STABILITY AND ERROR ANALYSIS FOR ABSOLUTELY CALIBRATED GEODETIC GPS RECEIVERS

摘要

The absence of absolute calibration data for geodetic-quality Global Positioning System (GPS) receivers and their associated equipment has limited the application of these instruments for time transfer. We have conducted a series of tests in order to calibrate two such geodetic receivers. Receiver calibration is made with respect to a dual-frequency GPS simulator. Inaccuracies due to the simulator itself are minimized by the calibration procedure. The antenna and associated cables are calibrated using a vector network analyzer. Incorporating this information, we are able to provide an error budget for the absolute delay of an entire GPS receiver site. Methods for reducing this error budget are proposed. The absolute calibration results for one receiver are compared to the results from a similar calibration performed 1 year earlier. Calibration stability is found to be on the order of 1 nanosecond for pseudoranges transmitted on both of the GPS frequencies (1575.42 and 1227.6 MHz). Laboratory calibration values are compared to in situ measurements in two modes: two receivers running off one antenna with a common clock, and two receivers running off two separate antennas with a common clock. Agreement is within the expected uncertainties based on the error budget described above. Using baselines on the order of 1000 km. calibrated GPS Carrier-Phase time transfer results are then compared to calibrated GPS Common View and Two-Way Satellite Time Transfer results. Agreement is again observed within the expected uncertainties. Calibration of the carrier-phase technique must also include errors due to modeling of the observations (e.g.. satellite orbits, atmospheric propagation) that are beyond the scope of this paper.