Observations of the directional characteristics of obliquely propagating HF radio signals and simultaneous HF radar measurements

In order to investigate the complex propagation mechanisms prevalent in the high latitude regions, a technique has been developed in which an ionospheric channel sounding signal from the DAMSON sounder system is received on a multi-channel, spaced antenna array receiving system. In addition to determining the channel scattering characteristics, the technique enables the direction of arrival of each of the components of the received signal, separated by time of flight and Doppler frequency shift, to be estimated. Initial measurements of this nature made over long paths from Isfjord and Harstadt to the UK yielded several interesting results. In particular, it was possible to relate the DF observations of the sounding signals to measurements made well to the side of the propagation paths with the CUTLASS HF radar located at Hankasalmi in Finland (this radar is a geophysical research instrument). In these tests, the directions of arrival of the various signal components were found to be related to the precise Doppler frequency of the various signal components, an observation attributed to a drift of ionospheric irregularities embedded in the background electron density over the mid point of the propagation path. This observation was consistent with measurements of ionospheric backscatter made with the CUTLASS HF radar