Combined channel sounding and direction finding studies of HF radio propagation effects observed over high latitude communication paths
The high latitude ionosphere is a dynamic region that is subject to a variety of disturbed conditions affecting oblique propagation of HF radio signals. Multipath and off great circle propagation resulting from rough ionospheric reflecting surfaces introduce time dispersion, while movements in the ionosphere impose Doppler dispersion. These effects, when combined, result in very complex signal characteristics, which cause degradation to the performance of communication systems if the delay or Doppler dispersion exceeds system dependent bounds.;Results are presented based around measurements of signals, produced by the Doppler And Multipath SOunding Network, recorded using a six channel spaced array receiving system, during a 10 day campaign at Kiruna in March 1998. In addition to delay and Doppler processing, the directions of arrival (DOA) of the signal components were estimated using a super resolution direction finding algorithm.;Most signals were narrow in delay and Doppler, however a significant statistical spread in their DOA was observed. Statistics on the occurrence and characteristics of the remaining signals revealed that trends in DOA are often associated with complex delay and Doppler propagation. A number of cases had Doppler spread exceeding a specified Doppler boundary of operation, while none exceeded the delay spread boundary.;Case studies revealed detailed propagation effects. Of particular interest is the often observed East â†’ West trend in DOA with Doppler offset. This is attributed to scattering from irregularities embedded in turbulent bulk convection flows. E-region modes with good spectral and geographic distribution of signal energy are often observed in otherwise complex cases.;Spatial filtering, using two or three element arrays was investigated and found to be effective at reducing large Doppler spreads to lower values more amenable to high data rate communication systems. Fast solver spatial filtering methods were found to be very robust, effective and a lot quicker.