• Smaller capital and investments required to start the program. A complete ground and space infrastructure is not needed to produce electricity. • Many identical and redundant elements of this infrastructure allow mass production cost reductions. • The feeding of many rectennas enables the organization responsible for commercialization of SPS electricity to diversify and stabilize its revenues. • A complete and well balanced geographical distribution of rectennas may allow shorter distances between the receiving sites and major consumer areas, and therefore, cheaper electricity transportation and distribution costs (this is mainly true for a high inclination constellation, the non coplanar case, where satellites fly over or near industrialized countries). 3- Adaptability: • This concept enables technology improvements to be gradually integrated to the infrastructure over the years. • Eventually, it can accommodate many design of SPS, as long as the power transmission subsystems are compatibles. • And it is fully adaptable to the growth of electricity demand. Disadvantages 1- Spacecraft: • As we use LEO, atmospheric drag losses are not negligible, especially for large structures. • Total illumination of the spacecraft by the Sun is significantly reduced in LEO compared to GEO. • The selection of the best orbit can be constrained by space debris or Van-Allen belt considerations. • This constellation will increase the number of spacecrafts in LEO orbit (and eventually, the density of space debris). • In LEO, rectennas are in the visibility of the SPS only for a few minutes and the antennas need very good tracking and/or deflection capabilities. 2- Rectenna: • With a constellation, we cannot always assure a continuous and stable feeding of all the rectennas (this can cause problems for basic energy supply, unless we use storage devices associated with the rectennas). • The rectennas must be able to collect efficiently the microwave power from many directions. • Because of the important number of receiving sites required by a LEO constellation of SPS, it could be difficult to meet the ideal geographical distribution since a large part of the Earth surface is not suitable for rectennas (oceans, mountains, etc.). Conclusion Many factors need to be taken into account when making orbit selection. We must optimize technical as well as economical parameters. The scope of this small study does not pretend to give the final and definitive answer to that question. Too many elements are missing, or are simply unknown. But, a constellation of SPS in equatorial LEO seems to be an attractive solution. Obviously, some technical problems, like the tracking of the beam, have to be solved. It should also be interesting to evaluate the more general problem of a 3D constellation (28.5' or polar orbits, for instance) to provide energy all over the world.
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