Oblique ionogram features associated with off-great-circle HF propagation at high latitudes

The presence of horizontal electron density gradients and irregularities in the ionosphere cause HF radio signals to deviate from the great-circle path joining transmitter and receiver. Such deviations have serious implications for the operation of position location systems operating within the HF band. In order to investigate the nature of these large scale bearing deviations and to relate them to the mode content of the signal, various experiments have been undertaken over paths for which simultaneous bearing measurements and forward oblique soundings were available. Two types of measurement are examined in this paper. In the first type, the soundings were made over the full HF band at 30 minute intervals together with the bearing measurements at 90 second intervals at three fixed frequencies. These measurements were obtained over two paths in Canada. In the second type of experiment, the bearing measurements were made on the sounder signal itself (a so-called sweeping-DF measurement). These latter experiments are similar to those made with an interferometric DF system which have previously been reported by Black et al. (1993, 1995), however in this instance a goniometric DF system was employed.