Fig. 10. Arecibo Observatory ionospheric heating test sphere have been predicted to occur. These nonlinear effects could possibly degrade communications and navigation system operations. Theoretical studies to date indicate that adverse interactions may occur at power densities greater than 23 mW/cm2. This level has been used as the design criterion for the microwave system studies. Experimental studies have been initiated to measure the threshold heating levels. A series of tests (16), illustrated in Fig. 10, were performed at the Arecibo Observatory in Puerto Rico in June 1977 to heat the ionosphere and to look for possible communications effect. Using the existing facilities at Arecibo, the heat input levels to the ionosphere were below the equivalent Solar Power Satellite levels and the heated volume was smaller. For these conditions, no nonlinear heating was observed and no communication effects were detected. Tests using higher power to equal or exceed the equivalent Solar Power Satellite heating levels are now being defined. These experiments, which will involve communications and diagnostic tests at several facilities over the next several years, will determine the extent and magnitude, if any, of the microwave beam/ionospheric interactions. Proposed biological studies of microwave beam effects include both intermittent exposure at levels greater than 1 mW/cm2 and chronic long-term exposure of less than 1 mW/cm2. The proposed studies have been developed after a literature survey (17) of more than 1000 published documents dealing with the potential effects of microwaves. Unfortunately, there have been very few investigations relating to long-term low-level effects, and, while there is no hard evidence to indicate that significant adverse effects will occur, some controversy does exist. The proposed research will hopefully provide definitive information in this area. Transportation The development of a commercial satellite network of Solar Power Satellites will probably require a new, tailored launch system to achieve the economies of scale and to preserve for reuse all vehicle elements, including propellant tanks. A vehicle design capable of a single-mission delivery of about 425 t to low Earth orbit is illustrated in Fig. 11. It has a height of 152 m (500 ft) and has wings on both stages for
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