HF fades caused by multiple wave fronts detected by a dipole antenna in the ionosphere

An experiment on transionospheric HF propagation was carried out in 1978 using a dedicated ground transmitter at Ottawa and the sounder receivers of the ISIS 1 and ISIS 2 spacecraft. This paper deals with resulting data from ISIS 2 at 1400 km altitude. Fade frequencies of one to a few hertz on 9.303-MHz fixed-frequency carrier signals were measured and were found to be confined to satellite locations between about 3° and 20° equatorward of the transmitter. The majority of the fades were of the Faraday type, involving ordinary (O) and extraordinary (X) wave components. There was also a smaller number of single-mode fades interpreted as a beat interference between two O-mode or two X-mode rays that take different routes to the spacecraft. Rays traced through model ionospheric density distributions based on tomographic data show that rays launched toward the equator are more susceptible to focusing by latitudinal periodicity than rays launched toward the pole. Such refractive effects can produce two same-mode rays with equal intensities and different propagation directions. Swept-frequency ionograms interleaved with fixed-frequency measurements have also been used to model density distributions in altitude and latitude. These distributions were used iteratively with three-dimensional ray tracing to find rays that connected the transmitter with the position of the satellite at times of interest along its path. Faraday fade rates thus predicted agree with those observed equatorward of the transmitter. Geometric optics cannot account for observed X-mode to O-mode signal ratios of at least 10 dB in other parts of the passes, ruling out deep Faraday fades. This research supports planning for coordinated ground-space radio experiments in the upcoming Enhanced Polar Outflow Probe satellite mission.