1,721,065 research outputs found
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
Extensive non-coherent averaging in doppler beam sharpened space-borne radar altimeters"
No full textGoing Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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
Global topography of the Moon by means of a nadir looking interferometric synthetic aperture radar altimeter
No full textImage 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)
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