(Firmain, et al, 1991). This frequency is selected for the system design, whether in GEO or at a 20000 km altitude. The following discussion on solid states versus tubes will consider the robustness of this latter choice (for the near term). Solid state usually saves a lot of mass and electrical power consumption, especially at low power levels and for frequencies in the range of a few GHz. Nevertheless, at higher frequency and power level, solid state devices are inefficient, unreliable and even not demonstrated at present. Furthermore, considering a power in the range of 1 MW, and solid state devices handling a reasonable 100 W (not existing today above a few MHz), the antenna would require 10000 of those elements. Even without taking into consideration the problem of control of such an amount of active devices, the foreseen efficiency (based on actual designs) is in the range of 50% at 2.45 GHz (Kaya, Matsumoto and Akiba, 1991). At 35 GHz, a better efficiency is not expected in the near term. The amount of heat dissipation is a serious limitation, especially effecting long term reliability. On the other hand, vacuum tubes have already demonstrated performances above 100 GHz (Firmain, et al, 1991) with power levels higher than 10 kW and an efficiency in the range of 80%. The main weak point is the lifetime of the cathode, limited to around one year for ground systems in the range of 1 MW. Nevertheless, low power tubes in X band are already space qualified and show a lifetime in the range of 10 years. It is therefore expectable that those performances will be reached by gyrotrons in the near future. Furthermore, the need to use many amplifiers would suggest a power level per tube from 1 to 10 kW, extending simultaneously the expected lifetime. For the long term it can not be excluded that solid state devices will become an attractive option. The geostationary orbit allows a fixed pointing antenna as long as the spacecraft is able to provide a stable platform. The antenna could be either parabolic or planar. For big dimensions the parabolic type is difficult to use due to size limitation by the launcher. A deployable technology could be considered but the uncertainty to obtain this kind of structure in a size large enough is high. Hence it seems more plausible to select a planar model, assembled in orbit. If a single micro waves source is used, this kind of antenna is relatively easy to obtain by distributing a set of slots on a planar surface. In our design a set of sources is used. These sources have to be phased with each other. The simplest way to obtain this result is to use a unique low power source feeding a set of amplifiers adjustable in phase to compensate for the relative discrepancies and aging effects. The design will then use 100 gyrotrons at 35 GHz. The antenna will be made of a set of 100 panels assembled in LEO and adjusted by optical interferometric means. The remaining deformations would be compensated by phase tuning of the amplifiers. The previous design is suitable for GEO where there is no strong requirement on beam mobility. For the 20309 km design it is necessary to have a total beam deflection over 24° side-to side. It is then expected that the previous design will no longer be usable as the fixed pattern of slots does not allow for the steering of the beam in various directions. Hence, a design using 1000 sources of 1 KW each will be used, these sources being fitted on a composite structure to ensure rigidity, each one having its own radiator. The need for an individual phase shifter drives the design to that of one gyrotron directly feeding one phase shifter connected to the antenna face. The considerations regarding solid state versus vacuum tubes remain unchanged. It is then necessary to assess the required pointing accuracy. The accuracy could be considered as the minimum angle variation of the beam that allow it to remain in a given area at ground level. Assuming that a safety ring of 1 km exists around the rectenna and considering the altitude of flight the angular accuracy should be in the order of magnitude of (1 120309) x 57.3°= 0.03°. This accuracy is directly related to pointing accuracy of the platform for the GEO design. For the mobile concept this becomes a dynamic accuracy to be provided at beam deflection level. Considering a total range of beaming over the rectenna of 24 , the number of steps for the phased array would be: 24 / 0.03 = 800. Assuming a binary coding of the angle for the phase shifters, the closest power of 2 is 1024. Then the phase shifter should be capable of 1024 steps of 3 / 100 . This requirement would be probably relaxed if it is possible to show that the beam is not sensitive to phase quantum changes of individual elements, allowing a reduced number of individual phase patterns. On the other hand, a graceful degradation of the antenna array would require an individual control of each phase shifter to be able to face any configuration occurring in flight. The speed for updating the array (assuming a central computer unit to control the phase) is determined by the time needed by the satellite to fly from one horizon of the rectenna to the opposite one. With the chosen orbit, and taking into account the Earth rotation, the time needed to deflect the beam 24 ° is 6h24' (to deflect it of 3 / 100° takes 28.8 s). Even with 10 bit phase shifters and 1000 devices, the data flow is well within the range of the present technology. This requirement could be increased if random phase shifts of each oscillator have to be compensated. However, even with 100
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