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Descripción
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| Orbiting satellites and other spatial vehicles have complex trajectories that can usually be precisely approximated with analytical or numerical trajectory estimation algorithms. However, some scenarios, such as LEOP (Launch and Early Orbit Phase) or critical manoeuvres, present greater angular uncertainty, around +/- 1º in elevation and cross-elevation axes, and beyond. During these, large dish antennas used for TT&C (Telemetry, Tracking & Command) may have too narrow a beamwidth to perform a reliable and fast acquisition. Traditionally, this problem has been solved using Conical Scan techniques, where a circular motion is superimposed to the main movement of the antenna so that the true DoA (Direction of Arrival) of the desired signal can be interpolated from the variation of its power levels. More recently, in X-band, a new system has been proposed by Astrium, termed XAA (X-band Acquisition Aid). In this case, a smaller dish is attached to the rim of the main reflector and, taking advantage of its broader beam, is tasked with the initial acquisition of the signal. However, Conical Scan on the one hand takes some time to converge, around 1 minute typically, and thus may be unsuitable in critical scenarios, where signal losses must be kept to a minimum. On the other hand, while XAA converges fast enough, below 10 seconds, it suffers from two drawbacks: the initial acquisition is thermal-noised limited due to the smaller G/T (antenna gain over noise temperature) of the auxiliary antenna, and its placement on the rim of the main reflector has a non-negligible mechanical impact on the whole system. Moreover, there is a trade-off between both drawbacks, since increasing the G/T of the auxiliary antenna can only be done at the expense of larger and heavier dishes, thus exacerbating mechanical concerns. In light of these, and taking into account the current trend to resort to ever smaller TT&C antennas in X-band, down to 4 - 5 metres in diameter so as to decrease costs, it would seem that the only solution is to increase the transmitted power of the satellite, at least during critical phases. While this option is currently contemplated, it carries with it normative and legal restrictions, mainly due to interference caused to other systems. Besides, this extra power is mostly unused during normal stages, and thus it places a wasted burden on requirements on the spatial segment. It is in this context that a novel acquisition aid system, termed SARAS and based on distributed array elements placed on the rim of the main antenna?s reflector, has been implemented and successfully tested with real satellite signals. The use of antenna arrays clearly reduces the mechanical impact on the main antenna, since the weight is distributed over the whole receptor. Moreover, the large separation between array elements and the use of digital super-resolution algorithms allows the system to work in very noisy scenarios, so that the radiating elements themselves can have very low G/T and be small-sized. Finally, since the array search for the true signal DoA with electronic scan techniques, rather than mechanical ones, convergence is much faster than Conical Scan. Thus, the proposed solution takes advantages from both Conical Scan and XAA, combining them in a versatile way that opens up the possible applications of SARAS. The first prototype has been implemented in S-band, mainly keeping in mind the acquisition of launchers, but a migration to X-band has also been considered for future designs, so that the system can cope with ever smaller TT&C antennas. This Thesis starts with a brief analysis of array history in various applications and, notably, in satellite communications. | |
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Tipo de Tesis
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Doctoral |
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Aprobado |
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Fecha
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