Tropospheric range error corrections for the NAVSTAR system

NAVSTAR is a triservice satellite navigation system designed to provide the means for military aircraft, ships, and ground users throughout the world to obtain precise position and velocity information at any time of day, in any kind of weather. Since the user's position is measured by the time delays between his location and several satellites, the additional delay due to tropospheric refraction must be taken into account. For angles above 5 degrees the time delay, or corresponding range error, can be estimated from the local surface refractivity, if known. The worldwide statistics of surface refractivity analyzed in this report show that refractivity is correlated with site latitude, height above sea level, and season, all of which should be readily known. Regression lines for range error corrections based on these parameters are derived, and the accuracy of the corrections are estimated. At 5 degrees, the minimum elevation angle at which NAVSTAR is intended to be used, the range error is about 80 ft.