Fig. 2.3.1: RETRODIRECTIVE BEAM STEERING Fig. 2.3.1 explains roughly the function of an active retro- directive array. The left figure represents an undeflected beam, while in the middle the beam is missing the rectifying element of the rectenna it is attached to. On the right figure, the displacement is measured on the ground and reported to the SPS. There the phase control system is able to redirect the deflected beam. The other antenna type is the horn fed 797 m diameter lenstype solid-state phased array design used with the GSSPS. The antenna consists of 2^6 crossed dipole elements with 6-bit digital diode phase shifters and about 100 beam steering computers. Each element operates at a power level of 98 W. Another potential antenna type is the electronically steerable multibeam antenna. 1.2.3.4 The Microwave Beam The geometry of a microwave beam depends on the size of the phased array transmitting antenna. Fig. 2.3.2 shows the transmission efficiency as a function of the areas of the transmitting and receiving antennas, the separation distance, and the wavelength. For the example of a 1 km diameter antenna and a 10 km diameter receiving antenna the reception efficiency
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