Figure 10.1.2 Tracking rates for Satellites at Various Altitudes. 10.1.2 Orbital Considerations Totally aside from the issue of what ground facilities could be used, it is worth remembering that there are two parts to any beamed power demonstration: a transmitter and a receiver. In the case of this particular demonstration, the receiver will be a small spacecraft. To determine the characteristics and cost of the receiving spacecraft, it is necessary to take a look at the impact of various orbit choices on the mission. On first consideration, taking into account the limitations of the Arecibo facilities, one would desire either a geostationary, sun-synchronous, or low equatorial orbit. A GEO orbit would overcome the tracking problem, but unfortunately the power received by any reasonably sized rectenna would only be on the order of milliwatts. As this power level is probably inadequate for the demonstration purposes, a lower orbit, likely under 2000 km, is called for. Ilie advantage of a sun-synchronous orbit would be that it would pass over Arecibo at the same time each day, and so ground station scheduling would be much easier. The difficulty involved in this type of orbit would be the high AV required for orbital insertion, which would result in higher costs. An equatorial orbit might then be considered, which would allow a satellite to pass within Arecibo’s arc several times a day. Figure 10.1.3 shows the problem with this concept. Arecibo can only see objects along the equator if they are over 70,000 km away. Figure 10.1.3 Arecibo Viewing Geometry Knowing that low equatorial orbits are inaccessible to the ground station and sun-synchronous orbits are relatively inaccessible to the spacecraft, a compromise must be made. A synchronous orbit is
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