Ionospheric modelling and HF radio systems at high latitudes

Successful planning of HF radio systems to operate at high latitudes requires some means of predicting ionospheric morphology, for both ambient electron density and ionospheric irregularities causing backscatter. The presence of several physical processes, and the sparsity of data, mean that reliable statistical models are not easily constructed. However, increasing knowledge of the processes operating, and the availability of powerful computers, makes possible the construction of physically-based numerical models of ionospheric structure. This paper describes such a model at a fairly advanced stage of development, and gives examples of calculated electron densities. The ingredients and short-comings of the model will be described, together with the steps to be taken to account for ionospheric irregularities, and to allow the prediction of HF system performance using ray-tracing.