Measured and simulated scattering functions and their impact on high frequency (HF) systems

The wide variety of propagation mechanisms which are apparent over high frequency (HF, 3-30 MHz) ionospheric propagation paths, can limit the performance of communication systems (especially data modems). These mechanisms are particularly pronounced at high latitudes, yet there has been little work done to characterise these paths in terms of the severity and the frequency of occurrence of the problems which they display.;Measurements of Doppler spread, multipath spread and signal to noise ratio have been made on four high latitude, HF communications paths with the DAMSON channel sounder. The measurement system and analysis techniques are outlined, and an analysis of the data pertinent to the design of robust HF data modems is presented. Physical interpretations of the data are made and the data is related to the measured performance characteristics of an HF data modem to estimate the modem availability on the paths considered.;A new HF propagation channel model has been investigated. The model differs from existing Watterson model simulations by including Lorentzian as well as Gaussian Doppler spreads, and by including the facility to model delay spreads. Its performance is assessed relative to a Watterson model simulator by testing both serial tone and FSK modems. The effects of correlation between the fading simulated at adjacent delay taps is investigated and discussed, and the effect of the delay spread shape used in the simulation is also investigated.;DAMSON data has been used to drive the new model in an HF simulation, and the performance of three different modems over a six hour period has been assessed. The chosen period contained both quiet and disturbed propagation conditions. The results are compared to existing techniques for determining modem availability using modem performance surfaces generated using a Watterson model simulator.