C. Power Transmission, Collection, and Conversion The SPS concept for the transfer of energy from geosynchronous orbit to earth is accomplished by means of microwave power transmission. This is basically a 3-step process which has as one of its major requirements high efficiency. The process consists of: 1. Conversion from DC power to microwave power 2. Focusing and transmission of the microwave power to Earth 3. Collection of the microwave power on Earth and conversion to DC power. Some of the more significant design and operational trade studies which have been made are summarized below. Microwave System Frequency - The primary frequency in the SPS microwave system which has been the subject of the majority of the trades thus far is the power beam frequency. (Trade-offs to determine the optimum set of uplink pilot frequencies for the phase control system are still being conducted.) The requirement for high efficiency over the microwave propagation path dictates the band of microwave frequencies which are acceptable; molecular absorption and rain attenuation through the Earth's atmosphere sets an upper limit on frequency selection of not much above 3 GHz. Alternate frequencies have been considered in most of the microwave system studies to date, the most notable being 5.8 GHz. Trades have been made from many points of view such as SPS system sizing of transmit antenna and rectenna, etc., (ref. 2,10,23); propagation effects through the ionosphere (ref. 26); hardware technology projections (ref. 2,10,23), etc. Although higher frequencies offer some advantages (smaller rectenna), the frequency of 2.45 GHz offers many more. This frequency is in the middle of the ISM band (industrial, scientific, and medical) of 100 MHz, there is lower attenuation when propagating through the atmosphere, and the projections for microwave equipment technologies are more promising.
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