The intrinsic high accuracy of modern HF direction finders cannot be realized in practice due to the perturbations and tilts which exist in the ionosphere. Particular attention is given in this dissertation to the effect of Travelling Ionospheric Disturbances (TIDs) since their occurrence is difficult to predict and the magnitude of the induced bearing error extremely variable.\ud \ud TID activity is quantified by monitoring simultaneously the bearings of a number of transmitters whose locations and frequencies are well known. This information is then employed to determine the expected bearing error on a target transmission reflected in the same area of the ionosphere. The major limitations of this correction scheme are investigated.\ud \ud The large-scale temporal and spatial variations of the quiet ionosphere (Systematic Ionospheric Tilts, SITs) produce bearing errors of similar magnitude to those due to TIDs. Rapid calculation of these errors for various path geometries and ionospheric conditions is possible by raytracing through 3-D ionospheric models. The limitations of a correction scheme which employs a 3-D model based on ionospheric predictions are examined.\ud \ud Variance is a particularly useful statistic in assessing the 'reliability' of bearings made on a particular HF circuit. A scheme is proposed in which the bearings of a network of reference transmitters are monitored continuously allowing a variance to be assigned to any target transmission, even if it is only present for a very short time. These variance values are of considerable value in practical DF applications.\ud \ud Whenever possible the experimental observations are related to the underlying physical processes in the ionosphere which generate these DF errors
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