Search

The SHAllow RADar (SHARAD) onboard Mars Reconnaissance Orbiter (MRO) is a 20 MHz Synthetic Aperture Radar (SAR) that probes the first hundreds of meters of the Martian subsurface. In order to interpret the detection of subsurface interfaces with ground penetrating radars, simulations using Digital Terrain Models (DTM) are necessary. This methodology paper focuses on the analysis of the first tens of meters of the Martian subsurface with SHARAD, comparing the use of different high-resolution DTMs for radar simulation, namely, from the High-Resolution Stereo Camera (HRSC) onboard the Mars Express and from the Context Camera (CTX) onboard MRO. The region of Terra Cimmeria was chosen as a demonstration area. It is a highly cratered southern midlatitude region, where, as will be discussed, the higher resolution of the aforementioned terrain models is mandatory to describe the surface at an acceptable level of detail for shallow subsurface radar interpretation. With a DTM corrected by photoclinometry using CTX imagery, we show that a reflector that was visible on SHARAD data but not on the simulation made with an HRSC DTM is, in fact, a surface echo that was not reproduced by the HRSC surface model. We also show that, unlike laser altimetry DTMs, optical DTMs are prone to artifacts that can make radar analysis more complicated for some scenarios. Reciprocally, we show that the comparison between radar and its corresponding simulated data is a way of assessing a DTM's quality, which is especially useful in missions where ground control points are lacking, unlike Martian observations. [ABSTRACT FROM AUTHOR]

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]

A ground-penetrating radar will be integrated inside the guiderope ballast of the Mars'98 balloon mission. A prototype of this impulse system working in the 5-15 MHz band was tested close to Dumont d'Urville, the French Antarctic Station, in February 1993. We obtained a large set of profiles in cold ice with a glacier thickness from 200 to about 900 m. For each depth, the measured echoes are very clear with a good signal-to-noise ratio (SNR). In this paper, we firstly describe the performances of the radar and we study the electromagnetic propagation in the Antarctica glacier. This knowledge allows us to use the frequency wavenumber migration to focus the signal, to increase the SNR and to rebuild an image of the subsurface. The migration process gives a good estimation of the real part of the permittivity (i.e., the velocity). The fact that the signal comes from different depths is used to measure the imaginary part of the permittivity which defines the ice attenuation. The spectral analysis gives its frequency dependence. Index Terms - Ground-penetrating radar, ice permittivity, migration.

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]

Our knowledge of the internal structure of asteroids is currently indirect and relies on inferences from remote sensing observations of surfaces. However, it is fundamental for understanding small bodies' history and for planetary defense missions. Radar observation of asteroids is the most mature technique available to characterize their inner structure, and Synthetic Aperture Radar Tomography (TomoSAR) allows 3D imaging of their interior. However, as the geometry of observation of small asteroids is complex, and TomoSAR studies have always been performed in the Earth observation geometry, its results in a small body geometry must be simulated to assess the methods' performances. We adopt here two different tomography algorithms and evaluate their performances in our geometry by assessing the resolution and the difference between the scatterer's position and its retrieved position. The first method, the Frequency Domain Back Projection (FDBP) is based on correcting the Fourier transform of the received signal by a phase function built from the geometry. While it can provide a good resolution, a bias remains in the imaged scatterer's position. Meanwhile, Compressive Sensing (CS) relies on the hypothesis that few scatterers lie in the same direction from the subsurface. Its application in the small body geometry is studied, which results in a slightly impoverished resolution but an improved localization of the scatterer. Key Points: High‐Frequency Radar (HFR) is an UWB Synthetic Aperture Radar (SAR) developed to retrieve the 3D structure of the first 10 m of an asteroid's subsurfaceSAR Tomography (TomoSAR) is crucial to improve the resolution in the vertical directionIn the specific asteroid geometry, simulations are necessary the assess the performances of the TomoSAR algorithms [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]

The context of space missions implies very strong constraints on the design of the instruments. For instance, mass, data rates, power consumption, clock frequency, and accuracy are very limited. These limitations degrade the performance and the accuracy of the measurements. This paper is related to the study of signals sampled at a low frequency. More exactly, we propose an oversampled pulse compression method for radar signals, which allows to reconstruction the signals with a much higher sampling rate. Consequently, this method allows significantly improving the multipath amplitude and the delay estimation accuracy. The efficiency of our method is clearly demonstrated with real measurements. These latter correspond to calibration measurements performed on ground with the radar COmets Nucleus Sounding Experiment by Radio-wave Transmission. The latter allows performing the tomography in the transmission of 67P Churyumov–Gerasimenko comets nucleus in the frame of the Rosetta/European Space Agency space mission. The results show that the proposed method allows estimating the delays with accuracy better than 10% of the sampling period in the case of a sampling frequency value close to the Nyquist limit. The analysis of the measured signals shows how a small frequency aliasing power degrades the delay estimation performance. This problem can be resolved using an estimation method based on an end-to-end signal modeling. [ABSTRACT FROM PUBLISHER]

On 12 November 2014 at 15:34 UTC, after a 7-h descent, the Philae lander made contact with the surface of the comet Churyumov-Gerasimenko (67P/CG) in the centre of the intended landing zone. Following the failure of its anchoring systems, it bounced and continued to fly ballistically. A couple of hours later, telemetry transmitted to Rosetta showed that Philae was stabilized on the surface and working properly although its precise and final location was and still is not known ( Biele et al, 2015 ). From the morning of 13 November 2014, CONSERT data enabled an initial estimate to be made of Philae’s location based on signals propagated through the 67P/CG nucleus. This rough location was then refined through three sets of measurements carried out during periods of direct visibility between Rosetta and Philae during the Philae’s First Science Sequence (FSS) on 13 and 14 November. This paper presents these measurements and the estimate of the final position of Philae in November 2014. [ABSTRACT FROM AUTHOR]

The Philae lander provides a unique opportunity to investigate the internal structure of a comet nucleus, providing information about its formation and evolution in the early solar system. We present Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT) measurements of the interior of Comet 67P/Churyumov-Gerasimenko. From the propagation time and form of the signals, the upper part of the "head" of 67P is fairly homogeneous on a spatial scale of tens of meters. CONSERT also reduced the size of the uncertainty of Philae's final landing site down to approximately 21 by 34 square meters. The average permittivity is about 1.27, suggesting that this region has a volumetric dust/ice ratio of 0.4 to 2.6 and a porosity of 75 to 85%. The dust component may be comparable to that of carbonaceous chondrites. [ABSTRACT FROM AUTHOR]

Presents information on the prototype testing of the ground-penetrating radar which is to be integrated inside the guiderope ballast of the Mars'98 balloon mission. Description of the performances of the radar; Examination of the electromagnetic propagation in the Antarctica glacier; What this knowledge allows the researchers to do; Spectral analysis of the signal.

The Philae lander, part of the Rosetta mission to investigate comet 67P/Churyumov-Gerasimenko, was delivered to the cometary surface in November 2014. Here we report the precise circumstances of the multiple landings of Philae, including the bouncing trajectory and rebound parameters, based on engineering data in conjunction with operational instrument data. These data also provide information on the mechanical properties (strength and layering) of the comet surface. The first touchdown site, Agilkia, appears to have a granular soft surface (with a compressive strength of 1 kilopascal) at least ~20 cm thick, possibly on top of a more rigid layer. The final landing site, Abydos, has a hard surface. [ABSTRACT FROM AUTHOR]

Abstract: Numerous scarps with similar characteristics have been observed in the polar layered deposits of Planum Australe, Mars. They are referred to as LAPSs (for Large Asymmetric Polar Scarps) because of their typical cross-section featuring a trough between a straight slope on one side with outcrops of layered deposits and a convex slope on the other side without any outcrops. These LAPSs are restricted to the outlying region of Ultimi Lobe. Topographic data, optical images, and subsurface radar observations have been analyzed and compared to produce a complete morphologic and stratigraphic description of these scarps. In all, 167 LAPS-like features have been identified. All have similar dimensions and characteristics and appear to be deep depressions in the ice. The polar deposits have an average thickness of 1km in this region and the LAPS depressions commonly reach half of that thickness. Subsurface data indicate that the depressions could reach bedrock at certain locations. Many surface features of the polar deposits of Mars are considered to be consequences of depositional and/or erosion processes. We propose a mechanical failure of the ice for the LAPSs origin, given the striking similarities in shape and size they show with rollover anticlines above listric faults commonly observed as a crustal extension mode on Earth. This tectonic scenario would imply a substantial outward sliding of the polar deposits in the region of Ultimi Lobe and a low basal shear stress. No information is available to determine whether such a system could be active at present. Confirmation of the “mechanical failure” hypothesis of these LAPSs on Mars is of major importance as it could be a macro-expression of fundamental differences between ice-sheet behavior under martian and terrestrial conditions. [Copyright &y& Elsevier]