High Power Oblique Incidence HF Ionospheric Modification

Abstract : To successfully perform the proposed ionospheric modification experiment (IONMOD) it is necessary to establish the range of the expected changes in the ionosphere, the parameters that offer the greatest likelihood of detection of these changes and the systems that can best carry out the appropriate measurements to detect them. The distributed region of the ionosphere is modeled as a spherical depletion region of varying depth and size. Three dimensional numerical ray tracing is then used to determine the expected magnitude and location of the amplitude and arrival angle deviations. These calculations indicate rather small changes, of the order of 3 dB in amplitude and about 0.5 deg in the elevation angle, with even smaller changes in the azimuth arrival angle. This leads to strong requirements for the measuring system in terms of antenna design and the signal to noise that must be achieved. Consider the basic concept of a probe system to make the required amplitude and arrival angle measurements. Such a system will be based upon the real time calculation of the Doppler spectrum of the signal on each antenna of the system and use the measured phase difference on each line in the spectrum, as an interferometer, to determine the arrival angle and amplitude. The requirements are determined for the signal to noise ration that will permit the measurements with sufficient accuracy. The results indicate that sufficient measurement sensitivity of the probe system can be achieved with a transmitter power of 100 W continuous and an antenna pre-amplifier (with a low noise front end) properly designed to be atmospherically noise limited. (JHD)