changes of 5 dB are seen when account is taken of the signal calibration. The fading rates and the coincidence of the onset in the fading period with the “ON” time of the facility are typical of all overdense heating cases observed on the LES-8 (and other earlier) signals. Figure 7 (also following from Ref. 11) shows the LES-8 signal recorded between the times of 0256 and 0335 h (UT) on March 12, 1980. The Platteville Facility was turned on at a frequency of 9.9 MHz with an input power of 1.5 MHz at 0300 h (UT). Between the times of 0045 and 0300 h (UT), the facility was “OFF.” At 0258 h (UT), f0F2 at Platteville was observed to be 8.1 MHz, and at 0312 h (UT),/0F2 was observed to be 7.9 MHz. The entire period of heating was therefore done in an underdense mode. It is readily apparent that, about five minutes after the onset of underdense heating, the LES-8 signal started to fluctuate considerably. Peak-to-peak fluctuations of 10 dB were observed, and at 0315 h (UT) when the facility was turned “OFF,” the LES-8 immediately started to settle back to its preheating level. The results shown in Fig. 7 tends to display obvious effects of underdense heating on the LES-8 satellite signals. The experiment at Carpenter, Wyoming, was repeated on April 14 and 15, 1980. At that time, the Platteville Facility was operating in an underdense mode for half-hour periods as opposed to the 15 min periods during the March, 1980 operation. The results obtained in April corroborate those obtained in March regarding underdense heating effects on satellite-to-ground telecommunication circuits. In extrapolating these results to the SPS scenario, the observed scintillations must be scaled to 2.45 GHz. The exact frequency scaling procedures have yet to be determined and the scale size of the irregularities that are created by self-focusing are dependent upon the wavelength of the perturbing (heating) wave. In addition, it must be borne in mind in interpreting these results for application to an SPS environment that the experimental observations were conducted in a manner designed to maximize the impact of any irregularities created by the heating of the ionosphere. The propagation path of the satellite signal was directed along the Earth's magnetic field, thereby reinforcing any scintillation-producing effects. It should be mentioned, however, that if the I//3 scaling for the self-focusing instability is correct, then, for the frequency (9.9 MHz) and power (1.5 MW) used in the experiments, the equivalent 2.45 GHz power density is about 90 mW/cm2 or almost four times the current SPS power density. Clearly, further study and observation are required. 5. CONCLUSIONS Studies directed toward assessing the performance of VLF, LF, and MF telecommunication systems operating in an experimentally simulated SPS environment have yielded results indicating that such systems will not be adversely impacted by SPS operation. These studies were conducted using actual operating telecommunication system signals. The effects of SPS heating on HF, VHF, and UHF systems need to be studied also. Results obtained indicate that underdense self-focusing can produce changes in the signal level of satellite transmissions received on the ground and in aircraft. The preliminary results show that changes in signal levels are much slower and much smaller than those associated with overdense (radio wave reflected from the ionosphere) heating processes. The initial results showing fluctuations in satellite signals passing through an underdense-heated ionosphere need to be studied further in terms
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