The amount of the polar cap that remains in view by a satellite at apogee over the pole depends on the height of apogee. As apogee is increased the latitude that can be seen decreases so that the polar cap coverage increases. Fig. 2 shows the north polar cap as seen by a satellite at three distances. The outer circle indicates the lowest latitude that can just be seen by line-of-sight from the satellite. For a satellite at 2 earth radii geocentric distance this is 30°; 4 earth radii, 15°; and 6 earth radii, 10°. The circumpolar motion of a rectenna is also illustrated. In order to avoid a very oblique angle of the beam to the rectenna, the rectenna should be located at as high a latitude as practical. At a latitude of 60° a satellite at a geocentric distance of 2 Earth radii above the pole is 35° above the horizon; at 4 Earth radii, 50°; and at 6 Earth radii, 55°. A latitude location somewhat above 60° would appear to be indicated. A site in the vicinity of the Arctic Circle would be appropriate. This system would be particularly well suited for countries with land above or near the Arctic Circle. Interestingly, these are not usually the countries for which solar energy is a viable option. Because of Kepler's equal area law, the elliptic orbit satellite spends more time near apogee than perigee. For example, for an apogee of 4 Earth radii the satellite will spend 16 times as much time within a given angular distance of apogee as for the same angular distance from perigee. This hang time at apogee increases with increasing apogee so the duty cycle during which a satellite is in view of a high latitude rectenna will increase with increasing apogee. If a 100% duty cycle is required, more than one satellite can be employed for a given rectenna. If energy storage through conversion to hydrogen is used, a 100% duty cycle may not be required. Modular or Incremental Construction Breaking free of the constraint of the fixed geostationary orbit distance introduces an important new possibility in SPS construction, namely modular design with incremental increases in satellite power capacity accompanied by increases in apogee while at the same time retaining a constant overall system efficiency. It is conceivable to think of a small low power system, with a low apogee. The low apogee permits sufficient beam power density to maintain high rectenna efficiency. As new power capability is added to the satellite, apogee may be gradually raised and the size of the rectenna increased. This could reduce the burden of very large front-end costs and long payback periods and make possible a pay-as-you-go financing plan. It also provides a viable plan for pilot or prototype plants without excessive investments.
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