1,720,985 research outputs found
Mission architecture for Mars exploration based on small satellites and planetary drones
Full text linkPurpose: The purpose of this paper is to deal with the study of an innovative unmanned mission to Mars, which is aimed at acquiring a great amount of detailed data related to both Mars’ atmosphere and surface. Design/methodology/approach: The Mars surface exploration is conceived by means of a fleet of drones flying among a set of reference points (acting also as entry capsules and charging stations) on the surface. The three key enabling technologies of the proposed mission are the use of small satellites (used in constellation with a minimum of three), the use of electric propulsion systems for the interplanetary transfer (to reduce the propellant mass fraction) and lightweight, efficient, drones designed to operate in the harsh Mars environment and with its tiny atmosphere. Findings: The low-thrust Earth-Mars transfer is designed by means of an optimization approach resulting in a duration of slightly more than 27 months with a propellant amount of about 125 kg, which is compatible with the choice of considering a 500 kg-class spacecraft. Four candidate drone configurations have been selected as the result of a sensitivity analysis. Flight endurance, weight and drone size have been considered as the driving design parameters for the selection of the final configuration, which is characterized by six rotors, a total mass of about 6.5 kg and a flight endurance of 28 minutes. In the mission scenario proposed, the drone is assumed to be delivered on the Mars surface by means of a passive entry capsule, which acts also as a docking station and charging base. Such a capsule has been sized both in terms of mass (68 kg) and power (80 W), showing to be compatible with 500 kg-class spacecraft. Research limitations/implications: As a general conclusion, the study shows the mission concept feasibility. Practical implications: The concept would return incomparable scientific data and can be also be potentially implemented with a relatively low budget exploiting of the shelf components to the larger extent, small identical spacecraft buses and modular low-cost drones. Originality/value: The innovative mission architecture proposed in this study aims at providing a complete coverage of the surface and lowest atmospheric layers. The main innovation factor of the proposed mission consists in the adoption of small multi-copter UAVs, also called “drones,” as remote-sensing platforms
Design Criteria of Remote Sensing Constellations of Small Satellites with Low Power Electric Propulsion and Distributed Payloads
Full text linkThe recent explosion in proposed microsatellite missions is based on the possibility to mass-produce cheap platforms capable to deliver acceptable performance over a limited lifetime. The assumption behind such scheme is that individual microsatellites are expected/allowed to fail in reasonable numbers, the resulting degradation of constellation performance being limited due to the large population of active spacecraft. We argue that cheap platforms do not necessarily need to be seen as disposable assets, so that low cost constellations featuring a low number of microsatellites may nevertheless be capable of remarkable performance. The key technology needed to enable such feat is low power electric propulsion, whereby microsatellites are allowed to acquire and maintain precisely tuned orbital locations, compensate atmospheric drag to fly longer, and de-orbit safely at end of life. A number of such microsatellites may be fitted with an instrument each from a suite of different sensors operating in various spectral bands. The constellation would operate as an actively controlled system, with the individual instruments providing well coordinated raw data that may be processed using data fusion techniques to yield the final product. Starting from the proven performance of a currently available low power Hall thruster, we present general design criteria for constellations based on a 50 kg-class microsatellite bus. The potential benefits of such technology are outlined with respect to applications such as precision farming, urban area monitoring, and dual use land surveillance
Analytical constellation design and link budget computation tool for EO missions
Full text linkThe recent trend for Earth Observation (EO) is to use constellations of small satellites for missions so far enabled only by large platforms, with relevant overall mission cost savings due to technology improvement and component miniaturization. The availability of a computationally light tool, capable of performing a parametric analysis of constellation performance as a function of number of satellites and orbital parameters and of estimating the link budget in a reliable way, is of fundamental importance in the preliminary phase of constellation design. Such a tool was recently developed in collaboration between SITAEL SpA and the University of Pisa and is presented and discussed in this paper.
The code developed consists of three inter-related modules. The first module is dedicated to constellation design. Assumption the use of sun-synchronous repeating ground track orbits, this tool needs as input only the instrument field of view (FOV), the mean latitude and the characteristic size of the area of interest. For repeat cycles (RC) between 1 to 30 days, the altitudes are iteratively computed along with the number of satellites needed to cover the area of interest. For a constellation in a single plane, the module also provides the satellites phasing to cover the area adjacent to the area covered by the previous satellite; for a multi-plane constellation, the number of orbital planes and their relative phasing is provided. The propellant mass needed for drag compensation is estimated at each altitude using the NRLMSISE-00 model. The constellation robustness, i.e. the of area covered in the case of failure of one or more satellites, is assessed considering a mean value of the ground track length in the area of interest.
The second module performs post-processing of data from NASA’s General Mission Analysis Tool (GMAT) to analyze coverage and link performance among satellites and ground stations, as well as inter-satellite links. Variables imported from GMAT include position, velocity and temporal information. Coverage status of user-specified targets is evaluated based on the optical instrument parameters. Considering the user specified points as ground stations (GS), the link budget is then computed. For specified antenna types, the position of the sub-satellite points are calculated relative to the electric field contour levels around a user-defined area. This approach therefore uses varying GS power output with fixed (maximum) gain, thus giving a range for uplink and downlink margins in terms of SNR and Eb/N0. Losses due to rain, cloud and atmospheric gases are accounted for from ITU-R models, while transmission losses in cables are calculated based on look-up tables for both the ground and the space segment. For the SNR method, the user specifies the bandwidths of the receivers or transmitters and the required SNR. For the Eb/N0 method, typical selectable modulation/demodulation schemes are incorporated in the tool.
The third module computes visibility among all members of the satellite constellation, allowing for the computation of inter-satellite link budgets under the assumption that the satellites are pointing towards each other in the direction of maximum gain.
In the paper, we present a number of selected EO mission test cases to show how the tool is used for the constellation design and the capabilities to performs link budgets in realistic mission scenarios, as well as the results of validation tests performed using the commercial STK software package. The advantages of using a simple, integrated tool for preliminary mission analysis, instead of a number of different software programs, are illustrated and discussed
Low-Thrust Propulsion Systems for Small Satellites
Full text linkSmall platforms represent valuable options for small scientific and Earth observation missions. One of the main challenges for microspacecrafts with launch mass below approx. 100 kg is the inclusion of a propulsive subsystem. Such addition would significantly enhance the performance of these platforms, broadening the possible applications and/or extending the operational lifetime. For these applications, electric propulsion systems are more suitable than classical chemical systems as they allow a larger payload mass fraction reducing the propellant mass requirement. The aim of this study is to investigate about state of the art of electric propulsion options for small spacecraft. A selection of possible electric propulsion systems for small satellite based on several requirements (power, thrust, specific impulse) is presented. This paper discusses four typical orbital manoeuvres of fundamental relevance for satellites in low Earth orbit: scenario No. 1 considers a 350 km decrease of orbital altitude; scenario No. 2 is about drag compensation for 90 days at very low altitude. Scenario No. 3 is a combined manoeuvre to change both semi major axis (by 150 km) and inclination (by 0.563 deg) at once. The fourth scenario is about orbit circularization. Results are thus normalized to obtain dimensionless parameters to be compared. Our analysis shows that an electric propulsion system offers significant advantages for small satellites in low Earth orbits in case of altitude variation, moderate inclination change, circularization and drag compensation at very low altitude
Recent Developments in Ionic Liquid Field Emission Electric Propulsion
Full text linkThe study herein reported was aimed at the characterization of the plume of a ionic liquid fed, linear slit FEEP thruster, in terms of composition and velocity of the constituents. Ionic liquid propellants are actively investigated as promising alternatives to alkali metals in field emission thrusters, in order to reduce system cost and ground operation complexity. To this end, a large number of tests was carried out using the EMI-BF4 ionic liquid as a propellant. The thruster was fired in either positive polarity or negative polarities to check the capability to extract anions and cations alone. Then, most of the testing was carried out in alternate polarity mode, in order to avoid electrochemical poisoning of the propellant, due to the unbalanced extraction of charged particles[1]. Such operating mode is believed to be the most promising candidate for flight operation, as it would allow to get rid of an external neutralizer to maintain electrical neutrality of the spacecraft.
Ion beam composition was investigated by means of a time-of-flight mass spectrometry technique. The measurements show that the emitted beam is mostly composed of monomers (BF4)-, dimers (C6H11BF4N2) (BF4)- and polymers (C6H11BF4N2)n (BF4)- (with n a function of applied extraction voltage). Under the assumption of a certain beam composition, propellant consumption was indirectly evaluated by means of time integration of the emitted current and independently verified by means of direct observation of the depletion of the propellant reservoir. The estimated resulting specific impulse is around 1400 s. The thruster behaviour resulted quite variable, especially when operated at high voltage levels in continuous polarity mode. Better performance was registered in alternate polarity operation with an alternation period of several tens of a seconds at extracted current of just a few μA
SSCAM: Micro-satellite platform for Earth observation
No full textThanks to recent technological advances in space component miniaturization, small spacecraft can assure a fast and affordable access to space, while providing a level of performance similar to larger ones. Nevertheless, only a few Earth observation micro-satellite have been demonstrated in orbit so far. This paper presents a small commercial platform concept tailored to the bservation of the Tuscany Region (Italy) area. A preliminary mission analysis is performed and some potential applications are
discussed. In particular, it is shown that several state-of-the-art imaging devices can be considered as the payload of the proposed platform
Going 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
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