power density to all ionospheric heights up to 700 km, however. Table 1 shows the SPS comparable power density based upon the I//2 relationship and size of the modified ionospheric region associated with the heating operation at Platteville. The details of the Platteville Facility and its operation are given elsewhere (9). 4. TELECOMMUNICATION STUDIES IN A SIMULATED SPS ENVIRONMENT Studies of the performance of a number of different telecommunication systems operating in an experimentally simulated SPS environment have been undertaken. For the most part, these studies were centered around the Platteville Heating Facility. Two series of experiments were conducted. The objectives of one series was to determine the degree to which ionospheric changes induced by SPS ohmic heating would impact upon telecommunication system performance. Since the Platteville Facility provides SPS-comparable power density due to ohmic heating (I//2) only to the lower ionosphere (see Table 1), telecommunication systems whose radio energy is reflected or substantially controlled by the lower ionosphere were investigated. The lower ionospheric studies were conducted during August, September, and October, 1979. The objective of the second series of experiments was to determine if thermal self-focusing effects could be produced by using underdense radio waves. The primary diagnostics of telecommunication system performance used were satellite transmissions in the very-high-frequency (VHF, 30-300 MHz) band. The self-focusing studies were in March and April, 1980. 4.1 Telecommunication System Impacts Due to Ohmic Heating The signals used to assess the potential impact of SPS operation on telecommunication systems operating in the lower ionosphere were 1. VLF signal sources - OMEGA 2. LF signal sources - LORAN-C stations 3. MF signal sources - AM broadcast stations. Receiving sites were located at Bennett, Colorado, and Brush, Colorado. At each of the sites, one or more of the source signals were recorded to test specific objectives. The telecommunication systems that were monitored were done so in a manner that realistically simulated the propagation of VLF, LF, and MF radio waves in an ionosphere impacted by the SPS microwave power beam. Details of these experiments are given in Ref. 10. No changes in system performance were noted that could be associated with ionospheric-heating induced disturbances. Figure 2 (taken from Ref. 10) shows an example of the received amplitude and phase of the OMEGA signal recorded at Brush, Colorado. The data were obtained on August 16, 1979, for the times indicated. The hatched blocks immediately above the time scale indicate that the Platteville Facility was operating in the “ON” mode. The amplitude and phase scales are indicated. The phase output from the receiver was designed such that, when either the zero or full scale (10 /as) outputs were reached, a reset occurs which placed the record pen at the opposite limit and another 10 /as of trace was then possible. During the time periods for which VLF, LF, and MF data were collected, large- scale geophysical disturbances — notably solar flares — occurred. These disturb-
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