Effect of inter-station height difference on GPS positioning precision

GPS, which recently has also become a main element in daily life and especially geodesy, is a three-dimensional triangulation system utilized in both locating time and position. Vertical and horizontal positions can be located simultaneously by a single method via GPS. However, some sources of error exist affecting the certainty of this system.In this project, we will examine the effects of refraction and deterioration of satellite signals on the accuracy of horizontal/vertical position, while passing through atmosphere. To effectuate this project, first we have determined on appropriate bases for IGS Stations and then downloaded suitable data for those bases over the internet. After that, these data has been evaluated by academic software, GIPSY Solution Model, and the results have been analyzed via Microsoft Excel.The selected bases were 10-16km at length and with varying altitudes between 50-1630m. There were a total of 26 bases, which were studied by 24-hour static data collection.Evaluation process has been performed in three phases:First evaluation aimed to investigate the effect of troposphere layer over the north component. To understand this first, the 24-hour data pertaining to 26 bases have been evaluated in Microsoft Excel; then their average component values and corresponding root mean squares have been calculated and finally we have reached a conclusion on the effect of increasing altitudes of bases over north component. Having completed the analysis, the same operation has been applied by 1, 2, 3, 4, 6, 8 and 12-hour data, and the results have been compared with those provided by 24-hour data.The second and the third evaluation aimed to investigate the effect of troposphere layer over east component and vertical component. The same operations in the first evaluation has been performed for this phase too and finally concluded by comparison of 1, 2, 3, 4, 6, 8 and 12-hour data with 24-hour data.Consequently, it has been observed that, as the observation time gets shorter, the GPS positioning accuracy decreases, which mostly affects the vertical component. Another important result is as the altitude difference between base points increases, the positioning accuracy gets even worse especially for the vertical component. The factors mentioned above also effects horizontal components, yet this affect remains at ignorable levels (<1cm).Keywords: continuous GPS applications, ellipsoidal height difference, GPS accuracy depending on length of session