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The increased in orbit lifetime of today’s telecommunication satellites combined with the rapid development of the offered services require always more flexible payloads for the satellite providers. A special interest was shown in the past years in antennas that can be reconfigured in orbit, in order to change coverages, use the same spacecraft at several orbital locations, and compensate for varying weather conditions. Shaped reflectors lack so far the capability of being reconfigured in orbit, while this can be obtained by an array-fed parabolic reflector.
The purpose of the present work is to study and quantify the properties, advantages and limitations provided by a shaped dual reflector equipped with a reconfigurable surface when applied to a realistic mission scenario in Ku band.
Mechanical repointing of the antenna will be assumed. A lateral face and Earth deck implementation of the antenna will be considered. The reconfigurable surface will be modelled by a mesh of interwoven flexible wires with circular cross section, supported by a number of control points and with a free rim.
The performances of the reconfigurable shaped reflector will be compared to the ones of a traditional fixed shaped reflector designed to illuminate the envelope of the desired coverages and of a shaped but fixed dual reflector optimized for each individual coverage.
The necessary number of actuators will be investigated and a sensitivity analysis to the actuator settings will be performed. The impact of imperfections of the reconfigurable reflecting surface such as anisotropy and reflectivity will be also taken into account.
Several concepts will be addressed during the study. In particular, the importance and advantages of reconfiguring the antenna in orbit by switching between the coverages and by varying the orbital location will be investigated. The performances of reconfiguring the subreflector while keeping the main reflector shaped and fixed (lateral face antenna) will be analyzed and compared to the ones given by a reconfigurable main reflector and a fixed ellipsoidal subreflector (Earth deck antenna).
The differences in directivities provided by a main reflector shaped for the coverages of interest or shaped for a general shaping will be highlighted.
The necessary number of actuators and their typical deviation from the parent surface will then be studied.
Shaped reflectors form one of the cornerstones of all satellite telecommunications today, and have in all but a few cases replaced array-fed parabolic reflector antennas for contoured beam generation.
While extremely attractive in terms of simplicity and associated costs relative to array-fed paraboloid, shaped reflectors lack the extra degree of freedom that an array has in the amplitude and phase excitation of each element. Consequently, the antenna community has considered the possibility of manufacturing shaped reflector antennas that can change its shape in space in order to meet the above requirements.
Theoretical studies have been conducted in the past, always considering simple coverages and antenna configurations without focusing on a real mission scenarios. It is the purpose of this ARTES 1 project to establish as realistic as possible near-future mission scenarios in Ku band where the added flexibility of a reconformable antenna will be of value, and study these scenarios in detail with respect to the requirements to the surface reconfigurability.
The results coming out from the present study will have significant implications on further programs and ARTES initiatives, providing guidelines and requirements for future developments.
First, for a dual reflector antenna it will be possible to quantify and understand the benefits given by a reconfigurable subreflector as opposed to a reconfigurable main reflector.
Second, a rule of thumb will be formulated in order to establish the necessary number of actuators once the expected shaping of the surface is known.
Third, the typical deviation from the parent surface and thus the requirements on the type of actuators will be clarified.
Fourth, guidelines on the effects on the radiated far-field due to a failure in the actuators behavior will be provided.
The project does not aim at providing a finite product or a system, but it constitutes a feasibility study the results and conclusions of which will be used in following projects and ARTES initiatives. A description of the expected results can be read in the paragraph 'Expected main benefits' while the structure of the investigations to be performed can be found in 'Project objectives'.
The project is expected to last six months. During that period five tasks are expected to be completed.
TASK 1 will deal with the consolidation of the mission and the antenna requirements.
TASK 2 will provide a survey of the possible technologies for the implementation of the reconfigurable surface, including suitable mechanical actuators and reflector surface technology.
During TASK 3 the performances of a reconfigurable dual reflector able to satisfy the requirements defined in TASK 1 will be studied and compared to the performances of a traditional fixed shaped reflector designed to illuminate the envelope of the desired coverages and of a shaped but fixed dual reflector optimized for each individual coverage.
TASK 4 will deal with a detailed RF design and analysis of the antenna configuration.
During TASK 5 the antenna requirements will be updated and the mechanism functional requirements will be elaborated.
The project started in January 2008. Now, in July 2008, the first three tasks have been completed and Task 4 has begun. Up to now, two mission scenarios have been defined in detail and two antenna configurations have been proposed.
A study on the possible technologies for the implementation of the reconfigurable surface, including suitable mechanical actuators and reflector surface technology, was conducted. A large analysis was performed in TASK 3 in order to compute the directivity levels for the two selected mission scenarios by the two antennas configurations.
The performances of a reconfigurable dual reflector were compared to the ones given by a traditional fixed dual reflector shaped to illuminate the envelope of the desired coverages and to the ones provided by a shaped but fixed dual reflector optimized for each individual coverage.
The number of actuators necessary to provide the same directivity results given by a traditional spline shaping was investigated. A detailed RF analysis of the dual reflector antenna will be now performed and a sensitivity analysis to the actuator settings will be considered. The impact of imperfections of the reconfigurable reflecting surface such as anisotropy and reflectivity will be then taken into account