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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

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. Source

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

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. Source

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

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. Source

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

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. Source

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

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. Source

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