GPS Satellite Geometry and Its Implications for Structural Deformation Monitoring

GPS is widely used in precise engineering applications, such as landslide monitoring, construction plant control and structural deformation monitoring. In these applications, uniform three dimensional positioning precision in a local coordinate system is required. Some of these applications have specific precision requirements in the vertical direction, for example when analyzing land subsidence, while others are concerned with the north or east direction such as evaluating the structural deformations caused by wind loading. It is important to analyze the achievable positioning precision with the current GPS constellation and potential augmentation techniques for such engineering applications. Using a newly developed GPS/Galileo performance assessment tool, GPS satellite geometry and its impact on precise positioning have been analyzed. The tool estimates dilution of precision (DOP) values, such as Easting DOP (EDOP), Northing DOP (NDOP) and Vertical DOP (VDOP) in a local coordinate system, using the precise GPS ephemeris as an input and site location and cutoff angle as constraint parameters. It demonstrates when a GPS only system is employed the worst VDOP can reach 37 in mid-latitude areas on a daily basis. For improving satellite geometry, new ephemerides for GPS and Galileo are used with the performance assessment software and similar DOP estimations are made. Calculation illustrates that the maximum VDOP value can drop significantly to less than 3. In this paper, the use of this GPS/Galileo performance assessment software is introduced in the context of structural deformation monitoring. The changes of DOP values on a daily basis for different areas and their potential effects on positioning precision are presented. It demonstrates that the launch of Galileo will improve the positioning precision and integrity of the whole monitoring system.