A reference set of efficiencies has been defined that represents reasonable goals for each step in the power conversion-transmission-reception chain (see figure 1). These efficiencies imply a power density limit of 21 kW/m2 at the transmitter which, together with a limit of 23 mW/cm2 at the ionosphere and the reference antenna taper, leads to a maximum power of 5 GW per microwave link delivered to the power grid. This is the value selected for the reference system. There is recent evidence that 23 mW/cm2 may be conservative; if so, the maximum power per link could be increased. A geostationary orbit, with zero eccentricity and inclination, is preferred on an overall basis, although a few other orbits offer some specific features that could prove to be advantageous. Solar radiation pressure is the dominant perturbative force, requiring on the order of 50 tonnes of propellant per year if eccentricity is to be held at zero. By differential thrusting, this orbitkeeping impulse can also be applied to altitude control, which would otherwise require nearly as much propellant itself. A major consideration in selection of the reference configuration (figure 2) was ease of construction. The scale of the program mandates the highest possible degree of automation in the construction process; this in turn places a premium on highly regular configurations that can be constructed with a small number of frequently repeated operations. Ease of construction was, for example, one consideration in the selection of an end-mounted, rather than central, antenna. The reference system is constructed in synchronous orbit using material transported from low earth orbit by electric orbit transfer vehicles. Construction in low orbit of sections of the satellite with subsequent self-powered transfer to synchronous orbit for assembly is an alternate approach.
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