This figure shows the microwave power density for the power distribution discussed in the last section of the previous chapter (Effects of SSPS RFI on Other Users), and Table 17 which will result in the formation of side lobes. It indicates a continuing decrease with distance from the beam center; e.g., at about 15 km, it is less than 0.01 mW/cm2. This is the lowest value set as a limit for microwave exposure by any country. The,permissible exposure value for humans or other forms of life by the time the SSPS becomes operational has yet to be determined on an internationally agreed-upon basis. In addition to these effects, interference with electronic equipment, medical instrumentation, or electro-explosive devices must be precluded as well. Their sensitivity to a low level of microwave exposure will have to be established. To realize this objective, industrywide standards may be necessary. The effects on birds flying through the beam is not known. Research on the effects of microwaves on birds at the level to be encountered in the microwave beam will have to be carried out. Preliminary evidence indicates that birds can be affected at levels of microwave exposure of 25 to 40 mW/cm2 in the X-band. The effects of microwave exposure on aircraft flying through the beam must also be considered. The shielding effects of the metal fuselage and the very short time of flight through the beam would preclude significant exposure to humans. Protection of aircraft fuel tanks from electrical discharges is a standard design feature, but the absence of microwave-induced effects will have to be confirmed. In addition, interference with aircraft communication and radar equipment will have to be precluded. Because of the concerns with potentially adverse effects being claimed to result even at low microwave power density, it is vital that the microwave system design requirements of the SSPS lead to the demonstration of low-exposure levels so that safe operations accepted on an international basis will result. The major elements of a microwave biological effects R&D program to support the SSPS development are listed below: • Limit R&D to SSPS specifics Frequency: 3.3 GHz cw Power flux density on ground: 30 mW/cm2 in central part of beam 0.01 mW/cm2 beyond a radius of 15 km On orbit: 25 kW/m2 in central portion of beam • Determine whether or not the near-optimum frequency and power flux densities for SSPS as derived for other considerations should be changed;
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