mil as viewed on the mil scale the accuracy of the baseline is related to the certainty in the position of the gps ground stations the p code of the gps system enables the gps ground stations to determine a more accurate position than does the ca code present gps ground stations can use the p code to determine its position within an uncertainty corresponding to a spherical volume of approximately 6 meters in diameter as compared to a spherical volume of approximately 30 meters in diameter using the ca code thus it is considered preferable for the present invention to use gps receivers that can receive the p code the long base line between the gps stations further minimizes any error in establishing the angle of the base line caused by the uncertainty in the position of the gps ground stations and may be established so as to obtain a desired accuracy the uncertainty in the location of the gps ground station however does not necessarily produce a corresponding error in the angle of the base line it is thought that the uncertainty in geographic position is approximately reciprocal for gps ground stations in close geographic proximity if the calculated position of neighboring gps ground stations is in error by the same amount in the same direction the angle of the base line connecting the stations does not change relative to an arbitrary direction such as north and the.