Identifying the degraded environment and bad receivers setting by using the GPS data quality indices

To ensure the accuracy of positioning, an accurate and efficient system for calibrating the Global Positioning System (GPS) receivers must be constructed, especially when thousands of GPS tracking stations around the world are established for a variety of applications. In Taiwan, an auto-analytical system for analysing data quality is already established to monitor the GPS stations. However, the relationships between the data quality and positioning precision are not clearly addressed yet, and this is the main goal we want to achieve. In this study, six indices are chosen to characterize the relationship between the data quality and GPS positioning precision. They are the number of observations, multipath on L1, multipath on L2, occurrence of cycle slips, relative frequency offset and frequency instability. To reduce common errors, fourteen types of GPS receivers were selected and set up at an Ultra-Short Distance Network of NML (National Measurement Laboratory, Taiwan) in the years 2002, 2003 and 2004. The results of this study indicate that the frequency instability is the most critical factor among the six indices of interest to influence the GPS positioning precision. The frequency instability may cause a positioning difference of as much as 17.5 mm in the precision of three-dimensional coordinates. The second and third most critical error sources are the number of observations and the multipath effect, respectively, with a maximum difference of 10.7 mm. In contrast, for the method of parameter estimation, the cycle slips and relative frequency offset have minor effects on positioning precision with a few occasional exceptions. The findings of this study will serve as a crucial indicator to classify the positioning error of the tracking stations for long term analysis.