Tropospheric range-error corrections for the Global Positioning System

The Global Positioning System (GPS) is a highly accurate navigation system that has a broad spectrum of military, civilian, and commercial applications. It uses a triangulation scheme based on the time delays of signals from the satellites to the user; these time delays are then equated to distances. However, as the timing signal passes through the Earth´s atmosphere it undergoes an additional time delay due to the index of refraction. The time delay produced by the troposphere approaches a value corresponding to a range error of about 25 m for an elevation angle of 5° and decreases with increasing elevation angle to less than a few meters at zenith. It has been shown that there is a good correlation between the range error and the surface index of refraction. Worldwide statistics of surface refractivity have been analyzed and shown to be correlated with site latitude, height above sea level, and time of year. Regression lines for range-error corrections based on these parameters are derived. Range-error accuracies vary from about 8% down to 3.7% of the total range error, depending on the amount of information that is available