energy transmitted during one pass is: 13 MWh beamed in 13 h for GEO and half of that for 20309 km orbit. It should be pointed out that the GEO option allows only to feed one receiving station when the 20309 km option could feed two stations and reach the same efficiency. The average level of delivered power during one pass is 1 MW for both cases. Figure 10.4.7 Sun Power Vs. Angular Position The power subsystem is organized around a set of solar cells arranged in a series-parallel cluster; the series structure is long enough to provide a voltage level allowing an easy DC/DC conversion (that is, over 160 V); the parallel structure is dimensioned by the current requirements. Except for the levels of voltage and current used, the architecture of the power subsystem is classical and could be similar to existing architecture, the input level varying only by 20%. It should be noticed that the variation of power level is directly related to the angle between the solar arrays. The analysis shows that an angle of 60 minimizes the difference between the maximum and first minimum of received power. In addition, this angle allows a symmetrical construction with elements of the same dimensions for each face. Transmitter Design The considerations for transmitter design are frequency selection, solid state or vacuum tubes, and a mechanically mobile structure or phased array. The frequency choice is driven by two considerations: size of the antenna and atmospheric attenuation. The size of the antenna is limited by the size of the spacecraft, to say: 100 m x 100 m. The atmospheric attenuation is an external factor implying a higher level of power at emission. With the antenna, assumed to be a phased array, it is difficult to predict accurately the transmission characteristics without using a simulation program. For any antenna a relationship exists between the diameter of emitter De and receiver Dr , the wavelength K and the distance of transmission D. The general shape of this relation is: Dr.De = KXD With K= 1.27 for parabola (considering power density at the receiving site) and 2.44 for a plane aperture (receiver in the first minimum of the diffraction pattern). The most stringent requirement is for the plane aperture that we would select for dimensioning purposes. With Dr=10 km, De=100 m, D=36OOO km, 1 should be less than 2 cm. This is a higher frequency than 15 GHz. To avoid interference with X band communications and to be in the first window of transmission , a possible frequency could be 35 GHz. A short survey of existing technologies shows that this frequency is the domain of the so called ‘Fast Wave Tubes'. In this family, the most promising device is the gyrotron
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