reflector into the smallest packaging volume at the least cost in mass. 1986 estimates place the mass of a 10 meter inflatable antenna at about 60 kg, and volume requirement at about 0.3 cubic meters. The use of a reflector also allows a two-part satellite structure which can be gravity-gradient stabilized, and enables the use of a small rectenna placed physically near to the satellite’s other electronics and measuring instruments. 110,11] There are also difficulties associated with the design type chosen. Parabolic reflectors are intended to focus radiation coming in directly along the boresight. This is fine for the cases where the satellite is coming in directly above Arecibo, but in most instances the satellite will appear somewhere else within the 20° from zenith toward which Arecibo can direct its transmission. To use the parabolic reflector properly in these situations, the satellite would have to align itself with its axis along the path of the incoming radiation. That, however, would require the satellite to give up its simple gravity gradient stabilized control system for a more complex, massive, and expensive system. An alternative to this would be to use the guide wires connecting the reflector to the satellite bus to tilt the reflector toward the incident radiation. Figure 7 shows an example of this situation. The incident radiation comes in at an angle but is still focused onto the rectenna surface. The change in inertia caused by tilting the reflector is not enough to affect the directions of the satellite’s principal axes significantly, so the overall spacecraft maintains the same orientation as before. Figure 8 shows the same situation with a more three-dimensional perspective. It is important to note that though the incident radiation is all concentrated onto the rectenna plate it is not all concentrated onto a point as it is when the incident radiation comes in along the boresight. Figure 9 is a close-up view of the situation in Figure 8, showing the pattern of light incident on the rectenna plate. The concentrated radio waves form a clear clover-like pattern which varies greatly in intensity with location. This non-uniform intensity pattern will impose difficulties on rectenna design and may result in lowered efficiency. In addition to the variation in intensity, however, the pattern shown in Figure 9 also implies a variation in phase. The special geometry allowing a parabolic reflector to focus light in phase only works when the light is incident along the boresight. For off- boresight geometries, the individual light rays have to travel differing distances and hence come in out of phase. This phase shift effect is highly dependent on the specific geometry of the satellite and can cause drops in efficiency of 50% or more. In addition, it is possible that the difficulty of predicting the exact extent of this effect would obscure measurements taken by the scientific instruments. Further investigation needs to be done to determine exactly what the effects of such an off-axis reflector based system would be. There is one further difficulty entailed by this reflector-based design. One degree or better pointing accuracy would be required to ensure that the incoming radiation was focused properly on the rectenna. This pointing requirement would also have to be achieved before the satellite enters the transmission beam, as the total time in the beam is only one or two tenths of a second. This does impose further restrictions
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