The directional diversity of HF signals received over high latitude paths and the possibility of improved data throughput by means of spatial filtering

The high latitude ionosphere is a disturbed region containing irregularities which may be regarded as providing a rough reflecting surface for obliquely propagating HF radio waves. Signals associated with each propagation mode may arrive at the receiver over a range of angles in both azimuth and in elevation. In addition, ionospheric movements at the reflection points impose Doppler shifts and Doppler spreads onto the signal. Various researchers have been engaged in an experimental programme of channel sounding over high latitude paths with the DAMSON pulse compression sounder (Davies and Cannon 1993). The system allows signal strength, time-of-flight, time dispersion due to multipath propagation and frequency dispersion Doppler spread and Doppler shift to be measured over point-to-point communications paths. All of these parameters are of great importance to the design and operation of HF communications equipment in the high latitude regions. It is suggested here that signals may be combined from two or more antennas in such a way as to, in effect, steer beam(s) or null(s) in the directional sensitivity pattern of the receiver antenna so as to exploit the directional diversity of the incident signals in order to reduce the delay and Doppler spreads. Such a technique is potentially important for increasing the data throughput of communications systems employing modern, high speed modems operating within the high latitude environment