1,721,013 research outputs found
Satellite de-orbiting by means of electrodynamic tethers - Part II: System configuration and performance
No full textTriple Probe System for in situ ionospheric plasma monitoring
No full textThe capability to establish the magnitudo, time and place of a forthcoming earthquake is still far from having a reliable and accurate prevision of practical interest. Different precursory phenomena of forthcoming earthquakes have been experimentally observed; they are mostly linked to a microfracturation of the rock due to stress accumulation during the pre-seismic phase in the hypocentral zone of an earthquake. The microcracks in the rocks can be considered seismic sources able to generate electromagnetic emissions, in particular in ULF-ELF bands (0-3000 Hz). The electromagnetic waves spread through the lithosphere to the ionosphere and the magnetosphere and they produce irregularities (Earth magnetic field variations, ionospheric plasma parameters disturbances and particles precipitation from Van Allen belts). In addition, the shallow earthquakes are capable to produce gravity oscillations, propagating upwards and coupling with neutral atmosphere and, eventually, with ionospheric layers. Significant variations of the total electron content in the ionosphere have been measured through GPS receivers. A space mission seems to be an affordable way to achieve in-situ observations, overall the Earth.
A plasma diagnostic system suitable to be hosted on board LEO satellites to investigate the presence of ionosphere disturbances has been designed, manufactured and tested. The system is an electrostatic probe, specifically a Triple Probe System (TPS), which allows the local plasma density and electron temperature determination through acquisition of instantaneous voltage and current measurements. A prototype has been arranged to be hosted on board UNISAT-4, a low-cost university satellite built by GAUSS (Gruppo di Astrodinamica – Università degli Studi di Roma “La Sapienza”), scheduled to be launched on December 2005.
The paper deals with the TPS basic concepts and some preliminary results, obtained in a test campaign carried out in a climatic room from -40°C to +60°C, are depicted
Electrodynamic tethers for deorbiting applications
No full textIn this paper the de-orbiting performance of various tether configurations have been studied and compared with each other. Three different configurations have been analyzed. An insulated conducting tether terminated with a balloon at the upper end (ITB), a bare tether only (BTO) and a bare tether terminated with a balloon (BTB). The drag forces obtained with the various systems have been computed taking into account the tether equivalent electric circuit which includes, besides the ohmic resistance of the wire, the effects of the plasma sheaths resulting from the interaction with the ionospheric plasma. The orbit decay times for a spacecraft of 500 kg mass equipped with a 5 km long tether, in the three different configurations, have been computed by using a simple model of the average ionospheric density profile. The BTB configuration turns out to be the one which exhibits the best de-orbiting performance. The total de-orbiting time from an altitude of 1300 km down to 200 km is 23 days for a system BTB, 32 days for ITB and 66 days for BTO
Electromagnetic measurements on Martian soil analogs: implications for MARSIS and SHARAD radars in detecting subsoil water
No full textIn this work a laboratory investigation on the electrical and magnetic properties of magnetite-bearing and water-bearing granular
mixtures, selected as Martian soil simulants, is presented. The objective of these measurements is the estimation of the performance of
space-borne ground penetrating radars for detection of subsurface water (both solid and liquid) on Mars (MARSIS on-board Mars
Express, and SHARAD on Mars Reconnaissance Orbiter). A simplified two-layers soil model is proposed to estimate the radio wave
maximum penetration depth. The upper layer contains different magnetite volume fractions, whereas the lower one, representing the
target to be detected, consists of pure water ice, or basaltic sand mixed with either liquid or solid water. The material constitutive
parameters are measured in the megahertz frequency region, to obtain the soil electromagnetic characterization in a band consistent with
the MARSIS and SHARAD sounders. Complex dielectric permittivity and magnetic permeability of the tested materials are obtained by
measuring the impedance of a capacitive cell (for the dielectric measurements), and a toroidal solenoid (for the magnetic measurements).
The attenuation of radar signals and the target detection depth are computed as a function of the operating frequency, the upper layer
magnetite content, and the receiver-to-transmitter signal power ratio
Determination of Frequency Dependent Electromagnetic Parameters Via Time Domain Reflectometry
No full text
“ATTENUATION, BANDWIDTH AND SCATTER FUNCTION MEASUREMENTS OF GLASS BEADS/MAGNETITE MIXTURES BY TIME DOMAIN REFLECTOMETRY”
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
Time Domain Reflectometry of Glass Beads/Magnetite Mixtures: a Time and Frequency Domain Study
No full textThe measurements of the time domain reflectometry sTDRd signal propagating along a coaxial
probe, filled with dry mixtures of glass beads and magnetite, are analyzed for deriving the
attenuation factor a and the bandwidth. Samples with different percentages s5%–25%d of magnetite
are considered. The a values are obtained by two different methods: sid wave amplitude at the
second reflection, siid electromagnetic parameters and widths of the passing bands. The two methods
provide consistent a values within the experimental uncertainties. The agreement supports the
possibility of measuring the attenuation factor from the second TDR reflection, for anhydrous
samples similar to the investigated mixtures
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