organizations, and other questions. A discussion of this kind of safeguard is presented in Appendix B. A prominant candidate institutional safeguard considered in this study is resident inspection. This involves the use of independent observers to monitor and inspect, as required, all space-based and land-based facilities belonging to the SPS project. This concept is discussed further in Section 5.4 below. A special type of agreement involves proximity rules in space. These can be described by analogy with civil aviation practices in international airspace. Legally, international airspace is not subject to national appropriation and is available to aircraft belonging to any country. However, in order to protect against collisions between aircraft, air traffic control services are provided. Depending on flight altitudes and distances from fixed navigation aids, substantial volumes of airspace surrounding each aircraft are protected from intrusion by other aircraft. Similarly, minimum distances of separation between spacecraft could be specified by international agreement. Another type of institutional safeguard is public discussion. This is primarily useful in safeguarding against threats and vulnerabilities which are based on misperceptions. For example, the public could fear that a system failure could cause diversion of the SPS power beam over a population center, thereby threatening the population. Open public discussion would counter such unjustified fears by accurately describing the flux density of the SPS power beam, by explaining how the SPS power beam is designed to home in upon a pilot beam located at the rectenna site, and by stressing the design features of the pilot beam system which make it secure and reliable. 5.3 Key Safeguards Possible safeguards noted in the threat and vulnerability matrices and in preceding discussions can be classed as active or passive, counterthreat or countervulnerability or both, and of varying degrees of provocativeness. Table 5-3 summarizes over thirty possible safeguards according to the above classifications. Three primary criteria for evaluating possible safeguards, in order of decreasing importance, are: 1) effectiveness; 2) provocativeness or acceptability; and 3) cost. While all safeguards identified in this study are thought to be realistic and potentially effective, total effectiveness is beyond reach in the
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