1,721,145 research outputs found
Oracle: a dual-smallsat mission to investigate the martian climate
No full textA mission concept to study Mars' climate and interior has been developed. The mission is based on a pair of SmallSats in the same orbit about Mars. The payload includes a novel inter-satellite tracking system that collects highly accurate data to characterize the permanent and seasonal polar deposits. Inter-satellite data also provides very accurate navigation performances, including autonomous and real-time navigation capabilities
Mars’ atmospheric calibration of radio tracking data for precise orbit determination
No full textThe accurate navigation of Mars' orbiters requires the precise modeling the radio tracking measurements. The Martian atmosphere perturbs the optical path of the radio links leading to significant Doppler shifts (i.e., up to 5–10 Hz) that affect the spacecraft orbit determination solutions. To process the data occulted by the Martian atmosphere, we present a method that fully calibrates the path delays induced by the neutral atmosphere and ionosphere. Mars’ atmospheric models are used to predict the refractive index of these media, and the estimation of scale factors enables a complete compensation of these perturbative effects. This technique allowed us to reanalyze MRO radio tracking data that were previously discarded to avoid aliasing in the results of our gravity investigation. This precise calibration of the Martian atmosphere will also impact the navigation of future missions during aerobraking phases and science operations at low altitudes
Precise pose estimation of the NASA Mars 2020 Perseverance rover through a stereo-vision-based approach
Full text linkVisual Odometry (VO) is a fundamental technique to enhance the navigation capabilities of planetary exploration rovers. By processing the images acquired during the motion, VO methods provide estimates of the relative position and attitude between navigation steps with the detection and tracking of two-dimensional (2D) image keypoints. This method allows one to mitigate trajectory inconsistencies associated with slippage conditions resulting from dead-reckoning techniques. We present here an independent analysis of the high-resolution stereo images of the NASA Mars 2020 Perseverance rover to retrieve its accurate localization on sols 65, 66, 72, and 120. The stereo pairs are processed by using a 3D-to-3D stereo-VO approach that is based on consolidated techniques and accounts for the main nonlinear optical effects characterizing real cameras. The algorithm is first validated through the analysis of rectified stereo images acquired by the NASA Mars Exploration Rover Opportunity, and then applied to the determination of Perseverance's path. The results suggest that our reconstructed path is consistent with the telemetered trajectory, which was directly retrieved onboard the rover's system. The estimated pose is in full agreement with the archived rover's position and attitude after short navigation steps. Significant differences (~10–30 cm) between our reconstructed and telemetered trajectories are observed when Perseverance traveled distances larger than 1 m between the acquisition of stereo pairs
Processing of altimetric data for precise orbit determination
No full textThe challenging science objectives of future planetary missions will require highly accurate trajectory reconstruction of deep space probes. Novel techniques to improve the navigation capabilities are being developed with the purpose to expand the scientific return of geophysical investigations across the Solar System. Science instruments that provide geodetic data from the spacecraft orbit may support the orbit determination process in combination with deep space radio tracking measurements. Altimetric data, for example, measure the relative distance of the spacecraft with respect to the celestial body's surface, yielding key constraints on the orbit evolution. Observations that are repeated over the same location (i.e., crossover) are less prone to errors associated with surface mismodeling, leading to significant improvements in the estimation of the spacecraft position. In this work, we present a method based on the combination of ground-based radio science and altimetric crossover measurements to enhance the estimation of the spacecraft orbit and geodetic parameters. The software is developed to carry out thorough numerical simulations of mission scenarios, including the generation of synthetic observables. We show the results of our covariance analysis by simulating and processing gravimetric and altimetric measurements that will be collected by future planetary missions
Rovers localization by using 3D-to-3D and 3D-to-2D visual odometry
Full text linkSpace robotic systems have been playing a crucial role in planetary exploration missions, expanding our access to harsh and hostile environments in the Solar System. Rovers' activities are still mainly controlled through ground operations, and our goal is to develop autonomous systems for navigation and path planning. The position estimates obtained by processing Wheel Odometry (WO) data induce significant errors because of wheels' loss of traction that is caused by, for example, high-slippage terrains (e.g., sandy-loose soils, steep slopes). Our work is focused on the implementation of a localization software based on Visual Odometry (VO). This is a technique developed for the estimation of rovers' position and attitude by using stereo images captured during the vehicle's motion. To determine the attainable accuracy of our software, we carried out a set of numerical simulations through a digitally-reproduced Martian-like environment. The results show that the algorithm allows reconstructing the rover's trajectory with higher accuracies compared to the localization system requirements of the NASA Mars Exploration Rovers (i.e., 10% error over a 100-m traverse)
Deep-space navigation with intersatellite radio tracking
No full textIn the past six decades the navigation of interplanetary space- craft has been accomplished through ground-based radio tracking only [1,2]. Deep-space probes have been equipped with sophisti- cated onboard radio subsystems to communicate with Earth’s sta- tions (e.g., NASA’s deep-space network [DSN] [3], ESA’s tracking network [ESTRACK] [4]) enabling Telemetry, Tracking, and Com- mand (TT&C) functionalities. Recent development and design of this instrumentation led to significant enhancements of the quality of the radio tracking data that have been used for precise orbit determination (POD) of interplanetary spacecraft [5].
Future space missions will require extremely accurate knowledge of spacecraft trajectories, and the intrinsic limitations of deep-space radio tracking data could not be fully adequate to fulfill those challenging operation goals. Alternative instruments have then been studied including optical systems that are expected to provide orders of magnitude improvements in the precision of probes positioning over ground-based radio [6]. Laser systems have been used so far in space applications (i.e., satellite laser ranging and lunar laser ranging) with passive corner cube retroreflectors [7]. These well-established passive techniques, however, would not be well-suited to enable spacecraft navigation over deep-space distances. Active optical systems may be possible in the near future by developing laser tran- sponders that would provide few centimeters interplanetary ranging accuracies [8].
An alternative technique of interplanetary orbit determination is based on satellite-to-satellite tracking (SST) with a multispacecraft configuration. Instrument architectures have been extensively inves- tigated for both radio (e.g., [9,10]) and laser (e.g., [11,12]) intersa- tellite systems. The missions Gravity Recovery and Climate Experiment (GRACE) [13] and Gravity Recovery and Interior Labo- ratory (GRAIL) [14] successfully used radio science systems for intersatellite tracking between a pair of spacecraft to precisely deter- mine the gravity fields of the Earth [15] and the Moon [16], respec- tively. Interferometric laser ranging system has also been designed to demonstrate the feasibility and the benefits of this technology. GRACE Follow-On (GRACE-FO) mission includes a laser ranging interferometer (LRI) as a demonstrator experiment with the goal to compare LRI data with microwave ranging data that are acquired by GRACE-FO intersatellite radio tracking instrument [17]. First LRI measurements have been collected in-orbit between GRACE-FO spacecraft, showing range biases comparable to those obtained through the microwave ranging instrument but also demonstrating a substantial improvement in the accuracy of the intersatellite mea- surements, thus confirming expectations [18].
The processing of these extremely accurate range and range-rate data between satellites orbiting the same celestial body strongly constrains the accuracies of the reconstructed trajectories [19]. These data types have a significant advantage compared with ground-based data because SST observations can also be processed by autonomous navigation systems onboard spacecraft. The analysis of intersatellite data only, however, leads to the determination of absolute orbits of two or more spacecraft only if one of the probes is in an orbit with unique size, shape, and orientation [20]. The Linked Autonomous Interplanetary Satellite Orbit Navigation (LiAISON) method dem- onstrated the benefits of SST observations by studying libration orbiters [21], and spacecraft orbiting a celestial body with an asym- metric internal gravity field [22]. Therefore, the combination of SST and deep-space tracking is fundamental to provide absolute orbit determination of spacecraft with unconstrained orbit configurations.
The radio and laser intersatellite systems based on GRACE and GRAIL technologies are high-mass payloads with a significant power demand. Future space robotic missions will only be able to host these instruments as mission-unique equipment devoted to gravity investigations. Deep-space navigation with highly accurate ground-based and SST observations will require a more compact instrument scheme. This work is based on a radio system architecture that enables intersatellite measurements few orders of magnitude more precise than deep-space tracking with significant mass and power savings with respect to the GRACE and GRAIL radio science instruments architecture [23,24]. This new intersatellite tracking system will be well-suited to dual- or multispacecraft configurations with SmallSats in the solar system
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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