This research outlines a low-cost, low-power, arc-minute accurate star tracker that is designed for use on a CubeSat. The device is being developed at the University of Texas at Austin for use on two different 3-unit CubeSat missions. The hardware consists of commercial off-the-shelf parts designed for use in industrial machine vision systems and employs a 1024x768 grey-scale charge coupled device (CCD) sensor. The software includes the three standard steps in star tracking: centroiding, star identification, and attitude determination. Centroiding algorithms were developed in-house. The star identification code was adapted from the voting method developed by Kolomenkin, et al. Attitude determination was performed using Markley's singular value decomposition method. The star tracker was then tested with internal simulated star-fields. The resulting accuracy was less than an arcminute. It was concluded that this system is a viable option for CubeSats looking to improve their attitude determination. On-orbit demonstration of the system is planned when the star tracker flies on the planned CubeSat missions in 2013 or later. Further testing with external simulated star fields and night sky tests are also planned. / text

Thèse (M.Sc.)--Université Laval, 2009. / Bibliogr.: f. [80]-82. Publié aussi en version électronique dans la Collection Mémoires et thèses électroniques.

Th.--Sci. phys.--Toulouse 3, 1977. N°: 760. / 761.

Dans cette thèse, nous proposons un mécanisme appelé Initial Spreading qui permet une optimisation remarquable des performances de TCP pour les connexions de petites tailles, représentant plus de 90% des connexions échangées dans l’Internet. Cette solution est d’autant plus intéressante que pour certaines technologies telles qu’un lien satellite, le temps d’aller-retour particulièrement long est très pénalisant, et des solutions spécifiques ont dû être implantées qui empêchent l’intégration du satellite dans un système de communication plus large. Nous montrons que l’Initial Spreading est non seulement plus performant, mais surtout plus général car pertinent dans toutes les situations. De plus, peu intrusif, il ne compromet aucune des évolutions de TCP passées ou à venir. / In this Ph.D. Thesis, we propose a mechanism called Initial Spreading that significantly improves the TCP short-lived connexions performance, and so more than 90% of the Internet connections. Indeed, if regular TCP without our mechanism can be considered as efficient for terrestrial networks, its behavior is strongly damaged by the long delay of a satellite communication. Satellite community developed then some satellite specific solutions that provide good performance, but prevent the joint use of satellite and other technologies. We show therefore that Initial Spreading is not only more efficient than regular solutions but enables also the use of an unique protocol whatever the context. Moreover, being non-intrusive, it is suitable for past and future TCP evolutions.

Dans le domaine de la surveillance maritime, les systèmes coopératifs d’identification et de positionnements tels que l’AIS (Automatic Identification System) sont souvent couplés à des systèmes d’observation de navires non coopératifs comme les Radars à Synthèse d’Ouverture (RSO). Dans ce contexte, la fusion des données AIS et Radar peut améliorer la détection de certains navires et d’identifier éventuels scénarii de surveillance. Le premier chapitre introduit les systèmes et détaille la structure des données AIS et Radar ainsi que le traitement du signal utilisé. Le deuxième chapitre présente l’apport potentiel de l’utilisation conjointe des données brutes Radar et AIS pour la détection de navires à l’aide d’un test basé sur le rapport des vraisemblances maximales (test GLRT). Bien que les performances soient encourageantes, la mise en pratique du détecteur en temps-réel semble compliquée. Le troisième chapitre présente une alternative sous-optimale qui explore les données brutes Radar et une carte des positions de bateaux provenant de l’AIS. Contrairement au chapitre deux, en plus de la détection simultanée par l’AIS et radar, les cas où seul l’un des systèmes détecte un objet peuvent maintenant être distinguées. Le problème est formalisé par deux tests d’hypothèses binaires successifs. Le test proposé est moins sensible à la proximité et à la densité des navires qu’un détecteur radar classique. Le quatrième chapitre présente le simulateur développé pour tester les algorithmes sur différents scénarii de surveillance, à savoir un scénario de piraterie sur un navire civil, un transbordement illégal et une navigation dans un environnement dense. / In the maritime surveillance domain, cooperative identification and positioning systems such as AIS (Automatic Identification System) are often coupled with non-cooperative systems for ship observation such as Synthetic Aperture Radar (SAR). In this context, the fusion of AIS and Radar data can improve the detection of certain vessels and possible identify some maritime surveillance scenarios. The first chapter introduces both AIS and Radar systems, details the data structure as well as the related signal processing. The second chapter presents the potential contribution of the joint use of raw Radar and AIS data for the detection of vessels using a generalized likelihood ratio test (GLRT). Although the performance is encouraging, in practice the implementation in real-time of the detector seems complicated. As alternative, the third chapter presents a suboptimal detection method that explores Radar raw data and a positioning map of vessels obtained from the AIS system. Differently from chapter two, in addition to the simultaneous detection by both AIS and radar, the cases where only one of the systems detects an object can now be distinguished. The problem is formalized by two successive binary hypotheses test. The results suggests that the proposed detector is less sensitive to the proximity and density of ships than a conventional radar detector. The fourth chapter presents the simulator developed to test the algorithms on different surveillance scenarios, namely a civilian ship piracy scenario, an illegal cargo transhipment and a scenario of navigation in a dense environment.

Resumo: Um abordagem analítica para a propagação de atitude de satélites estabilizados por rotação em órbita circular é apresentada, incluindo o torque magnético residual (torque devido ao momento magnético ao longo do eixo de rotação do satélite). O modelo de quadripolo é utilizado para descrever o campo geomagnético. O método da média é aplicado para determinar o torque ao longo de um período orbital. Observa-se que Torque Magnético Residual Médio não possui componente ao longo do eixo de rotação, de modo que não afeta o módulo da velocidade de rotação do satélite. Para um período orbital uma solução analítica é apresentada. Esta solução mostra que o torque residual contribui para a deriva e precessão do eixo de rotação. Simulações são realizadas com os dados dos Satélites de Coleta de Dados Brasileiros (SCD1 e SCD2). / Abstract: An analytical approach for the attitude motion of spin stabilized artificial satellite in circular orbit is presented, including residual torque. The quadripolo model is used to described the geomagnetic field. The averaged residual torque is computed for one orbital period. It is observed that the residual magnetic torque does not have a component along the spin axis. The inclusion of this torque on the rotational motion differential equations of spin stabilized spacecraft's yields the conditions to derive an analytical solution. The solution shows that residual torque does not affect the spin velocity magnitude, contributing only for the precession and the drift of the spin axis of the spacecraft. Applications are shown for the spin stabilized Brazilian satellites SCD1 and SCD2. / Orientador: Maria Cecília F. P. Zanardi / Coorientador: Hélio K. Kuga / Banca: Rodolpho Vilhena de Moraes / Banca: Valcir Orlando / Mestre

The recent generation of Agile Earth Observation Satellite (AEOS) has emerged to be highly effective due to its increased attitude maneuvering capabilities. However, due to these increased degrees of freedom in maneuverability, the scheduling problem has become increasingly difficult than its non-agile predecessors. The AEOS scheduling problem consists of finding an optimal assignment of user-requested imaging tasks to the respective AEOSs in their orbits by satisfying the operational resource constraints in a specified time frame. Some of these tasks might require imaging the same area of interest (AOI) multiple times, while in some tasks, the AOIs are too large for the AEOS to image in a single attempt. Some tasks might even arise while the AEOSs are preoccupied with existing tasks. This thesis focuses on formulating the AEOS scheduling models where onboard energy and memory constraints while operating and the task specifications are diverse. A mixed-integer non-linear scheduling problem with a reward factor has been considered in order to handle multiple scan requirements for a task. Although initially, it is assumed that the AOIs are small, this work is extended to a three-stage optimization framework to handle the segmentation of large AOIs into smaller regions that can be imaged in a single scan. The uncertainty regarding scan failure is handled through a Markov Decision Process (MDP). These two proposed methods have significant benefits when tasks are available to schedule prior to the mission. However, they lack the flexibility to accommodate newly arrived tasks during the mission. When multiple new tasks arrive during the mission, predictive scheduling based on learning historical data of task arrivals is proposed, which can schedule tasks in an online manner faster than complete rescheduling and minimize disruption from the original schedule. Evolutionary optimization-based solution methodologies are proposed to solve these models and are validated with simulations. / Thesis / Doctor of Philosophy (PhD)

In global navigation satellite systems (GNSS) a fundamental operational component is the calculation of the orbits of the system spacecraft. This requires understanding and modelling the forces that act on the spacecraft. Solar radiation pressure (SRP) is the force caused by the impact of solar photons on the spacecraft surface. For GNSS spacecraft this is a significant force. If SRP is not included in the force model, then the calculated position of the spacecraft can be in error by between one and two hundred metres after one 12-hour orbit. SRP can be modelled using either analytical or empirical methods, or by some combination of the two. Historically, analytical SRP modelling has been somewhat neglected and high precision orbit estimation has relied upon empirical methods to account for SRP. Even so, most of these empirical methods start the estimation process with an a priori analytical model. The success of this empirical approach relies upon having many observations of the range between the system spacecraft and ground-based tracking stations, and works well within the context of the International Global Positioning System Service (IGS) network, which provides the necessary data volume. However, empirical methods do not work as well in operational GNSS, as these typically have a relatively small number of tracking stations. Moreover, empirical methods cannot be applied at the GNSS design stage, where knowledge of the system dynamics plays a key role. Existing methods for calculating analytical SRP models can only be used with relatively simple spacecraft structures, and lack flexibility as tools for analysis. In this study a new method is developed for calculating analytical SRP models that can cope with a high level of complexity in the spacecraft structure. The method is based upon simulating the solar photon flux with a pixel array. Using the method, models are calculated and tested for the Russian GLONASS IIv spacecraft. This particular spacecraft was used as the testbed because, at the time the study was being conducted, an international scientific campaign - called IGEX-98, the International GLONASS Experiment - was being carried out to analyse the Russian system. Developing force models for the spacecraft was one of the campaign goals, and the IGEX-98 steering committee accepted a proposal to use SRP models for GLONASS from this study. A detailed description is given of all the mathematics and physics that was used to develop the modelling technique. The method by which the models can be calculated and applied in practical orbit determination is also provided. In order to test the performance of the SRP models computed for the GLONASS spacecraft using the new method, comparisons were made between two kinds of trajectory. The first kind was calculated by numerical integration of the spacecraft's second order differential equation of motion, where this force model included the custom SRP models developed in the thesis. The second kind of trajectory, which is used as a 'truth' model in the study, was a precise orbit computed by the University of Berne using IGS range data and an empirical SRP model. Such precise orbits are the best estimates available of the true trajectories, as they are derived from the simultaneous estimation of multiple receiver tracking station network positions and spacecraft force model parameters. The repeatability of the Berne orbit is circa 0.75m. The RMS differences between the two trajectories over one twelve-hour orbit (an arc length of circa 160,000km) were 0.7m in height, 1.3m across track and 3.5m along track. This shows that the trajectory derived from the force model alone is very close to the precise orbit. The time-varying pattern of the differences between the two trajectories strongly indicates that the residual mismodelling of the forces acting on the spacecraft is due to thermal re-radiation effects. Further tests of the method were also conducted using satellite laser ranging (SLR) data to calculate arc lengths of 400 days, again using SRP models from the study. This enabled the calculation of model scale factors and additional empirical terms. The average SRP model scale factor was circa 1.01, which implies that the average error in the a priori SRP models calculated for the GLONASS IIv spacecraft is at the 1% level. This is consistent with an error budget based on an assessment of the accuracy of the source data supplied by the Russian authorities. The magnitude and parameterisation of the SLR empirical terms again strongly suggest that most of the remaining mis-modelling is caused by thermal effects. An analysis is given of the effect on the a priori SRP model of unmodelled, SRP-related forces acting along the spacecraft Y-axis. This is the so-called Y-bias. It is shown that whilst Y-bias effects are important in orbit determination, they are less critical in the process of calculating the a priori SRP model. A discussion is provided on how the new method can be adapted to improve the modelling and understanding of thermal re-radiation and Y-bias effects, and also on what benefits might accrue from such studies. The new method is an improvement over existing techniques as it enables the calculation of high precision SRP models that can be applied in the design, operation and scientific analysis of GNSS. A UK patent application has been made in respect of the new method.

International Telemetering Conference Proceedings / November 04-07, 1991 / Riviera Hotel and Convention Center, Las Vegas, Nevada / The Telemetry Test Station has been developed at the Digital Systems Division, ISRO Satellite Centre, to test House Keeping Telemetry Packages which will be flown onboard satellites. The main feature of the Test Staion is its configurability, since it is intended to be used with different types of (low bit rate) telemetry packages. Other features include automation of test procedure, and reduction in testing time/increase in repeatability due to minimisation of manual interaction. Since the test station comprises GPIB controlled equipment, hardware and software, thf configurability should be built in at all levels and stages, from design to implementation. Thus since the specification is subject to change, the configuration for structure of the system is traosferred from the designer to the user. This results in a large part of the system being devoted to user interface design, since the computer which is an integral part of the setup must be ‘invisible’ to the user. Due to specialised requirements the user and the system are treated at peer level i.e, at any time during the entire test session, the user can override system process and verify/correct it and equally, the system must also verify/correct user input especially with regard to critical test procedures. The test tession itself (after configuration) is divided into three equally important sections: preprocessing, test and analysis. All the data required during the test run should be made ready before the actual test. Since it has been found by experience that this is not always convenient, the preprocessing software is integrated in to the test session so that the user can perform this at any time. Similarly, the results of the test can also be analysed immediately or even during the test to improve subsequent procedures. This paper describes the prototype telemetry test station which has been built and tested over the past two years. This performance has been satisfactory. The system integration features and design problems and solution are high lighted.

International Telemetering Conference Proceedings / October 17-20, 1994 / Town & Country Hotel and Conference Center, San Diego, California / Small satellites have been perceived as having limited access to NASA's Space Network consisting of the TDR satellites and associated ground terminals. This paper presents the potential for access of the space network using basic small satellite design constraints and a simple helical antenna for the communications links. From the analysis derived through simulation of the orbit of both satellites, small satellites can be shown to have up to 30 minutes per orbit of single-TDRS access. Data rates on the order of 100 kbps are possible in this configuration with total daily data volumes in excess of 100 Mbits being achievable. Design parameters are given for a variety of orbital inclination angles and spacecraft transmission powers to illustrate the expected available contact time for such small satellites to the Space Network. This is compared with typical access time through a fixed ground station.