and the intermodulation of components of the interferer with the desired signal. Scoring procedures applicable to this problem can be derived by extrapolation of existing empirical and analytical degradation models for receiver systems employed for communications, radar, or a general range of metering applications. These define a range of performance characteristics, in terms of probability density functions for signal to interference ratios. Most of the EMC problems related to the SPS that represent a significant expenditure in victim equipment modification will be concerned with nonlinear response category. Computer devices, optical equipment and medical instrumentation present unique scoring descriptives that relate to effective apertures and energy coupling into signal and control circuitry. These types of problems can, to a lesser degree, be extrapolated from experience with high power radar illumination of surveillance and monitoring equipment required for military operations. Limited measurements will be required for the SPS problem to assure credibility in the predicted degradation and recommended functional modifications for these classes of equipment. Additional EMC-related problems that will be addressed concern the infrared (IR) source and optical reflection characteristics of the SPS vehicle in temporary and permanent orbit locations. Functional impacts are anticipated for astronomy and space tracking/monitoring equipment because of the probable magnitudes of the emissions and reflections from the large orbiting platform. 4.1.4 State of Knowledge Looking at the attenuation by the troposphere, it is expected on theoretical grounds that at a frequency of 2.45 GHz, radio waves suffer little attenuation, by the lower atmosphere of the earth. Gaseous absorption, due to both oxygen and water vapor absorption, is expected to contribute no more than about 0.1 dB, (Ref. 4.1.8) and rain absorption, no more than another 2 dB. This "worst case" total of 2.1 dB, although small, represents a loss of 38% of the originally transmitted power. On the average, one would expect about 0. 02 dB of attenuation at a frequency of 2.45 GHz on a 30° elevation angle path to a satellite in a temperate climate characterized by a gaseous atmosphere with no rain. Even this, however, represents a 4.5% loss of power.
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