An investigation of hf data communication circuits at high latitudes.

The high latitude ionosphere forms a highly dynamic and disturbed medium which can severely degrade the performance of hf radio links. Characteristics of the signal which may be affected include signal strength, fading rate and the frequency range over which signals may propagate. Furthermore, large Doppler frequency shifts and spreads may be imposed on the signal due to movements in the ionosphere near to the reflection point. To investigate these effects, signals from a transmitter located at Clyde River on Baffin Island in the Canadian North West Territories were received near Fairbanks, Alaska and at Alert, Elsmere Island. Doppler spread, signalling error rates and error distribution patterns occurring were measured during one month campaigns in summer 1988 and winter 1989. Several interesting features were observed: (a) Large Doppler spreads, often in excess of 10 Hz were frequently observed on the trans-auroral path and their occurrence was well correlated with the average position of the auroral oval relative to the path reflection points. (b) The bit error rate of low speed FSK signals was not influenced by the level of Doppler spreading observed. Error rate was well correlated with the received signal to noise ratio and was in good agreement with theory. Signal to noise ratio was well correlated with predicted variations in ionisation due to changes in solar zenith angle. (c) Distribution of errors were related to changes in Doppler spreading. Large Doppler spreads, were correlated with significantly shorter bursts of error was less than when Doppler spreading was small. However, there was no significant correlation with the length of guard space between errors. This work is of particular relevance to researchers developing high latitude prediction programs and those employing channel simulators to assess communication systems.