studies undertaken to assess the impact of the operation of the SPS upon ionospheric-dependent telecommunication systems are discussed. The material presented here was obtained, for the most part, as the result of experimentally simulating SPS ionospheric effects by the use of a ground-based ionospheric heating facility. 2. SPECIFIC BACKGROUND The power density associated with the passage of the SPS microwave power beam as it is transmitted from geostationary orbit to the surface of the earth is to be 23 mW/cm2 at the beam center. Such power densities are of sufficient intensity to give rise to changes in the electron temperature and electron density in the Earth's ionosphere. These changes, in turn, can affect the performance of telecommunication systems that rely upon the ionosphere as a medium for electromagnetic propagation. The ionosphere is commonly defined as that portion of the Earth's upper atmosphere where sufficient numbers of free electrons exist so as to affect radio-wave propagation. Electromagnetic radiation propagating through this region is collision- ally damped by the free electrons. For microwave frequencies, the fraction of wave energy absorbed in the ionosphere is expected to be relatively small. However, the resulting heating of the plasma can significantly affect the local ionosphere thermal budget. Numerous telecommunication systems rely on ionospheric reflections or transionospheric propagation as part of their communication-signal path. Any system that can significantly modify the ionosphere has the potential to produce wide- ranging telecommunication interferences. The power density associated with the passage of the SPS power beam through the ionosphere may lead to substantial heating of the lower ionosphere and to the creation of irregularities and striations in the ionospheric plasma at F region heights (250 and 400 km) (2,3). The heating of the lower ionosphere that is associated with the passage of the SPS microwave power beam is believed to result from ohmic type interactions between the microwave beam and ionospheric electrons. In the upper ionosphere (the F region), the heating effects associated with the passage of the SPS beam are believed to give rise to the phenomenon of thermal self-focusing. Figure 1 provides an artist's concept of how telecommunication systems could be impacted by the operation of the satellite power system. The figure shows an SPS beaming energy to the surface of the earth and giving rise to enhanced electron temperature in the D region, and the formation of irregularities in the F region. Telecommunication systems, whether they be situated on the surface of the earth or in space, can be affected by the modified ionosphere. 3. IONOSPHERIC HEATING RESULTING FROM SPS OPERATION Heating the ionosphere with high-powered radio waves has been conducted on an intentional basis since the late 1960's (4,5). This type of heating has been undertaken principally using radio waves whose frequencies are confined to the HF (3-30 MHz) portion of the spectrum. The radio waves used in most of the HF heating experiments were transmitted into the ionosphere, and at a height where the ionospheric plasma frequency, fv, equalled the frequency of the heater wave,/; the heater wave
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