130,639 research outputs found
Cold Atom Interferometry for Enhancing the Radio Science Gravity Experiment: A Phobos Case Study
No full textInterplanetary missions have typically relied on Radio Science (RS) to recover gravity fields by detecting their signatures on the spacecraft trajectory. The weak gravitational fields of small bodies, coupled with the prominent influence of confounding accelerations, hinder the efficacy of this method. Meanwhile, quantum sensors based on Cold Atom Interferometry (CAI) have demonstrated absolute measurements with inherent stability and repeatability, reaching the utmost accuracy in microgravity. This work addresses the potential of CAI-based Gradiometry (CG) as a means to strengthen the RS gravity experiment for small-body missions. Phobos represents an ideal science case as astronomic observations and recent flybys have conferred enough information to define a robust orbiting strategy, whilst promoting studies linking its geodetic observables to its origin. A covariance analysis was adopted to evaluate the contribution of RS and CG in the gravity field solution, for a coupled Phobos-spacecraft state estimation incorporating one week of data. The favourable observational geometry and the small characteristic period of the gravity signal add to the competitiveness of Doppler observables. Provided that empirical accelerations can be modelled below the nm/s 2 level, RS is able to infer the 6 × 6 spherical harmonic spectrum to an accuracy of 0.1–1% with respect to the homogeneous interior values. If this correlates to a density anomaly beneath the Stickney crater, RS would suffice to constrain Phobos’ origin. Yet, in event of a rubble pile or icy moon interior (or a combination thereof) CG remains imperative, enabling an accuracy below 0.1% for most of the 10 × 10 spectrum. Nevertheless, technological advancements will be needed to alleviate the current logistical challenges associated with CG operation. This work also reflects on the sensitivity of the candidate orbits with regard to dynamical model uncertainties, which are common in small-body environments. This brings confidence in the applicability of the identified geodetic estimation strategy for missions targeting other moons, particularly those of the giant planets, which are targets for robotic exploration in the coming decades. Astrodynamics & Space Mission
Robust Multi-Disciplinary Optimization of Unmanned Entry Capsules
No full textThe conceptual design of entry vehicles is commonly done in a number of sequential steps. One usually begins with a generic shape to get a first estimate of the aerodynamic properties and uses a mass-point model for the initial trajectory design. Gradually, more detail is added and the outer shape is changed to accommodate specific mission and/or trajectory requirements. This shape will largely define the aerothermodynamic characteristics of the vehicle. Since aerothermodynamic challenges, such as vehicle heating, remain one of the most difficult problems in atmospheric re-entry, an exploration of the possible shapes for a vehicle early in the design is advisable. It is advantageous to use a continuous model for the analysis, so that one is not limited to the analysis and comparison of a limited number of shapes, but is instead free to analyze any shape in the design space. With only 5 geometric parameters it is possible to already model the geometry of Apollo-like shapes, as demonstrated later in Section IV. The internal layout of the subsystems is usually addressed only at a later stage and the designers have to make sure that the mass properties (total mass, location of the center of mass and inertia tensor) meet the requirements. Deviations from these requirements can jeopardize the entire mission, because the loads on the vehicle may change, or the stability and control properties cannot be handled by the Guidance, Navigation, and Control (GNC) system any more. Further, uncertainties related to the entry conditions, environment, the characteristics of the thermal protection system, and the design characteristics and allocation of the equipments on board, pose the multidisciplinary problem to be particularly cumbersome.
In this paper we propose a multidisciplinary, robust optimization approach for the design of unmanned entry capsules in support of the activities of the International Space Station. This problem is handled by minimizing the total mass of the capsules, while maximizing the internal available volume for carrying payload. As a third objective, we propose the maximization of the re-usability of the capsules, which can be seen as an attempt to push towards cheaper and more efficient solutions.
The shape, aerothermodynamic, and dynamic mathematical models are adapted from the work of Dirkx and Mooij. It was demonstrated that the proposed simplified aerodynamic model can predict the aerodynamic forces and moments for ballistic shapes sufficiently well for use at a conceptual design stage. The multidisciplinary design framework is now enriched with a Thermal-Protection System (TPS) model, encompassing re-usable and ablative materials, as well as active cooling mechanisms. This allows for a complete conceptual design of an entry capsule.
Uncertainties of the design variables and environmental factors are integrated into the optimization process to handle probabilistic constraints. A probabilistic constraint is a constraint in the design or objective space that shall be satisfied with a pre-defined confidence level. The optimizer thus drives the search of optimal capsules towards those solutions that have the best expected performance under uncertain conditions, and that also meet the constraints with a given confidence level, pre-selected by the designer/decision-maker.
A sampling-based approach is used to estimate the expected performance of the capsules and to determine the compliance with the probabilistic constraints. For each design point to be evaluated by the optimizer, a set of additional design points is generated around it, according to the joint Probability Density Function (PDF) of the uncertain variables and uncertain environmental factors, and evaluated. To limit the computational effort of the robust optimization, we adopt a double-repository archive maintenance scheme to save all the design-variable combinations computed during the process such that previous design points can be reused at future steps. The double-repository scheme allows to preserve the joint PDF of the input uncertain variables, therefore it is generally applicable with any type of multivariate distribution as input.
This paper is structured as follows. In Section II we provide a brief overview of related work. We then introduce the robust optimization approach and the double-repository archive maintenance scheme in Section III. A short overview of the mathematical models used for the analysis can be found in Section IV. In Section V results are discussed and in Section VI we provide conclusions and recommendations. Two appendix sections, namely Sections VII and VIII provide the thermophysical properties of the materials used for the TPS concepts and the results of the validation of the thermal models respectively
MeSH term explosion and author rank improve expert recommendations
No full textInformation overload is an often-cited phenomenon that reduces the productivity, efficiency and efficacy of scientists. One challenge for scientists is to find appropriate collaborators in their research. The literature describes various solutions to the problem of expertise location, but most current approaches do not appear to be very suitable for expert recommendations in biomedical research. In this study, we present the development and initial evaluation of a vector space model-based algorithm to calculate researcher similarity using four inputs: 1) MeSH terms of publications; 2) MeSH terms and author rank; 3) exploded MeSH terms; and 4) exploded MeSH terms and author rank. We developed and evaluated the algorithm using a data set of 17,525 authors and their 22,542 papers. On average, our algorithms correctly predicted 2.5 of the top 5/10 coauthors of individual scientists. Exploded MeSH and author rank outperformed all other algorithms in accuracy, followed closely by MeSH and author rank. Our results show that the accuracy of MeSH term-based matching can be enhanced with other metadata such as author rank
Preliminary Navigation Analysis for the Flyby Tour of ESA’s JUICE mission: An investigation on the trajectory correction maneuvers design
No full textThe JUICE spacecraft is an ESA mission to the Jovian system that will be launched in 2022. It will collect scientific data about Jupiter and its Galilean moons thanks to a flyby tour of Europa, Ganymede, Callisto and a final Ganymede orbiting phase. A high number of flybys will be performed with minimumaltitudes of 200 km. The tracking of the spacecraft during the flybys allows the estimation of the position of the spacecraft and the moons; in particular the knowledge of the moons positions can improve noticeably. However, due to the uncertainties in the positions of the moons and the spacecraft itself, a robust trajectory control is required for precise flyby targeting, to ensure that the nominal mission plan is achieved with the required margins. A preliminary navigation analysis for the mission JUICE has been developed, with the objective of the studying the influence of the spacecraft and moons positions uncertainties on the trajectory correction maneuvers (TCMs). The navigation analysis is composed of an orbit determination covariance analysis and a guidance simulator. The covariance analysis determines the standard deviations of the parameters of interest, (spacecraft and moons initial states) from the observations computed along the trajectory of the spacecraft. Different parameters can be chosen for the covariance analysis in order to represent different mission conditions and tracking data performances. The guidance has the purpose of computing the necessary actions to bring back the spacecraft to the desired path after the nominal one has been perturbed with the spacecraft and moons positions uncertainties; these action are the TCMs, which are represented by a ¢V due to the high thrust engine of JUICE. Three different targeting algorithms have been implemented for the guidance, with different characteristics of accuracy and speed. Two statistical maneuvers per flybys have been implemented, the pre-flyby (targeting) maneuver and the post-flyby (cleanup) maneuver. The navigation analysis is performed using a Monte Carlo method, sampling with a normal distribution the results of the covariance analysis to obtain perturbed trajectories; statistical results are then computed. This is necessary because the results of the OD are to be interpreted in a statistical way; the real perturbed trajectory is always affected by an uncertainty. A sensitivity analysis with respect to the tracking data types has been performed for the covariance analysis. The results show that for JUICE an uncertainty of the level of a few hundred meters can be reached, if range and VLBI data are used. However these results are just indicative since Doppler data have not been included in the simulations. The uncertainty in the positions of the Galilean moons obtained thanks to the flybys depends highly from the tracking strategy used and the data types. With themost realistic configuration (tracking interrupted before the flyby and inclusion of all the past data), the level of uncertainty can arrive to a few tens of meters, with data accuracies of 0.2 m for the range and 0.1 nrad for the VLBI. These results are for the formal errors and it is known that they are usually overly optimistic; the true error will be higher. Moreover, the importance of optical data for a further uncertainty reduction has been proven. The improvement of the Galilean moons uncertainties along the Tour as data are continuously added is also observed. Sensitivity analysis with respect to the maneuver time and the maneuver execution errors have been performed for the Monte Carlo navigation analysis. The maneuver time has the highest influence on the size of a maneuver; each flyby presents a time for which the ¢V required to correct the trajectory is minimum. Very often this time is of 3 days before the flyby (for the targeting maneuver), as found often in literature; however many flybys have this minimum around 2 days before the flyby. The maneuver execution error has a limited impact on the total mission ¢V , but it can decrease the accuracy of the targeting, causing an higher miss distance at flyby. The total ¢V required to performthe flyby Tour varies considerably upon the different parameters chosen; using current level accuracies (1mfor the range and 1 nrad for VLBI) and a maneuver time of 3 days before (and after) the flyby, the result is ¢V Æ 13.5 m/s (around 0.67 m/s per flyby). This value includes the targeting and cleanup maneuvers; it is easily obtainable by the propulsion system of JUICE, concluding that it is possible to correct the trajectory for the uncertainties of JUICE and the moons with a small effort.Aerospace EngineeringSpace EngineeringAstrodynamics & Space Mission
Interplanetary laser ranging: Analysis for implementation in planetary science missions
No full textMeasurements of the motion of natural (and artificial) bodies in the solar system provide key input on their interior structre and properties. Currently, the most accurate measurements of solar system dynamics are performed using radiometric tracking systems on planetary missions, providing range measurement with an accuracy in the order of 1 m. Laser ranging to Earth-orbiting satellites equipped with laser retroreflectors provides range data with (sub-)cm accuracy. Extending this technology to planetary missions, however, requires the use of an active space segment equipped with a laser detector and transmitter (for a two-way system). The feasibility of such measurements have been demonstrated at planetary distances, and used operationally (with a one-way system) for the Lunar Reconaissance Orbiter (LRO) mission. The topic of this dissertation is the analysis of the application of interplanetary laser ranging (ILR) to improve the science return from next-generation space missions, with a focus on planetary science objectives. We have simulated laser ranging data for a variety of mission and system architectures, analyzing the influence of both model and measurement uncertainties. Our simulations show that the single-shot measurement precision is relatively inconsequential compared to the systematic range errors, providing a strong rationale for the consistent use of single-photon signal-intensity operation. We find that great advances in planetary geodesy (tidal, rotational characteristics, etc.) could be achieved by ILR. However, the laser data should be accompanied by commensurate improvements in other measurements and data analysis models to maximize the system's science return. The science return from laser ranging data will be especially strong for planetary landers, with a radio system remaining the preferred choice for many orbiter missions. Furthermore, we conclude that the science case for a one-way laser ranging is relatively weak compared to next-generation radiometric tracking systems, requiring the development of much more accurate space-based clocks.Space EngineeringAerospace Engineerin
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe 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
"Closing the R&D Gap, Evaluating the Sources of R&D Spending"
No full textBoth spending and tax policies have been implemented in the United States with the goal of stimulating private sector research and development (R&D). Karier questions whether current R&D policy, especially the research and experimentation tax credit, can contribute to closing the gap between nondefense expenditures on R&D in the United States and such expenditures in other countries, such as Japan and Germany. He also explores possible changes to our current R&D policy to make it more effective.
- …
