desired with inclination over Arecibo's 18.3°and accessible with minimal AV. The orbit need not pass over Arecibo every day, but as much regularity as possible would be desired for scheduling and facilitating possible tracking systems modifications. The exact orbital parameters depend on the results of several trade-offs described below, but the height and inclination should be calculated with a view toward making the orbital period (taking into account the regression of the nodes) an integral fraction of a sidereal day. The choice of orbital height of the satellite is closely related to the specific objectives of its mission. As shown in Figure 10.1.4 below, as orbital height increases, the time that the satellite will be in the beam increases while the power received decreases. A second trade-off is made when deciding which of the radars to use. If the 430 MHz beam is used instead of the 2.38 GHz beam, the amount of power received decreases, but the time in the beam is increased by about a factor of six. Several other factors help to determine the orbit selected for the spacecraft. For a small satellite with a relatively large collector such as the one considered here, drag is a very significant parameter in choosing the orbital height. In an orbit up to 400-500 km, the satellite's lifetime would be relatively short. For altitudes of 700 km or greater, drag ceases to be a significant problem and larger structures can be employed. Another factor which tends to push the satellite toward a higher altitude is that drag forces at lower altitudes tend to disrupt attempts to use gravity gradient stabilization. Finally, one must consider the issue of orbital debris, perhaps a serious problem if an inflatable reflector as described below is to be used. From this perspective, an orbit of around 1200 km would be desirable. A high altitude such as this would also giving the advantage of having a transit time through the 430 MHz beam of a little over a second. Unfortunately, the trade-offs described above are largely irrelevant for a satellite demonstration aimed at costs under $10 million. To launch a satellite within that sort of budget there are only a few possible vehicles. Of these, the major candidates would appear to be the space shuttle launched “Get- Away Special” (GAS) and the Ariane ASAP ring. As both of these systems are severely weight and volume restrained, carrying any sort of propulsion system would drastically reduce the usable system mass. Figure 10.1.4 Trade-offs for Altitude Choice. Calculations are for 10 m Diameter Collecting Area Receiving Power from the 2.38 GHz, 400 KW Arecibo Radar Thus the orbit of the satellite is basically determined by the orbit of the main spacecraft. In the case of Ariane ASAP launches, it is possible to achieve a sun-synchronous orbit, but not one tailored to pass over the proper spot each day. In the case of the GAS, the orbit is that of the shuttle. Figure 10.1.5 shows the frequency with which typical orbits of these types might pass within view of
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