New Improvements in HF Ionospheric Communication and Direction Finding Systems

This chapter presents new HF (3–30 MHz) systems dedicated to ground to ground radio links with applications to ionospheric characterisation, channel modelling, radio communications, direction finding and single site localisation. The received signals result from the vectorial addition of the multipaths generated by the ionosphere. Considering the acquisitions at the outputs of an array of identical antennas (homogeneous array), a high level of spatial and temporal correlation can be observed. Therefore, it appears relevant to additionally discriminate the incoming modes by considering their polarisations. The purposes of the different systems which are described in the following sections are the use of a heterogeneous array. This polarisation-sensitive solution for array processing is principally characterized by the spatial distribution of non identical antennas. Consequently, the applications to digital communication involve a multi channel processing in the receiver as a SIMO (single input multiple output) structures. The correlation factors depend on the polarisation characteristics of the incident wave. Moreover, the heterogeneous array is still efficient with a reduced space diversity (set up in a limited place), the differences in the polarisation parameters balancing the weak values of the differential geometrical phases. In the following developments, the suggested techniques aim to take some better advantage of the ionospheric medium in several applications.

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