VARIABILITY OF GPS SATELLITE DIFFERENTIAL GROUP DELAY BIASES

Global Positioning System (GPS) satellites can be used to measure the total electron columnar content of the ionosphere with relatively high accuracy and low cost. One of the most important limiting factors in using this technique, however, is the assessment of the differential instrumental delay biases between the L band frequencies in both the satellites and the receivers. These differential L2-L1 biases must be measured and removed from the GPS measurements before an accurate estimate of the total electron content can be obtained. A series of measurements from the Block I GPS satellites indicates that the combined satellite and receiver differential L2-L1 biases can be measured with an accuracy of 0.5 ns differential delay. An investigation of the variation of the satellite differential L2-L1 biases over a time span or five weeks shows that these biases are quite stable with time with less than 0.3 ns variation. A follow-up experiment conducted two years later showed that the satellite biases had not changed significantly over this longer time span. A comparison of the measured satellite differential biases with the prelaunch calibration values was made for four satellites. The measured biases for two of the satellites agreed with the prelaunch values to within the error estimates of the measurements, but the biases for the other two differed significantly.