Traceability in metrology and uncertainty evaluation of a calibration system for GPS receivers

GPS is already a main method of positioning measurement in geodesy and is applied widely in many fields. For maintaining and ensuring the accuracy of positioning, an accurate and efficient system for calibrating the GPS receivers must be established. A highly accurate GPS calibration network, tied to the ITRF coordinates of IGS stations, can be effectively used to evaluate the performance of GPS receivers. This study addresses the feasibility of establishing a system for calibrating GPS receivers and the system's traceability in metrology. Uncertainties of the GPS calibration network established and maintained by NML (National Measurement Laboratory, Taiwan) are evaluated based on the method suggested by the ISO (International Organization for Standardization). The uncertainties of NML network coordinates are obtained and used as a basis for calibration. The results of the slope distances between pillars measured by the GPS processing units and the precise EDM units are discussed. Analytical results indicate that the 3-D expanded uncertainty of the main station TNML of the network in the ITRF system is around 33.2 mm at the 95% confidence level. The 3-D expanded uncertainties of the calibration points of ultra-short distance network and short distance network are evaluated to be about 2.2 mm, and 3.4 mm in relation to the main station TNML, respectively, at the 95% confidence level. The precision of the NML network coordinates suffices to calibrate the geodetic and navigational GPS receivers of regional users and is available through the Internet.