Observations of Doppler spreading and FSK signalling errors on HF signals propagating over a high latitude path

Several interesting features of the propagation of HF signals over high latitude paths have been observed: (a) large Doppler spreads, often in excess of 10 Hz, are frequently observed on all paths indicating the presence of a turbulent 'rough' reflector which produces multiple sub-modes. (b) The Doppler spreading of trans-auroral oval signals is well correlated with the position of the average auroral oval relative to the propagation path. This region is a source of major disturbance. (c) The bit error rate of low speed FSK signals is not influenced by the high level of Doppler spreading observed on trans-auroral oval paths. The error rate is well related to the received SNR and is in good agreement with the theoretical distribution. (d) The distribution of bit errors is markedly affected by changes in the Doppler spreading of the received signals. At high Doppler spreads, the length of the error bursts is significantly less than for the cases where the Doppler spreading is small. However, no significant affect on the length of the guard space between error bursts is evident. The large spread in the frequency spectrum of the received signals is a well known feature of high latitude propagation. However, the present studies show that the Doppler spread is sensitive to auroral activity and to the geometry of the propagation path in relation to the auroral oval. Furthermore, it has proved possible to relate the Doppler spreading imposed on the signal by the ionospheric reflection processes to the error distribution characteristics of low speed FSK signals.<
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