The paper describes a method of estimating a priori the probable accuracy of high-frequency direction-finding bearings based on the estimation of the probable value of the standard deviation or variance to be associated with the observation. To apply the method the bearing observations must be made according to a prescribed form, which consists in taking about ten ????????snap???????? readings of the bearing spread over about five minutes. The arithmetic mean of this group of bearings is taken as the observation, and the scatter or spread of the readings in the group provides one of the quantities necessary for the estimation of the probable variance. Other components entering into the estimation of the variance are (a) site error component based on past history of the direction-finder, (b) ionospheric lateral deviation component based on known data, and (c) observational error component based on the quality (i.e. flatness) of the minimum. In a practical trial of the method, good agreement was obtained between estimated and actual bearing errors. For example, a check applied at the 90% probability level showed that 85????????90% of the bearings did have their variance correctly estimated.
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