Abstract: The accuracy of GPS relative positioning is limitted by several sources of errors.The effects of the following biases are studied in this paper:relative and absolute trop-ospheric refraction,ionospheric refraction,and the deviations in the height and horizontal coordinates of the fixed station from their true values.To estimate their effects the for-mulae are derived for the condition that the visible satellites are uniformly distributed.If the above condition is not satisfactory a program GPS-RPBE developeb dy the author can be used.The investigation made in this paper leads to several rules,which provide batter understanding of the nature of the effects of different types of errors and the measures to be taken in order to improve the accuracy of relative positioning.