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1D-FDTD Characterization of Ionosphere Influence on Ground Penetrating Radar Data Inversion
No full textA finite-difference time-domain (FDTD) simulator is employed to evaluate distortions induced by the ionosphere on chirp signals transmitted by orbiting ground penetrating radars like the Mars advanced radar for subsurface and ionosphere sounding (MARSIS) and the shallow radar (SHARAD). The main goal is to support the data inversion process, i.e., the estimation of the permittivity of subsurface layers. Also proposed is an approach to study the influence of the ionosphere on chirp signals for present and future missions. Since data inversion relies on both crust attenuation estimation and rough geometries compensation, procedures that could be both strongly influenced by the ionosphere, several ionosphere compensation schemes are compared and their impact on data inversion is discussed
A phase-gradient-autofocus algorithm for the recovery of MARSIS subsurface data
No full textThe Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) is currently operating, providing information of the subsurface structure of the planet Mars. The transmitted chirp signals pass through the ionosphere, resulting delayed, attenuated, and, in the phase, distorted. MARSIS signals are routinely phase corrected during ground processing by adopting a predefined model based on the Chapman theory. In this letter, phase distortions are compensated by using a phase-gradient-autofocus algorithm, which is capable of correcting distortions of different nature by exploiting their redundancy along the orbit. For the first time, images presenting saturated regions have been recovered
Volume scattering influence on MARSIS and SHARAD data inversion
No full textThe Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) and the Shallow Radar (SHARAD) are currently operating on Mars transmitting an HF chirp signal to retrieve information regarding the Martian crust structure. An important task to be performed is the data inversion, i.e., the permittivity estimation of the detected subsurface layers. In order to be properly performed, degrading effects on attenuation estimation and geometric term correction have to be understood. This paper aims to simulate a few representative scenarios, using the Finite-Difference Time-Domain formulation, pointing out the volume scattering influence on MARSIS and SHARAD data. Influence on both attenuation estimation and geometric term correction will be discussed
Subsurface geometry influence on radar returns in the orbiting ground penetrating radar context
No full textThe Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) and the Shallow Radar (SHARAD) are low frequency, pulse limited ground penetrating radars selected to investigate the Mars subsurface as payloads of the ESA Mars Express and NASA MRO missions respectively. Radar echoes coming from both surface and subsurface are strongly affected by geometry. The proposed work aims to produce a theoretical analysis for the various scenarios of interest, evaluating the impact on the data inversion process that aims to estimate the dielectric constant of materials composing the different detected sub-superficial interfaces
Impact of uncompensated ionospheric distortions on MARSIS data
No full textThe Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) currently operating on Mars needs a fine ionospheric correction in order to deliver products useful for geological investigations. Ionosphere influence can be assessed using a new approach based on the finite-difference time-domain (FDTD) method. The proposed work aims to underline the errors introduced by a not perfect knowledge of the ionospheric electron density profile on the chirp signal range compression. Such effect has a great impact on the data inversion process that aims to estimate the permittivity of the subsurface detected interfaces
Martian magnetic minerals signature detection by Shallow Radar (SHARAD)
No full textNear-global thermal infrared mapping by the Thermal Emission Spectrometer (TES) on Mars Global Surveyor
has revealed unique deposits of crystalline gray hematite (a-Fe2O3) exposed at the Martian surface in the Sinus
Meridiani region. The material is an in-place, rock stratigraphic sedimentary unit characterized by smooth, friable
layers composed primarily of basaltic sediments with 0-20% crystalline gray hematite.
Shallow Radar (SHARAD) is a ground penetrating radar (GPR) provided by the Italian Space Agency
(ASI) and selected by NASA for the Mars Reconnaissance Orbiter (MRO) mission. The goal of this nadir-looking
altimeter with synthetic aperture capabilities is to investigate the surface and subsurface of Mars providing data
about the crustal composition of the planet. The sounder operates using a 20 MHz carrier and a bandwidth of
10 MHz (from 15 to 25 MHz) to achieve a theoretical vertical resolution of 15 m in free space, maintaining an
acceptable penetration capability of approximately 1500 m. Performance of the instrument can however be highly
dependent on the operating environment and in particular on the reflectivity of the surface and the subsurface, on
the effect of the ionosphere and on the level of clutter echoes, which in turn depend on the surface topography.
Laboratory measurements of electrical and magnetic properties of grey hematite at Mars ambient temperatures
in the ground penetrating radar frequency range have produced surprisingly strong dielectric relaxations
as well as the expected magnetic properties. At the average Mars surface temperature of 213 K hematite has a
strong dielectric relaxation near 15 MHz which is strongly temperature dependent. Between day and night this
relaxation will move through the frequency range of SHARAD that may be capable of identifying the temperature
dependence.
Several works regarding the effect that magnetic materials should have on the signal transmitted by ground
penetrating radars like SHARAD have been proposed in the past. Since a vast data set has been acquired by the
sensor over Sinus Meridiani the present study aims to validate previous works underlining the limitation that
surface geometry induces on the data analysis
Image resolution enhancing in the MARSIS experiment
No full textMARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) is a low frequency, pulse-limited radar
sounder and altimeter selected by ESA as a payload of the Mars Express mission. Synthetic aperture technique is
required to reduce the wide ground footprint (due to the low operating frequency and the small allowable antenna
dimensions) and, thus, the unwanted echo from other surface objects.
MARSIS primary objective is to detect, map and characterize subsurface material discontinuities in the upper crust
of Mars. These may include boundaries of liquid water-bearing zones, icy layers and geologic structures. Past
studies have shown polar caps stratifications, in terms of depth structure and composition, ground ice abundance
and seasonal variations (thickness of seasonal deposits, thermal effects).
MARSIS is the first instrument able to detect what lies beneath the surface of Mars. MARSIS operates with a very
high fractional bandwidth: 1MHz bandwidth allows a vertical resolution of 150 m in free space which corresponds
to a lower resolution in the subsurface, depending on the electromagnetic wave propagation speed in the crust.
The centre frequency of the pulses transmitted by MARSIS can be set to 1.8 MHz, 3MHz, 4 MHz and 5MHz.
On day side operations, it operates only in 4MHz and 5MHz due to the ionosphere plasma frequencies of Mars
that reflects all the frequencies lower than 4 MHz. All the four carrier frequencies are available for subsurface
sounding on night side.
This paper propose a modified version of the well known stepped frequency processing to improve the vertical
resolution of MARSIS in order to allow the detection of thinner interfaces that could not be discriminated by the
present processing because of its coarse vertical resolution. In fact, range resolution in SAR images is inversely
proportional to the transmitted signal bandwidth. Since there is a limit in the transmitted bandwidth that can
be supported by the radar hardware, there is a limit in range resolution that can be achieved by processing the
SAR data in conventional way. However, if the frequency band of the received signal is widened with a group of
sub-pulses, close in frequency (e.g. 3Mhz and 4 MHz), and properly combined, the composite signal increases the
bandwidth and hence the improvement in range resolution can be achieved.
The algorithm proposed modifies the standard stepped frequency processing introducing ionosphere effects
compensation necessary for a correct data processing . Thanks to improved data set it will be possible to have
either a deeper knowledge of the subsurface stratifications as well as additional information about the nature of
the volume scattering useful in the data inversion process (estimation of the materials composing the surface and
the subsurface by the estimation of the dielectric constants)
Planetary radar data inversion techniques improvement
No full textThe planetary radar (e.g. MARSIS) data inversion is based on the selection of groups of stationary frames, within
the area under investigation, that shall be statistically analyzed after suitable correction. The selection step includes
the recovery of bad/poor data and the estimation of the geometrical surface and subsurface features; these feature
shall be utilized in order to obtain data that are only dependent by the material nature of the inclusion, within the
layer, and of the interface. This paper is addressed to the techniques used for the frames selection, recovery and
their geometric estimation content.
As first step, frames have been selected in Mars areas where the surface and subsurface have a physical
optics behavior (i.e. quite flat); the surface flatness has been estimated according to a simulator based on MOLA
(Mars Orbiter Laser Altimeter) data while the subsurface has been estimated taking into account the Doppler filters
content (i.e. filter 0, +1, -1). Being the surface and subsurface quite flat only small geometric contribution have
been estimated and used for correction of the received echoes. To perform this task surface and subsurface models
have been developed, under the Kirchhoff approximation hypothesis, to be compared with the experimental data.
A figure showing the different material nature of different areas of the Mars South Pole has been drawn.
The discovery of areas with an high dielectric constant led geologists to analyze those areas with other instrument
to confirm the results obtained by MARSIS. This paper outlines also the way out for future works in
order to analyze more complex surface and subsurface scenarios where conditions for geometric optics or fractal
can be present. In this case, it will be mandatory to develop a clutter cancellation technique to avoid the presence
of false subsurface echoes generated by surface and subsurface features not immediately below the nadir direction
of observation. It will be also necessary to improve the access to the simulator for a quickly accomplishment of
this task. In addition it is necessary a deeper analysis of the volumetric clutter including its distribution within the
layer thickness. A fully automation of the frames selection will be set for a fast analysis of wide areas of Mars.
Finally a data fusion with SHARAD data will improve the reliability and the validity of the obtained results
Weighting network influence on the geometric term correction in MARSIS data inversion
No full textMars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) is a low frequency, pulse-limited radar sounder and altimeter selected by ESA as a payload of the Mars Express mission. This work retraces the processing that leads to the extraction of parameters needed to perform the data inversion pointing out an effect caused by the weighting network application in presence of volume scattering that could jeopardize the backscattering-related geometry interpretation on a specific set of data
Ionosphere compensation and stepped frequency processing in the MARSIS experiment
No full textThis paper is addressed to the improvement of the range resolution of MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) by means of a modified version of the stepped frequency processing algorithm. MARSIS is a low frequency, pulse-limited radar sounder and altimeter selected by ESA as a payload of the Mars Express mission. The ionosphere affects MARSIS operation in terms of phase distortion, attenuation and Faraday rotation. The ionosphere fine compensation is obtained according to the uniform model, allowing, with the correctly compensated data, the production of MARSIS images at higher resolution. In this way it is possible to detect hidden interfaces never seen before due to MARSIS coarse vertical resolution. © 2011 IEEE
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