of the time measurement while taking into account the on-board processing. We present the time analysis of such a radar by developing a behavior model of the system for a long time scale which is implemented in a software simulator. A clock test bench is then developed to provide simulator inputs from true oven controlled crystal oscillators clocks under selection for the mission. The simulator coupled to the test bench allows us to better quantify the impact of the time errors on the measurement, to support clock selection and to improve science return.

The proposal deals with the production of a dual-mode, low-frequency, radar installed on board of a leight-weight UAV. The radar will operate into two modes: as Synthetic Aperture Radar (SAR) and as ground penetrating radar. Both instruments provide extremely interesting images that are extensively used in Earth and Planetary observations. However, these airborne systems are bulky and can be operated only from manned aircraft both planes and helicopters. On the other hand, the few drones that can sustain such a equipment are large and heavy. In both cases, the operations are expensive and with a complicate logistic. The quantum leap of FlyRadar consists in installing this radar system onboard a small and light electric octocopters, providing low cost utilisation and easy operations. This affordable system will enlarge the user communities generating the possibility for an extensive use of FlyRadar taking advantage of the potentiality of this long-lasting innovation. The use of SAR and penetrating radar is widespread in Earth observation spanning from geological survey to archeological prospecting, from agricultural assessment to artefact detection. However, FlyRadar could be used also in planetary exploration. The NASA mission Mars2020 will test a drone and it will pave the way for the utilisation of UAVs in the in-situ exploration of Mars and Titan. The project is split in two phase. The first half of the project will be devoted in identifying the requirement, in designing instruments and in building them. The second half will be dedicated to the evaluation of FlyRadar with three experiments lasting a month each. The tests will performed in dry desert areas to avoid the negative effect of ground water and humidity on the radar signal. The test executions will be matched by month-long joint sessions for the analysis of the data, assess the scientific value and investigate the technological aspects.

Electromagnetic full waveform inversion (FWI) is a high-resolution method to reveal the distribution of dielectric parameters of the medium. Traditionally, the electromagnetic FWI is usually performed using the L-2 norm misfit function, which suffers severely local minimum problems since the L-2 norm misfit function is nonconvex in the model space. To mitigate the local minimum problem, the quadratic Wasserstein metric (W-2) derived from the optimal transport theory is proposed to replace the L-2 norm to define the misfit function. Since the computation of the W-2 metric requires the signal to be mass conservation and nonnegativity, the softplus function is applied to rescale the signal to meet these two requirements. Based on the softplus scaling method, a hierarchy inversion strategy that involves multiple scaling coefficients is proposed to mitigate the local minimum problem without sacrificing the inversion resolution. The numerical experiments prove that the FWI based on W-2 (W-2-FWI) is more independent on the initial model than the FWI based on the L-2 norm (L-2-FWI), and the W-2-FWI could be more robust to zero-mean random noise than the L-2-FWI. Finally, a comprehensive numerical experiment related to asteroid radar tomography is carried out to show the potential of W-2-FWI in realistic applications.

Abstract Radar data interpretation for subsurface analysis is based upon the comparison with simulations, to discriminate a subsurface echo from a surface reflector. Our goal is to detect subsurface ice at Mars’s midlatitudes — that have been shown to be present in the first tens of meters below the surface [1]—, using data from the SHAllow RADar (SHARAD) onboard Mars Reconnaissance Orbiter (MRO). We are specifically interested in looking into the southern midlatitudes of Mars, a highly craterized area with a variety of structures on the surface. This is why we have to be extra careful in the radar data analysis with the simulations, and we are faced with issues that are not necessarily present for the study of the poles. We will present a method that allows us to complete the simulations and helps us to resolve further ambiguities. Sounding the close subsurface of Mars’s midlatitudes The MARSIS and SHARAD Nadir-looking radars have been sounding the surface of Mars since respectively 2005 and 2006, at different frequencies — between 1.8 and 5 MHz for MARSIS, and 20 MHz for SHARAD [2]—. Their data have allowed us to better understand the composition and structure of the Martian subsurface. On fathom radar profiles, echo coming from the subsurface from nadir could arrive with the same delay that surface echoes arriving from a slant direction. This ambiguity is classically resolved by simulation of the measurement from Digital Terrain Models (DTMs) as developed for MARSIS [3,4]: given a surface model and an orbitography, we can simulate the surface-related component of the signal. By comparing the radar data to the simulation, we can therefore — in theory — fully identify the subsurface reflectors. The question is especially complex when the objective is to detect ice at Martian midlatitudes at depths lower than a few tens of meters. Furthermore, given that the radar signal mostly comes from the nadir and a limited area around, we will have a high sensibility for the variations in local slopes in the surface models used for the simulations. In order to accurately simulate the surface component of the signal, the surface model resolution must be in the order of the wavelength or lower (15m for the case of SHARAD). This is why we will need higher resolution DTMs than the one mainly used for MARSIS simulations (MOLA). DTMs can be generated from different data types, but to have high resolution models, the privileged method is stereo photogrammetry [5]. As an example, with the CTX camera onboard MRO, we can reach a typical resolution of 12 meters, compared to the 463 meters of MOLA. The area that we are focusing on is located in Terra Cimmeria, around (172°E ; 39°S), and shows a number of canal-like structures, along with numerous craters. Those structures are located a few degrees off-nadir, therefore very close to the main surface echo, complicating the analysis with the simulations. In fact, a slight variation of the model from the actual topography can change the result of the simulation, and the process of stereo photogrammetry introducing noise, we will necessarily be faced with some differences between simulations and radar data. Another issue is that on this specific area, only MOLA and HRSC DTMs are available, with a maximum resolution of 200m per pixel, far from the 15m of the wavelength. With the aforementioned issues, we are left with ambiguities that we cannot resolve as is, so we must find a way to get around it. We will therefore present a method that allows to complete the simulation by resolving the remaining ambiguities. This method applied to the area discussed above allowed us to study in details the echoes analyzed by [6] and to revisit the results. References [1] C. M. Stuurman et al., « SHARAD detection and characterization of subsurface water ice deposits in Utopia Planitia, Mars: SHARAD DETECTION OF ICE UTOPIA PLANITIA », Geophys. Res. Lett., vol. 43, no 18, p. 9484‑9491, sept. 2016 [2] R. Seu et al., « SHARAD sounding radar on the Mars Reconnaissance Orbiter », J. Geophys. Res., vol. 112, no E5, p. E05S05, may 2007 [3] J. -. Nouvel, A. Herique, W. Kofman and A. Safaeinili, "Radar signal simulation: Surface modeling with the Facet Method," in Radio Science, vol. 39, no. 1, pp. 1-17, Feb. 2004 [4] Y. Berquin, A. Herique, W. Kofman, et E. Heggy, « Computing low-frequency radar surface echoes for planetary radar using Huygens-Fresnel’s principle: COMPUTING RADAR SURFACE ECHOES », Radio Sci., vol. 50, no 10, p. 1097‑1109, oct. 2015 [5] R. A. Beyer, O. Alexandrov, et S. McMichael, « The Ames Stereo Pipeline: NASA’s Open Source Software for Deriving and Processing Terrain Data », Earth and Space Science, vol. 5, no 9, p. 537‑548, sept. 2018 [6] S. Adeli, E. Hauber, G. G. Michael, P. Fawdon, I. B. Smith, et R. Jaumann, « Geomorphological Evidence of Localized Stagnant Ice Deposits in Terra Cimmeria, Mars », J. Geophys. Res. Planets, p. 2018JE005772, june 2019

Studying the presence of water and characterizing the tectonic structure in the firsts tens of kilometers of the crust of the Galilean icy moons are crucial to understand the formation and evolution of these bodies and could provide an insight into the setting for extra-terrestrial life within our solar system (Nimmo and Pappalardo, 2016). The most promising technique for directly detecting subsurface oceans is a penetrating radar and the use of low frequencies (< 30 MHz) is preferred to probe the moons' subsurfaces since the losses due to the surface roughness and absorption of the ice are reduced (Bruzzone et al., 2011). However, Jupiter has a loud radio environment for frequencies < 40 MHz (Cecconi et al., 2012), and the use of a passive mode, which would exploit Jupiter’s decametric radio emissions, is considered to operate the radar with low frequencies in the sub-Jovian hemispheres. However, the passive radar operates in a complex bistatic 3D geometry where Doppler and delay information are not separable. This justifies the use of simulations with realistic orbitographies to identify which configurations will lead to the best and worst performances, which is necessary to establish the scenarios of observation of the radar. In this paper, we compute the impact of the geometry on the final bistatic performances, using planned Juice orbits. In order to study the influence of the geometry only, we do not take into consideration the stochastic character of Jupiter’s noise. The emission is then a simple impulsion located in the Jupiter auroral coronas: we regard four sources, at the Eastern and Western borders of Jupiter’s North and South auroral coronas, as presented in Figure 1. In this geometry, the passive radar is orbiting around Ganymede and studies the reflection of the Jovian signal by a point target at the nadir in a monostatic geometry, or a point making a specular reflection in the bistatic case. To recognize weak, delayed, and Doppler-shifted contributions of the Jovian signal (also called reference signal), cross-correlation is carried out between the registered Jovian signal and the measured reflection. The next signal processing is the computation of the SAR synthesis to retrieve a Range-Doppler map of the surface probed. Figure 1. Considered location for the four sources of emission of Jupiter’s radio emissions, and Jupiter. The sources will then be referred to as red source, blue source, yellow source, and green source. To compute the performances of the radar, we simulate the signal scattered in monostatic and bistatic cases, for three different scenarios (figure 2, table 1) using different integration times, perform SAR synthesis and compute on the images the size of the resolution cell. We depict in table 2 the area of the resolution cell for each scenario, as well as the size of the Fresnel zone. The area of the resolution cell is always better than the size of the Fresnel zone, but table 2 depicts that with an integration time of one minute, probing close to the center of the sub-Jovian hemisphere provides the best resolution (1.33 km²). However, this conclusion changes when the integration times are increased to two minutes, and probing with an incidence angle of 38° becomes more favorable than along the nadir (0.28 km²). Indeed, in this geometry, the Range and Doppler sidelobes are not separated, and this non-separability increases with the integration. Thus, while the along-track resolution is predictably worsened in the bistatic case compared to the monostatic scenario and improves with increasing integration time, the resolution in across-track varies as well with the integration time. For orbit 2, it improves with an increasing integration time, while, for orbit 1 and 3, which are aligned with nadir, the integration time does not seem to largely impact it. This means that, while probing along the center of the sub-Jovian hemisphere seems more efficient with small integration times, probing different parts of the sub-Jovian hemisphere might become of interest if increasing the integration time is possible. This study showed that, as expected, the resolutions obtained are largely dependent on the geometry, but as well on the integration time. These conclusions will have to be kept in mind will preparing the probing strategy of Jupiter’s moons. Figure 2.Left: JUICE orbits as visualized on Cosmographia (https://www.cosmos.esa.int/web/spice/cosmographia)Right: Position of the three considered orbits ( orbit n°1 in red, orbit n°2 in green, and orbit n°3 in cyan), with the bistatic specular point in bright color, and the nadir point with an opacity. Table 1 Description of each scenario. Orbit Associated color Date Center Time Incidence Orbit n°1 Red 18/03/2033 15:41 2 Orbit n°2 Green 21/03/2033 09:46 38 Orbit n°3 Cyan 27/04/2033 08:02 27 Table 2 Performances resulting from each orbit. ρ2 is the area of the resolution cell while Fresnel depicts the area of the Fresnel zone. ρ2 (km2) Fresnel (km2) Orbit n°1 1 mn Monostatic 0.503 22 Bistatic 1.33 38 2 mn Monostatic 0.25 22 Bistatic 0.655 38 Orbit n°2 1 mn Monostatic 0.518 22 Bistatic 1.78 55 2 mn Monostatic 0.25 22 Bistatic 0.28 55 Orbit n°3 1 mn Monostatic 0.518 22 Bistatic 1.92 46 2 mn Monostatic 0.28 22 Bistatic 0.953 46

Mars Express has been exploring Mars since late 2003 giving us fascinating views of the red planet as well as unique science insights. The long-life of Mars Express is an asset in order to understand the long-term evolution of the atmosphere of Mars, and in particular, of the ionosphere and upper atmosphere. Currently, we have about 16 years of ionospheric observations at Mars, which cover near two cycles of solar activity. This is the first time that such a long time series observations can be investigated in other planet rather than Earth. In particular, this work focuses on Mars’ ionospheric behaviour during two consecutive solar minima (23/24 and 24/25) using the same dataset. We use the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on board Mars Express to investigate the total electron content behaviour of the whole atmosphere in relation to key parameters, such as the solar flux, solar illumination, dust storms and Sun-Mars distance. The topside variability of the ionosphere is also investigated through variations in the neutral scale height, and results are supported by ionospheric modelling. The main goal of this study is to investigate whether the ionosphere during the recent solar minimum 24/25 was similar (and to which degree) to the ionosphere at solar minimum 23/24, which was characterised by a pronounced reduction of both the topside and bottomside regions.

EI+FWI Method for Reconstructing Interior Structure of Asteroid Using Lander-to-Orbiter Bistatic Radar System Jian Deng1,Wlodek Kofman2,3, Pei-min Zhu1, Alain Herique 2,Ruidong Liu4, Shi Zheng4 1 China University of Geoscience AQ:5 (CUG), Wuhan 430074, China (e-mail: dengjian_cug@126.com). 2 CNRS, CNES, IPAG, Université Grenoble Alpes, 38000 Grenoble, France, wlodek.kofman@univ-grenoble-alpes.fr). 3 Centrum Badan Kosmicznych Polskiej Akademii Nauk (CBK PAN), 00-716 Warsaw, Poland (e-mail: 4 China Academy of Space Technology, Xi’an Branch, Xi’an 710100, China (e-mail: shizheng_xjtu@163.com). Since the missions to asteroids including Dawn, Hayabusa2 [2 - 4], OSIRIS-Rex [5] and so on, we had accumulated some preliminary knowledge on the surface structure and components of asteroids. But the internal structure which is the key to understanding and modeling asteroid accretion and evolution history is still fairly unknown and has never been measured accurately for any asteroid. In view of this, China National Space Administration (CNSA) is actively preparing a mission named “Zhenghe” [6] aiming to reveal the inner structure of Asteroid 2016HO3. Consequently, study on radar tomography for imaging the interior of asteroids is becoming one of the most in-demand technologies. The conventional radar tomography is usually based on ray tracing method (RTM), which utilizes only a small portion of the signal information and is unable to generate complete results especially under the acquisition geometry of limited data density. To get the accurate dielectric permittivity distribution within the asteroid, full waveform inversion (FWI) [7] which utilizes the whole information (including time, amplitude and phase) of the signal, is proposed as a promising method which could be applied in the future asteroid interior detection missions. Despite the high resolution, FWI is a highly nonlinear and ill-posed inverse problem which is sensitive to inaccuracies of the starting model especially when bandwidth is limited. Because of the bandwidth limitation, the range-compressed radar signal with carrier oscillates rapidly in the time domain, which makes the object function of FWI contains numerous local minimums. Thus, it is hard for FWI to converge to the global minimum of model space without an accurate initial model. Concerning this problem, we propose the envelope inversion (EI) [8], which is more robust to the choice of initial model because the time-domain envelope signal is in the baseband frequency range and varies slower than the range-compressed signal with carrier, as a supplement to FWI to recover the inner permittivity distribution of asteroid. The conventional FWI compares in the object function complete wave forms, while EI compares the time-domain envelope between the synthesized and observed signal. It has been discussed that asteroid radar tomography could be performed in different observation geometries, such as monostatic orbiter, bistatic orbiters and lander-to-orbiter system, and our work mainly focuses on the bistatic lander-to-orbiter scenario. Such analogous scenario (Fig. 1) has been applied in the CONSERT (Comet Nucleus Sounding Experiment by radio wave transmission) [9] mission to the Comet 67P/CG. Fig. 1. Lander-to-orbiter configuration. This artistic view is based on CONSERT/Rosetta In our work, the effects of initial model, noise level and carrier-frequency on FWI and EI algorithm for asteroid tomography are analyzed based on 2D numerical experiments [8],. The 2D numerical experiments shows that the EI+FWI combination, where EI serves to define the initial model for FWI, constrained by total variation regularization, has good potential for asteroid tomography. Based on EI+FWI strategy, a series of 3D numerical experiments are conducted to test the influence of orbital measurement density and landing site on the tomography. The synthetic asteroid model and the observation geometry are presented in Fig. 2. The 3D numerical experiments demonstrate that, if the signal is less contaminated by noise, EI+FWI has the ability to reconstruct the interior structure with a sufficient accuracy even under a sparse observation (like only 20 orbital measurement points); but if the signal is highly contaminated by noise, dense observation will be indispensable to ensure the reconstruction accuracy. Besides, the results also indicate that the single lander system has the potential to reveal the interior of asteroid, but the landing site could have a significant influence on the tomography. The partial results of the 3D experiments are shown as Fig. 3. Fig. 2. (a) 3D synthetic asteroid model, the model contains 3 anomalies (red) embedded in the background (green), the anomalies and the background is 4 and 3, respectively. (b) Schematic illustration of spiral observation geometry, it contains 480 measurement points (blue dots), and the angle θ is between the rotation axis and the line defined by the lander and the barycenter of the asteroid Fig. 3. EI+FWI reconstructions under the spiral observation illustrated in Fig. 2(b). (a) and (b) present the reconstructions without noise and with the SNR= –10dB (before the processing of range compressing the BPSK signal), respectively. The inversions are started with a homogenous asteroid of which the (i.e. 10% lower than the background of true model). References. [1] C. T. Russell and C. A. Raymond, “The dawn mission to vesta and ceres,” Space Sci. Rev., vol. 163, nos. 1–4, pp. 3–23, Dec. 2011. [2] Y. Tsuda, S. et al., “Flight status of robotic asteroid sample return mission Hayabusa2,” Acta Astronautica, vol. 127, pp. 702–709, Oct. 2016. [3] S.-I. Watanabe et al., “Hayabusa2 mission overview,” Space Sci. Rev., vol. 208, nos. 1–4, pp. 3–16, Jul. 2017. [4] D. Perna et al., “Spectral and rotational properties of near-Earth asteroid (162173) Ryugu, target of the Hayabusa2 sample return mission,” Astron. Astrophys., vol. 599, p. L1, Mar. 2017. [5] D. S. Lauretta, “OSIRIS-REx asteroid sample-return mission,” in Hand- book of Cosmic Hazards and Planetary Defense, Eds. Cham, Switzerland: Springer, 2015, pp. 543–567. [6] E. Gibney, “China plans mission to Earth’s pet asteroid,” Nature, to be published. [7] P. Sava and E. Asphaug, “3D radar wavefield tomography of comet interiors,” Adv. Space Res., vol. 61, no. 8, pp. 2198–2213, Apr. 2018. [8], J. Deng,et al., “EI + FWI Method for Reconstructing Interior Structure of Asteroid Using Lander-to-Orbiter Bistatic Radar System” IEEE TGRS in press [9] W. Kofman et al., “Properties of the 67P/Churyumov-Gerasimenko interior revealed by CONSERT radar,” Science, vol. 349, no. 6247, Jul. 2015, Art. no. aab0639.

The Mars' ionospheric behavior during two consecutive solar minima (23/24 and 24/25) is investigated with the same dataset. In particular, we use the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on board Mars Express to investigate the total electron content behavior of the whole atmosphere in relation to the solar irradiance (EUV and X-ray fluxes), the solar zenith angle and the heliocentric distance. The topside variability of the electron density profiles is also investigated through variations in the peak density and neutral scale height. Moreover, the equations of the NeMars empirical model of the Martian ionosphere for low solar activity are tested for both minima. We have found that the topside ionosphere of Mars behaved similarly at both solar minima. However, when considering the bottomside, a pronounced reduction in ionization in particular cases is suggested. In addition, larger TEC values are found during the solar minimum 24/25 in the nightside sector that may indicate possible larger plasma transport than during the minimum 23/24. Finally, this study confirms that the ionospheric empirical NeMars model equations derived by Sanchez-Cano et al. (2016) for the low solar activity period during the solar minimum 23/24 are also valid and accurate for the solar minimum 24/25. The long duration of Mars Express is a critical factor for determining the long-term Martian ionospheric variability, which in turn, is essential for understanding the global evolution of the planet's atmosphere.

The ESA Hera mission approved by the last ESA council Sapec19+ will be launched in 2024 to deeply investigate the Didymos binary system and especially its moonlet. Onboard the Juventas small platform, the JuRa radar will fathom Didymoon and provide the first direct observation of an asteroid deep interior. The question of asteroids’ internal structure is crucial for science, planetary defense and exploration. After several rendezvous missions, our knowledge relies entirely on inferences from remote sensing observations of the surface and theoretical modeling [1]. Didymos binary asteroid system is an S-type asteroid system orbiting the sun with a semi-major axis of 1.64 AU. The primary body has a diameter of 800 m and Didymoon, the secondary body with a diameter of 160 meters is orbiting around the main at a distance of 1.2 km [2]. In 2022, DART, the NASA contribution to the AIDA program, will impact the moonlet to quantify the mechanical response of the body, mainly from ground-based observation [3]. Five years later, Hera is a unique opportunity to observe in detail the bodies, the crater and the ejecta in order to better constraints mechanical models [2], [4]. Hera will provide a global characterization of the binary system: shape, density, dynamic properties, thermal properties, composition, … JuRa, aboard the small platform Cubsat Juventas, will probe the moonlet. The JuRa instrument is a monostaic radar, BPSK coded at 60MHz carrier frequency and 20MHz bandwidth, inherited from CONSERT/Rosetta [5], [6] and redesigned in the frame of the AIDA/AIM phase A/B [2], [7]. JuRa maps the backscatter coefficient (sigma zero - s0) of the surface or subsurface, which quantifies the returned power per surface or volume unit. It is related to the degree of heterogeneity at the scale of the wavelength and to the dielectric contrast of heterogeneities, giving access to both, the sub-meter texture of the constituent material and larger scale structure. The first objective of JuRA is to characterize the moonlet’s interior, to identify internal geological structure such as layers, voids, sub-aggregate, to bring out the aggregate structure and to characterize its constituent blocks in terms of size distribution and heterogeneity at different scales (from submetric to global). The second objective is to estimate the average permittivity and to monitor its spatial variation in order to retrieve information on its composition and porosity, especially in the area of the impact crater. Radar bypasses the near surface alteration by space-weathering and thermal-cycling as observed with optical remote sensing. The same characterization applied to the main asteroid of the binary system is among the secondary objectives, to detect differences in texture and composition and to support the modeling of the binary system’s formation. Supporting shape modeling and the determination of the dynamic state through radar ranging are other secondary objectives. Thus, the observation of the structure and composition of moonlet will provide constraints on the mechanical model of the impact process. By When compared to the observation of the main body, it will constraint the model of binary system formation to discriminate between progressive versus catastrophic process and more generally on the stability condition of the system. Acknowledgments Hera is the ESA contribution to the AIDA collaboration. Juventas and JuRa are developed under ESA contract supported by national agencies. JuRa is built by EmTroniX (Lux), UGA/IPAG (Fr), TUD (Gr), Astronika (Pl) and BUT (Cz). Juventas is built by Gomspace (Lux). This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 870377 (project NEO-MAPP). References [1] A. Herique et al., « Direct observations of asteroid interior and regolith structure: Science measurement requirements », Advances in Space Research, vol. 62, no 8, p. 2141‑2162, oct. 2018, doi: 10.1016/j.asr.2017.10.020. [2] P. Michel et al., « Science case for the Asteroid Impact Mission (AIM): A component of the Asteroid Impact & Deflection Assessment (AIDA) mission », Advances in Space Research, vol. 57, no 12, p. 2529‑2547, juin 2016, doi: 10.1016/j.asr.2016.03.031. [3] A. F. Cheng et al., « Asteroid Impact & Deflection Assessment mission: Kinetic impactor », Planetary and Space Science, vol. 121, p. 27‑35, févr. 2016, doi: 10.1016/j.pss.2015.12.004. [4] S. D. Raducan, T. M. Davison, et G. S. Collins, « The effects of asteroid layering on ejecta mass-velocity distribution and implications for impact momentum transfer », Planetary and Space Science, vol. 180, p. 104756, janv. 2020, doi: 10.1016/j.pss.2019.104756. [5] W. Kofman et al., « The Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT): A Short Description of the Instrument and of the Commissioning Stages », Space Science Reviews, vol. 128, no 1‑4, p. 413‑432, mai 2007, doi: 10.1007/s11214-006-9034-9. [6] W. Kofman et al., « Properties of the 67P/Churyumov-Gerasimenko interior revealed by CONSERT radar », Science, vol. 349, no 6247, p. aab0639, juill. 2015, doi: 10.1126/science.aab0639. [7] A. Herique et al., « A radar package for asteroid subsurface investigations: Implications of implementing and integration into the MASCOT nanoscale landing platform from science requirements to baseline design », Acta Astronautica, mars 2018, doi: 10.1016/j.actaastro.2018.03.058.

Our knowledge of the internal structure of asteroids is currently indirect and relies entirely on inferences from remote sensing observations of the surfaces. Radar observation of asteroids is the most mature technique available in order to characterize their structure, which is fundamental for understanding the small bodies' history and for planetary defense missions. However, as the small bodies' geometry is complex, simulation of the radar design and the signal is required in order to assess the instrument performance and thus the mission science return. SPRATS is a software package designed for mission and operation analysis for planetary bodies. It provides the tools to evaluate the radar concepts and mission scenarios, evaluate the processing performances, validate data processing techniques on the final data, and process real radar data when facing complex geometries different from the Earth observation geometry. The paper presents the architecture of the SPRATS toolbox along with relevant examples to illustrate its interest.

The Low Frequency Radar (LFR) on the JUVENTAS CubeSat for HERA / ESA mission to Didymos Binary Asteroid is a unique opportunity to perform direct measurements of its internal structure and regolith. LFR has been developed to fathom asteroid from a small platform. This instrument is inherited from CONSERT/Rosetta and has been redesigned in the frame of the AIDA and HERA ESA mission.Onboard JUVENTAS, LFR is operating in monostatic mode to probe down to the first hundreds of meters into the subsurface and to achieve a full tomography of the Didymos' moonlet. Direct observations of the internal structure of asteroids can solve still open basic questions like: Is the body a monolithic piece of rock or a rubble-pile? How high is the porosity? What is the typical size of the constituent blocks? Are these blocks homogeneous or heterogeneous? How is the regolith covering its surface constituted?The low frequency aboard the Juventas CubeSat will contribute to the solution of these open and for planetary defense crucial questions.- The first LRF objective is the characterization of the moonlet interior, to identify internal structure and to analyze the size distribution and heterogeneity of constitutive blocks from sub metric to global- The second objective is the estimation of average permittivity and mapping of its spatial variation especially in the crater area.- The same characterization applied to the main of the binary system is among secondary objectives.- Supporting shape modeling and determination of the dynamical state by radar ranging is a further secondary objective.This paper will present the instrument concept and measurement strategy, its performances and the expected science return.

The NASA InSight mission is operating from the surface of Mars for more than a year. RISE (for Rotation and Interior Structure Experiment) is one of the scientific payloads of InSight. This radio-science experiment consisting in an X-band transponder and two horn-antennas enabling two-way coherent radio-link between Mars and the Earth [Folkner et al., 2018]. The main goal of RISE is to measure the slight modulations of the nutational motion of the spin axis of Mars induced by the liquid core of the planet in order to constrain its interior structure and core properties. To increase our chance to achieve this challenging goal, we must calibrate the RISE Doppler data by accounting to 2nd order effects like the Mars atmospheric noise. This study shows the predicted contribution of the Martian ionosphere to the RISE data collected so far. To do so, we use a new empirical model of the Mars’ ionosphere called MoMo [Bergeot et al. 2019]. This model is based on the large database of Total Electron Content (TEC) derived from the subsurface mode of the Mars Express MARSIS radar. The model provides the vertical TEC as a function of solar zenith angle, solar activity, solar longitude and the location. Using MoMo, we produce vTEC maps for Mars that are then used to estimate the slant TEC in the Earth line of sight, enabling to infer the phase delay and Doppler shift affecting the RISE X-band measurements. These computed effects are shown to be of the order of 10-3mm.s-1 in Doppler observables, with a larger effect around sunrise and sunset. This is about one order of magnitude below the typical measurement noise of RISE, but it is comparable to the contribution of the liquid core in the Doppler (~10-3-10-2mm.s-1).The MoMo model is suitable for any Mars radio-science data calibration, and in particular the forthcoming ExoMars 2020 LaRa measurements [Dehant et al. 2019]. The predictions made with MoMo will be of great use either for the data corrections or to define the timing of observations in order to avoid operating when the TEC rapidly varies (i.e. close to sunrise and sunset). The model output is further discussed here in terms of climatologic behavior of the Mars’ ionosphere. For comparison, we also investigate the contribution of the Earth ionosphere using Global Ionospheric Maps (GIMs) based on GNSS data.

The interior structures of the comets and asteroids, still poorly known, might hold a unique key to understand the early Solar System. Considering the interaction of an illuminated electromagnetic wave with this kind of targets, these ”objects” are very large compared to the applicable wavelength. Consequently, tomographic imaging of such targets, i.e., reconstructing their interior structure via multiple measurements, constitutes a challenging inverse problem. To reach this objective and to develop and test inverse algorithms, we need to investigate electromagnetic fields that have interacted with structures analogous to real asteroids and comets. In this study, we focus on the acquisition of these fields considering three methods: calculated fields obtained with (1) time and (2) frequency domain methods and (3) microwave measurements performed for an analogue model, i.e., a small-scale asteroid model.

This study uses time-series Sentinel-1(S-1) synthetic aperture radar images to evaluate the impact of multi-temporal polarimetric processing on land-cover classification. Various polarimetr...

This study uses the ray tracing method to simulate synthetic aperture radar (SAR) images of urban areas. The images are constructed for polarisations: horizontal-horizontal (HH) and vertical-vertic...

Many future space missions to asteroids and comets will implement autonomous or near-autonomous navigation, in order to save costly observation time from Earth tracking stations, improve the security of spacecraft and perform real-time operations. Existing Earth-Spacecraft-Earth tracking modes rely on severely limited Earth tracking station resources, with back-and-forth delays of up to several hours. In this paper, we investigate the use of CONSERT ranging data acquired in direct visibility between the lander Philae and the Rosetta orbiter, in the frame of the ESA space mission to comet 67P/Churyumov-Gerasimenko, as a proxy of autonomous navigation and orbitography science capability.

Since Rosetta's lander Philae touched down on comet 67P/Churyumov-Gerasimenko on November 12, 2014, many tools have been applied to reconstruct Philae's flight path and attitude between separation, the touchdowns, collision and the final landing at Abydos. In addition to images from the cameras onboard both orbiter and lander (“OSIRIS”, “CIVA” and “ROLIS”), radio tracking results, solar array and radio data link housekeeping data, one of the major sources for timing and attitude information were two point magnetic field measurements by the magnetometers “ROMAP” and “RPC-MAG” aboard Philae and Rosetta. In this study all the different results are combined to determine in further detail what happened to Philae during its travel above the surface of 67P/Churyumov-Gerasimenko. In addition to a description of the descent dynamics and the attitude during rebound, the approximate coordinates for the collision at 16:20 UTC with the rim of the Hatmehit crater and the second touchdown are estimated. It is also shown, that Philae did not change attitude between the end of the first-science sequence and September 2, 2016.

International audience; The internal properties of cometary nuclei are the best clues to the size distribution and properties of the early planetesimals that formed the planets during the early solar system nebula processes. The CONSERT radar was designed to probe the interior of comet 67P/Churyumov-Gerasimenko with a wavelength of about 3 meters. It was successfully operated during the First Science Sequence of the Rosetta mission when the CONSERT’s wave propagated through part of the nucleus’ small lobe. The shape of the received signals provides some hints about the internal structure of the comet's small lobe. The limited broadening observed on the received pulses is compared to simulations performed on a series of non-homogeneous nucleus models to put constrains on the potential structures at the wavelength's scale. The study allows to exclude structures showing a permittivity contrast larger than 0.25 inside the sounded part of the nucleus at a few-metre size scale.

CONSERT, a bistatic radar onboard the Rosetta spacecraft and its Philae lander, was designed to probe the nucleus of comet 67P/Churyumov–Gerasimenko with radio waves at 90 MHz frequency. In 2016 September, the exact position of Philae was retrieved, within the region previously identified by CONSERT. This allowed us to revisit the measurements and improve our analysis of the properties of the interior, the results of which we present here. The relative permittivity of the materials is found to range from about 1.7 to 1.95 in the shallow subsurface (

Due to its significance in astrobiology, assessing the amount and state of liquid water present on Mars today has become one of the drivers of its exploration. Subglacial water was identified by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) aboard the European Space Agency spacecraft Mars Express through the analysis of echoes, coming from a depth of about 1.5 km, which were stronger than surface echoes. The cause of this anomalous characteristic is the high relative permittivity of water-bearing materials, resulting in a high reflection coefficient. A determining factor in the occurrence of such strong echoes is the low attenuation of the MARSIS radar pulse in cold water ice, the main constituent of the Martian polar caps. The present analysis clarifies that the conditions causing exceptionally strong subsurface echoes occur solely in the Martian polar caps, and that the detection of subsurface water under a predominantly rocky surface layer using radar sounding will require thorough electromagnetic modeling, complicated by the lack of knowledge of many subsurface physical parameters. Higher-frequency radar sounders such as SHARAD cannot penetrate deep enough to detect basal echoes over the thickest part of the polar caps. Alternative methods such as rover-borne Ground Penetrating Radar and time-domain electromagnetic sounding are not capable of providing global coverage. MARSIS observations over the Martian polar caps have been limited by the need to downlink data before on-board processing, but their number will increase in coming years. The Chinese mission to Mars that is to be launched in 2020, Tianwen-1, will carry a subsurface sounding radar operating at frequencies that are close to those of MARSIS, and the expected signal-to-noise ratio of subsurface detection will likely be sufficient for identifying anomalously bright subsurface reflectors. The search for subsurface water through radar sounding is thus far from being concluded.

We present a new method for determining the total electron content (TEC) in the Martian ionosphere based on the time delay of received radar pulses of the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on board the Mars Express spacecraft. Previous studies of the same dataset have produced differing results for the day-side ionosphere, so it is useful to have an alternative way to compute the TEC in this region. This method iterates a model ionosphere in order to simultaneously match the ionospheric delays of the signals received by the radar’s two channels by finding the model which minimizes the root mean square error (RMSE) between the measured and simulated delays. Topographical information is obtained from data from the Mars Orbiter Laser Altimeter (MOLA) instrument. The model parameters are held constant for a given orbit, and a very good agreement between the simulated and measured delays is obtained. The TEC can then be inverted from the ionospheric model. Matching the delays of both channels simultaneously applies an additional constraint to the model which has not been made in previous studies. The model is additionally validated by matching the simulated pulses with the raw range-compressed measurements for one orbit. Finally, typical model parameters are compared to those obtained by previous studies, which are also simulated. The method is applied to orbits during moderate solar activity, and results show very good agreement with previous studies.

Comets and asteroids, the small bodies of the Solar system, contain unique keys to understand the Early Solar System the posterior evolution of which is still poorly known. Our knowledge of the internal structure of comets and asteroids is, unfortunately, so far only indirect and is based on remote sensing observations of their surfaces and on theoretical modelling of their formation and evolution [1].