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Duke R.,Surrey Space Center | Bridges C.P.,Surrey Space Center | Stewart B.,Surrey Space Center | Taylor B.,Surrey Space Center | And 3 more authors.
Proceedings of the International Astronautical Congress, IAC | Year: 2016

Flight and ground segment software in university missions is often developed after hardware has matured sufficiently and also as bespoke codebases with very little common software to address key subsystems in power, communications, attitude, and payload control. This bespoke software process is often hardware specific, highly sequential, and costly in staff/monitory resources and, ultimately, development time. Within Surrey Space Centre (SSC), there are a number of satellite missions under development with similar delivery timelines that have overlapping requirements for the common tasks and additional pay-load handling. To address the needs of multiple missions with limited staff resources in a given delivery schedule, computing commonality for both flight and ground segment software is exploited by implementing a common set of flight tasks (or modules) which can be automatically generated into ground segment databases to deliver for advanced debugging support during system end-to-end test (SEET) and operations. This paper focuses on the development, implementation, and testing of SSC's common software framework on the Stellenbosch ADCS stack and emulators for numerous missions including Alsat-1N, Re-moveDebris, SME-SAT, and InflateSail. The framework uses a combination of open-source embedded and enterprise tools such as the FreeRTOS operating system coupled with rapid development templates used to auto-generate C and python codes offline from "message databases". In the flight software, a "core" packet router thread forwards messages to and from threads for inter process communication (IPC). On the ground, this is complemented with an auto-generated PostgreSQL database and web interface to test, log, and display results in our satellite operations centre. Profiling is performed using FreeRTOS primitives to manage module behaviour, context, time and memory - especially important during integration. This new framework has allowed for flight and ground software to be developed in parallel across SSC's current and future missions faster, with fewer propagated errors, and increased consistency between the flight software, ground station and project documentation. Copyright © 2016 by Surrey Space Centre - University of Surrey.

Maqsood M.,Surrey Space Center | Maqsood M.,Institute of Space Technology | Gao S.,Surrey Space Center | Gao S.,University of Kent | And 5 more authors.
IEEE Transactions on Antennas and Propagation | Year: 2014

This paper presents the design and development of a dual-band switched-beam microstrip array for global navigation satellite system (GNSS) applications such as ocean reflectometry and remote sensing. In contrast to the traditional Butler matrix, a simple, low cost, broadband and low insertion loss beam switching feed network is proposed, designed and integrated with a dual band antenna array to achieve continuous beam coverage of ±25° around the boresight at the L1 (1.575 GHz) and L2 (1.227 GHz) bands. To reduce the cost, microstrip lines and PIN diode based switches are employed. The proposed switched-beam network is then integrated with dual-band step-shorted annular ring (S-SAR) antenna elements in order to produce a fully integrated compact-sized switched-beam array. Antenna simulation results show that the switched-beam array achieves a maximum gain of 12 dBic at the L1 band and 10 dBic at the L2 band. In order to validate the concept, a scaled down prototype of the simulated design is fabricated and measured. The prototype operates at twice of the original design frequency, i.e., 3.15 GHz and 2.454 GHz and the measured results confirm that the integrated array achieves beam switching and good performance at both bands. © 1963-2012 IEEE.

Verbin D.,University of Surrey | Verbin D.,Israel Aerospace Industries | Lappas V.J.,University of Surrey | Lappas V.J.,Surrey Space Center | Ben-Asher J.Z.,Technion - Israel Institute of Technology
Journal of Guidance, Control, and Dynamics | Year: 2011

This paper proposes a new integrative control logic for rapid maneuvering of a rigid satellite using reaction wheels. The proposed control algorithm is of a state feedback nature and is designed to accommodate a variety of conditions, such as the general three-dimensional direction of rotation, initial and/or final angular rates, general alignment of three or four reaction wheels, torque and angular rate limits, and system time response limits. Simulations indicate that the new algorithm allows smooth control, free of chattering. The new controller also uses a tuning capability to compensate for time delays and parasitic dynamics. The tuning capability can be varied in order to enhance performance or robustness. Robustness of the algorithm is proven through a stability analysis and supported by detailed and practical simulations. Copyright © 2010 by Dov Verbin.

Herrera-Sucarrat E.,University of Surrey | Roberts R.M.,Surrey Space Center
Journal of Guidance, Control, and Dynamics | Year: 2013

In this paper a simple and very general approximation of the gravitational potential for a nonspherical body is presented. The gravitational potential is expanded using spherical harmonics and spherical Bessel functions, and it satisfies Laplace's equation outside the circumscribing sphere and Poisson's equation inside the circumscribing sphere. Therefore, trajectories can be integrated near the surface of the asteroid, as well as far away from it. This paper focuses on the construction of a simple expansion of the gravitational potential that preserves the critical nonlinear dynamical behavior of other gravitational models for a nonspherical asteroid that are more complex and computationally more demanding. Copyright © 2012 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Verbin D.,University of Surrey | Verbin D.,Israel Aerospace Industries | Lappas V.J.,University of Surrey | Lappas V.J.,Surrey Space Center
Journal of Guidance, Control, and Dynamics | Year: 2013

A new attitude control method for agile rigid spacecrafts that is based on combining single gimbal control moment gyros together with reaction wheels is presented. The method is expected to suit remote sensing spacecrafts that are required to perform multiple rapid retargeting of their line of sight. The main advantage of single gimbal control moment gyros is rapid rotational maneuvering, but their application for high quality pointing requires a very accurate gimbal mechanism.Onthe other hand, the reaction wheels may be more easily applied for accurate pointing, but their torque-to-power performance is inferior for maneuvering compared to single gimbal control moment gyros. The paper shows that careful coordination between reaction wheels and single gimbal control moment gyros, together in a hybrid configuration, draws more performance from single gimbal control moment gyros in terms of agility and achieves quality pointing between maneuvers by using only reaction wheels. The high-level control is based on a braking curve that relates angular rate to any given three-dimensional altitude, which is calculated according to the satellite eigenaxis deceleration capability. The required angular rate as produced by the braking curve is converted to angular momentum and is then translated into gimbal angles. The lower level control is based on a steering law in the gimbal angles' domain, where the gimbals are steered to track angular commands. Copyright © 2012 by Dov Verbin. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

Horri N.M.,University of Surrey | Horri N.M.,Surrey Space Center | Palmer P.,University of Surrey | Palmer P.,Surrey Space Center
Journal of Guidance, Control, and Dynamics | Year: 2012

The challenging problem of controlling the attitude of satellites subject to actuator failures has been the subject of increased attention in recent years. The problem of controlling the attitude of a satellite on all three axes with two reaction wheels is addressed in this paper. This system is controllable in a zero-momentum mode. Three-axis attitude stability is proven by imposing a singular quaternion feedback law to the angular velocity trajectories. Two approaches are proposed and compared to achieve three-axis control: The first one does not require angular velocity measurements and is based on the assumption of a perfect zero momentum, while the second approach consists of tracking the desired angular velocity trajectories. The full-state feedback is a nonlinear singular controller. In-orbit tests of the first approach provide an unprecedented practical proof of three-axis stability with two control torques. The angular velocity tracking approach is shown to be less efficient using the nonlinear singular controller. However, when inverse optimization theory is applied to enhance the nonlinear singular controller, the angular velocity tracking approach is shown to be the most efficient. The resulting switched inverse optimal controller allows for a significant enhancement of settling time, for a prescribed level of the integrated torque. Copyright © 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Sauter L.,University of Surrey | Sauter L.,Surrey Space Center | Palmer P.,University of Surrey | Palmer P.,Surrey Space Center
Journal of Guidance, Control, and Dynamics | Year: 2012

Large formations of satellites currently require extensive ground-based planning to enable formation-wide collision-free reconfiguration. Allowing satellite formations the flexibility to execute collision-free reconfiguration operations onboard each spacecraft can significantly reduce the ground operations burden and increase the responsiveness of the formation to reconfiguration events. An analytic model predictive controller for fuelminimized, collision-free trajectory following is developed. The controller exploits the natural dynamics for relativemotion path-following using minimal fuel and requires a minimal computational burden. Constraints are handled implicitly and performance provides 423 times the fuel savings compared with traditional proportional-integralderivative- type control at the same computational speed. Through hardware testing and comparison with other approaches, results also show that constraints can also be handled explicitly while still performing significantly faster than similar forms of model predictive control for formation reconfiguration. Copyright © 2012 by the American Institute of Aeronautics and Astronautics, Inc.

Maqsood M.,University of Surrey | Gao S.,Surrey Space Center | Gao S.,University of Kent | Brown T.W.C.,University of Surrey | And 3 more authors.
IEEE Transactions on Antennas and Propagation | Year: 2013

This paper presents the design of a novel multipath mitigating ground plane for global navigation satellite system (GNSS) antennas. First, the concept of a compact low multipath cross-plate reflector ground plane (CPRGP) is presented. In comparison with the choke ring and electromagnetic band gap (EBG) ground planes, the proposed CPRGP has compact size, low mass, wide operational bandwidth, and simple configuration. The proposed CPRGP is then integrated with a circularly polarized dual-band GNSS antenna in order to assess the multipath mitigating performance over two frequency bands. Measurement results of the proposed CPRGP with GNSS antenna achieves a front-to-back ratio (FBR) over 25 dB at L1 (1.575 GHz) and L2 (1.227 GHz) bands and maximum backward cross-polarization levels below-23 dB at both bands. Antenna phase center variation remains less than 2 mm across both L1 and L2 bands. Furthermore, the performance comparison of the proposed CPRGP with the commercially available pinwheel antenna and the shallow corrugated ground plane is presented, showing the advantages of CPRGP for high precision GNSS applications. © 1963-2012 IEEE.

Klokocnik J.,Academy of Sciences of the Czech Republic | Gooding R.H.,Surrey Space Center | Wagner C.A.,National Oceanic and Atmospheric Administration | Kostelecky J.,Research Institute of Geodesy | And 2 more authors.
Surveys in Geophysics | Year: 2013

Dynamic resonance, arising from commensurate (orbital or rotational) periods of satellites or planets with each other, has been a strong force in the development of the solar system. The repetition of conditions over the commensurate periods can result in amplified long-term changes in the positions of the bodies involved. Such resonant phenomena driven by the commensurability between the mean motion of certain artificial Earth satellites and the Earth's rotation originally contributed to the evaluation and assessment of the Stokes parameters (harmonic geopotential coefficients) that specify the Earth's gravitational field. The technique constrains linear combinations of the harmonic coefficients that are of relevant resonant order (lumped coefficients). The attraction of the method eventually dwindled, but the very accurate orbits of CHAMP and GRACE have recently led to more general insights for commensurate orbits applied to satellite geodesy involving the best resolution for all coefficients, not just resonant ones. From the GRACE mission, we learnt how to explain and predict temporary decreases in the resolution and accuracy of derived geopotential parameters, due to passages through low-order commensurabilities, which lead to low-density ground-track patterns. For GOCE we suggest how to change a repeat orbit height slightly, to achieve the best feasible recovery of the field parameters derived from on-board gradiometric measurements by direct inversion from the measurements to the harmonic geopotential coefficients, not by the way of lumped coefficients. For orbiters of Mars, we have suggestions which orbits should be avoided. The slow rotation of Venus results in dense ground-tracks and excellent gravitational recovery for almost all orbiters. © 2012 Springer Science+Business Media B.V.

Wokes D.S.,Surrey Space Center | Palmer P.L.,Surrey Space Center
Journal of Guidance, Control, and Dynamics | Year: 2011

A new method of filling in the heuristic gap between image acquisition and edge-line correspondence/specific target recognition is introduced. Under the assumption that the target's dimensions are known to an acceptable level of accuracy, it is possible to create a spheroid to model that target. Comparing the accuracies with a target's reconstruction when modeled as a spheroid or sphere, there is a radial boundary beyond which it is better to model the target globally. The smaller the features used for detecting and tracking, relative to the target's global dimensions, the greater the distance for which heuristic modeling methods is applicable when the former method is not. The results show that at larger distances, a global description of a target gives better pose estimation technique than attempting to identify smaller features, whose accuracy is affected to a greater extent by pixelation.

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