English: Diagram illustrating the operation of a solar power satellite, a satellite that would harvest solar energy and transmit it to Earth as a beam of microwaves, where it would be received and converted to electric power. The satellite would have a large area of W:solar cells, to convert sunlight to electricity. This would be converted to microwaves by high frequency vacuum tubes and beamed down to Earth by a phased array antenna, where it would be received by a rectenna array several kilometers square. A "pilot beam" of light from a laser would keep the phased array aimed at the rectenna. The rectenna would convert the microwaves into DC electric power, which would be converted to AC power and added to the electric power grid. The advantage of the solar power satellite, invented in the 1960s by Peter Glaser, over ground-based solar cells is that it is never "night" or cloudy in space so the satellite could produce power 24 hours a day.