1,720,972 research outputs found
Analysis of radio occultation data to determine atmospheric profiles and associated uncertainties
No full textThe analysis of atmospheric radio occultations enables an in-depth investigation of planetary ionosphere and neutral atmospheres, by measuring the radio frequency shift that affects a signal propagating through the medium. A precise characterization of the atmospheric layers requires a thorough processing of the radio tracking data to estimate the thermodynamic properties of the atmosphere and their related uncertainties.
A standard procedure to process radio occultation data requires a preliminary knowledge of the spacecraft trajectory. In this work, we present a technique to retrieve refractivity, density, pressure, and temperature profiles with their associated uncertainties through the analysis of raw radio tracking data occulted by the atmosphere. By integrating the algorithm for radio occultation processing with a Precise Orbit Determination (POD) software, an enhanced reconstruction of the spacecraft trajectory is obtained to recover the frequency shift due to the medium refraction. The resulting radio signal is then processed to yield information regarding atmospheric properties. A Monte Carlo simulation algorithm is also included to provide the formal uncertainties of the estimated parameters.
We applied this technique to radio occultation profiles of the NASA mission Mars Reconnaissance Orbiter (MRO). To validate the method, our estimated atmospheric profiles are compared to the numerical predictions of the Mars Global Reference Atmospheric Model (GRAM) and the Mars Climate Database (MCD)
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
A Technique for the Analysis of Radio Occultation Data to Retrieve Atmospheric Properties and Associated Uncertainties
Full text linkAmong the techniques for atmospheric sounding, radio occultation enables an in depth investigation of vertical profiles from the ionosphere to the troposphere by measuring the radio frequency signal associated to the propagation medium. A precise characterization of the atmospheric layers requires a thorough processing of the raw radio tracking data to estimate the thermodynamic properties of the atmosphere and their related uncertainties. In this work, we present a method to retrieve refractivity, density, pressure, and temperature profiles with the associated uncertainties by analyzing a set of raw radio tracking data occulted by the atmosphere. This technique is also well suited to process two-way Doppler measurements that are not acquired during dedicated occultation campaigns. The NASA mission Mars Reconnaissance Orbiter (MRO) provided a significant amount of radio occultation data that were not planned for atmospheric sounding, but were caused by the spacecraft orbit geometry. Our analysis of one of these occultation profiles with the proposed method allows indicating that MRO occultation datasets provide crucial information regarding Mars’ troposphere that can be used as input of general circulation models
Constraining the Internal Structures of Venus and Mars from the Gravity Response to Atmospheric Loading
Full text linkThe gravity fields of celestial bodies that possess an atmosphere are periodically perturbed by the redistribution of fluid mass associated with the atmospheric dynamics. A component of this perturbation is due to the gravitational response of the body to the deformation of its surface induced by the atmospheric pressure loading. The magnitude of this effect depends on the relation between the loading and the response in terms of geopotential variations measured by the load Love numbers. In this work, we simulate and analyze the gravity field generated by the atmospheres of Venus and Mars by accounting for different models of their internal structure. By precisely characterizing the phenomena that drive the mass transportation in the atmosphere through general circulation models, we determine the effect of the interior structure on the response to the atmospheric loading. An accurate estimation of the time-varying gravity field, which measures the atmospheric contribution, may provide significant constraints on the interior structure through the measurement of the load Love numbers. A combined determination of tidal and load Love numbers would enhance our knowledge of the interior of planetary bodies, providing further geophysical constraints in the inversion of internal structure models.The gravity fields of celestial bodies that possess an atmosphere are periodically perturbed by the redistribution of fluid mass associated with the atmospheric dynamics. A component of this perturbation is due to the gravitational response of the body to the deformation of its surface induced by the atmospheric pressure loading. The magnitude of this effect depends on the relation between the loading and the response in terms of geopotential variations measured by the load Love numbers. In this work, we simulate and analyze the gravity field generated by the atmospheres of Venus and Mars by accounting for different models of their internal structure. By precisely characterizing the phenomena that drive the mass transportation in the atmosphere through general circulation models, we determine the effect of the interior structure on the response to the atmospheric loading. An accurate estimation of the time-varying gravity field, which measures the atmospheric contribution, may provide significant constraints on the interior structure through the measurement of the load Love numbers. A combined determination of tidal and load Love numbers would enhance our knowledge of the interior of planetary bodies, providing further geophysical constraints in the inversion of internal structure models
Safe navigation and visual odometry-based localization for planetary exploration rovers
No full textThe future robotic exploration of planetary surfaces will require autonomous and safe operations to accomplish outstanding scientific objectives. The main goal of space robotic systems consists in expanding our access capability to harsh environments in the solar system (e.g., Martian polar caps, icy moons). However, the operations of systems onboard landers and rovers are still mainly commanded and controlled by ground operators. To enhance the efficiency of future rovers, we are developing a robust guidance, navigation and control system that enables safe mobility on different terrain and slopes conditions, including the presence of obstacles.
High slippery terrains, such as sandy-loose soils, could prevent the rover locomotion, affecting its safety. Furthermore, the presence of these demanding terrains may impact on the rover navigation, leading to inaccuracies in the Wheel Odometry (WO) measurements because of wheels’ loss of traction. Therefore, we implemented a navigation algorithm based on Visual Odometry (VO) that is the technique based on the processing of stereo-camera images captured at successive times during the vehicle’s motion. This method is fundamental to help WO during operations that require fast responses and a high-accurate positioning. We also adopted a LIDAR sensor to improve the position estimate accuracy by processing measurements associated with well-known terrain features.
We present here numerical simulations of rover navigation across different terrain conditions by using accurate dynamical models, including the deformabilities of both wheel and terrain. VO and LIDAR data are simulated and processed to determine the positioning accuracies that enable safe navigation. The results are in full agreement with the existing (i.e. MER rovers) and future (i.e. ExoMars) rover performances. Our algorithm allows reconstructing the rover trajectory with higher accuracies compared to the localization system requirement of the NASA MER rovers (i.e., 10% error over a 100 meters traverse)
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
Sequential Processing of Inter-Satellite Doppler Tracking for a Dual-Spacecraft Configuration
Full text linkThe navigation of future interplanetary spacecraft will require an increasing degree of autonomy to enhance space system performance. A real-time trajectory determination is of paramount importance to reduce the risks of operations devoted to the exploration of celestial bodies in the solar system and to reduce the dependence and the loading on the ground systems. We present a technique for a sequential estimation of spacecraft orbits through the processing of line-of-sight relative velocity measurements that are acquired by the novel inter-satellite tracking system. This estimation scheme is based on the extended Kalman filter and is tested and validated in a realistic Mars mission scenario. Our numerical simulations suggest that the proposed navigation system can provide accuracies of a few meters in position and a few millimeters per second in velocity
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