Factors affecting the estimation of GPS receiver instrumental biases

Global positioning system (GPS) has been widely used to investigate the ionosphere through the estimation of the total electron content (TEC) and its distributions in space. One of the important factors affecting the ionosphere TEC estimation accuracy is the hardware differential code biases (DCBs) inherited in both GPS satellites and receivers. This paper investigates various factors affecting GPS receiver instrumental bias estimation accuracy. Through a number of designed tests, we concluded that the most important factor is the ionosphere model accuracy. Some of large daily bias variations of receiver DCB detected by other studies, such as receivers in low latitude regions, are not due to the DCB changes, but the estimation errors. The DCB estimation values can vary significantly for different ionospheric models and different sizes of networks. For example, the receiver DCB values obtained from the global and the station- specific models exhibit difference from -2·5 to 14·3 TEC unit (TECU) for different stations. Different data processing methods also contribute to DCB estimation errors. The results from smoothing and non-smoothing GPS observation show that the difference of DCB reaches up to 6·8 TECU for some stations, with the mean difference of 3 TECU. On the other hand, the elevation cut-off angle does not play an import role in ionospheric delay estimation. For elevation cut-off angles from 10 to 30°, our tests show that the DCB estimation differences are <0·4 TECU.