A special ray tracing computer will be used to provide real-time frequencies and elevation beam steering during the experiment. Real-time signal processing will permit the site operators to evaluate the performance of the system and make adjustments during the experiment. At the midpath point a vertical sounder will provide the ionospheric information necessary for the frequency management of the experiment. The three antenna array will be in an L configuration to measure the phase differences between the antennas. The probe receiving system uses multiple antennas to measure the the vertical and azimuthal angle of arrival as well the Doppler frequency shift of the arriving probe signal. The transmitter for the probe system will be at the same location as the high power transmitter while the probe receiver will be 2400 km down range. The HF probe system will use a low power (150 W) CW signal shifted by approximately 40 kHz from the frequency used by the high power system. This concept was first used by soviet researchers more » to insure that this diagnostic signal always passes through the modified region of the ionosphere. For these experiments we are introducing a new ET probe system, a low power oblique sounder, to be used along the same propagation path as the high power disturbing transmitter. An important part of this program is to determine the existence of a threshold for non-linear effects by varying the transmitter output. The output of this large transmitting system will approach 90 dBW. Plans are now underway to carry out new HF oblique ionospheric modification experiments with increased radiated power using a new high gain antenna system and a 1 MW transmitter. The observations are discussed in the context of a simplified model of the coupling from the Siple antenna into the ionosphere, which provides reasonable agreement with observations. Dynamics Explorer I satellite recordings of unducted Siple signals showed trends similar to the ground data on ducted signals. A combination of L shell data and models of antenna coupling into the whistler mode may aid in the location of ducts. Propagation paths of ducted Siple signals observed at Lake Mistissini were identified with propagation paths deduced from natural whistlers, from which the L shell values and equatorial number densities for the paths were calculated. Where whistler mode growth and emission triggering occurred, saturated peak values of received signals were independent of antenna polarization and initial injected power more » levels, in agreement with previous findings. Excitation of a multiplicity of magnetospheric propagation paths and the received signal strength were observed to depend on the transmitter antenna polarization. Crossed 21-km horizontal dipole antennas on top of the 2-km-thick ice sheet were used to transmit 2- to 4-kHz waves alternately with right-hand circular, left-hand circular, and linear polarizations. Whistler mode waves of various polarizations were transmitted by the Siple Station, Antarctica, VLF transmitter and received near the geomagnetic conjugate point at Lake Mistissini, Quebec.
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