A limited test program was conducted between 2-15 June 1977 using the existing ionospheric heating facilities at the Arecibo Observatory and a portable radar placed to observe at normal incidence to the Earth’s magnetic field lines any field-aligned irregularities of electron density produced over Arecibo (17). The portable radar was provided and operated by the Aeronomy Laboratory of NOAA at Tarare, near Point-a-Pitre on the French island of Guadeloupe, made available by the Institut de Physique du Globe in Paris. The results are that the available transmitters (HF, 430 MHz, 2380 MHz) feeding the 1000-foot antenna produced no striations that could be detected at Guadeloupe where the minimum detectable cross section was HL3 m2. The radio wave heating of the ionosphere for the available transmitters is about 1% for HF, 40% for 430 MHz, and 5% for 2380 MHz of the ohmic heating that the proposed 5 GW SPS system will produce. The horizontal dimensions of the heated ionosphere are 25 km at HF, 0.7 km at 430 MHz, and 0.2 km at 2380 MHz. The latter two are less than the predicted scale size to produce thermal self-focusing instabilities and the power densities are less than the critical power densities predicted for thermal self-focusing instabilities. The negative results are consistent with the theoretical predictions. As a check of the system, strong echoes (cross sections up to the order of 103 m2) from striations over Arecibo were observed at Guadeloupe when the Arecibo heating frequency was equal to the plasma frequency at some height in the ionosphere. In this case the striations are the result of instabilities based on resonances between the natural plasma frequency and the heater frequency, a problem that will not be encountered with the SPS. The surprising scintillation of communication satellite signals at gigahertz frequencies observed at ground stations has been reported and summarized by Taur (22). The early observations are found in COMSAT memoranda and in the COMSAT Technical Review (23). Scintillations of radio signals propagating through the ionosphere are observed at all frequencies, but the scintillation amplitudes decrease rapidly with frequency (fading amplitude/signal amplitude proportional to inverse frequency squared) and it was thought that by assigning satellite communication channels in the microwave region of the radio spectrum the problem would be avoided. The installation of satellite communication systems at 4 and 6 GHz demonstrated that scintillations (peak-to-peak excursions up to 10 dB) did occur at times in the tropics, the preferred times being early evenings in the months near the equinoxes. The occurrence of scintillations has been well mapped observationally in latitude, time of day, and season. Microwave scintillations are observed in the. tropics and in northern latitudes associated with aurora (24). Taur (23) suggests that the scintillations are within 25° latitude during periods of maximum solar activity and within 20° during periods of minimum solar activity. Scintillations are observed at a given station on more days during the preferred periods when solar activity is high. The aurora region is far enough north to be of no immediate interest for the SPS problem. The SPS will produce radio interference [1] directly on the frequency 2.45 GHz, although current users of this frequency band will not be upset by it, [2] directly on harmonics of 2.45 GHz and some current users may be troubled by it, and [3] indirectly if the SPS generates scatterers in the ionosphere that open communication links where they currently do not exist. These problems are being assessed by the Institute of Telecommunications Sciences and Battelle Northwest Laboratory for the Department of Energy.
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