Net Present Value method calculates the total monetary value of a project over its duration in present day dollars [Oxnevad 91], by incorporating the effects of inflation and the value of money (interest or discount rate). Baseline values for these variables are: discount rate 20% (typical for high risk investment) and inflation 5%. The Net Present value is the sum of the Present Values (PV's) for the pertinent variables. Those that we consider are: We may express these as follows: where y is the number of years after program start, PE is the program end in years, r is the discount rate, and Invy is the investment required in that year. It should be noted that although inflation is included explicitly, it is meaningless to consider it in isolation as it directly affects the real discount rate: higher inflation is equivalent to a lower discount rate. Many NPV models include the effects of inflation implicitly through its effect on the net discount rate. Ground to Space Power Beaming Ground to space laser transmission is the only application technically viable in the near term (see 7.2.2), and this time-scale is considered initially. We therefore consider here the cost of ground based lasers, of which the free electron laser provides the only near term solution to the power and frequency requirements (optical frequencies for solar array conversion). Project Selene studies, under way at NASA Marshall Space Flight Center, have costed a 10MW ground based free electron laser at $500M for the first ground station, and $225M for subsequent stations. Many consider these figures optimistic, however, and we will assume technology development costs to be of order $1B, with subsequent construction costs of $25OM per station. While most near term uses will require much lower power levels than those considered for Selene, we will use these cost figures for a generic ground based laser system. Maintenance costs per year, again based on Project Selene figures, will be taken as $40M. Since a ground based laser is limited in practice to beaming within 60° of zenith [Landis 92], to provide coverage over 360° of longitude a ring of at least 3 ground laser stations would be required (4 would be required for coverage of geostationary orbit). For low Earth orbit customers, however, many more stations would be required along the orbital ground track, since a 500 km altitude orbit reaches an azimuth of 60° with respect to a point on the ground when only 900 km away laterally. Approximately 40 ground stations would be required for continuous illumination of a satellite at 500 km altitude and equatorial inclination. For the space station orbit, however, the 28° orbital inclination renders totally impractical the supply of ground based laser energy on a continuous basis as discussed insection 3.1.1. Moreover, even the areas with the world's best weather factors have 50 to 100 cloudy days per year. If more continuous power must be supplied in order to attract customers, as many as double the number of ground stations may be required, separated by sufficient distance as to be in different weather systems, and this would increase power availability to nearly 98%, assuming no correlation between the weather at different ground station locations.
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