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With the development of the JUpiter ICy moons Explorer (JUICE)/Radar for Icy Moons Exploration (RIME) [European Space Agency (ESA)] and Europa Clipper/Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) [National Aeronautics and Space Administration (NASA)] instruments, designed to study the subsurface of the Galilean moons, interest has been growing to study the performances of sounding radar orbiting these bodies. In the presence of strong Jupiter’s radio emissions, probing in a passive mode using these emissions as the emitter is considered. However, radar performances in this bistatic mode are dependent on the entire geometry of observation: the position of the source of emission, the spacecraft trajectory, and on the region probed. The 3-D Simulations are necessary to estimate the performances of these measurements. We analyze the influence of the geometry of observation by approximating Jovian radio bursts as radio impulses, simulating the signal scattered by a point target in a realistic 3-D geometry and computing the resolution. This allows a preliminary identification of scenarios of observation best suited for this radar mode. The influence of the correct localization of Jupiter’s emissions on the synthetic aperture radar (SAR) image is investigated as well, and interest in recovering the true position of Jupiter’s source is highlighted. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 (<25 m) and about 1.2 to 1.32 in the interior. These differences indicate different average densities between the shallow subsurface and the interior of comet. They can be explained by various physical phenomena such as different porosities, the possible compaction of surface materials, or even perhaps different proportions of the same materials. This strongly suggests that the less dense interior has kept its pristine nature. [ABSTRACT FROM AUTHOR]

Abstract The internal structure of asteroids is still poorly known and has never been analyzed directly by measurements. Our knowledge relies entirely on inferences from remote sensing observations of the surface and theoretical modeling. Direct measurements are crucial to characterize an asteroid's internal structure and heterogeneity from sub-metric to global scale. The radar package developed in the frame of the phase A/B1 of the Asteroid Impact Mission (AIM) as part of the larger Asteroid Impact & Deflection Assessment (AIDA) mission is a mature instrument suite to answer this question and to improve our ability to understand and model the mechanisms driving Near Earth Asteroids (NEA). It is of main interest for science, exploration and planetary defense. This instrument suite consists of a monostatic high frequency radar (HFR) to investigate the stratigraphy of surface regolith and a bistatic low frequency radar (LFR) to characterize the deep interior. The chosen platform to deliver the surface unit of the LFR and other instruments for a close-up study of the target asteroid is the MASCOT nanolander, which already flies on Hayabusa2 (HY2) in a mineralogy scout configuration. In this paper, we present the chosen instrumentation for radar science, baseline mission requirements and the initial design for integration into the lander platform, including all peculiarities and constraints. Highlights • Direct observations of asteroids by radar to understand accretion and evolution. • The baseline design of two radars is derived from mission science requirements. • Surface and subsurface maps are provided by a monostatic high frequency radar. • Deep interior 2D and 3D tomography is provided by a bistatic low frequency radar. • Aspects of integrating the bistatic radar onto the reusable MASCOT lander platform. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]