Analysis and modelling of pseudorange and carrier-phase biases in GNSS Precise Point Positioning

Unlike GPS, GLONASS observations are affected by the Frequency Division Multiple Access (FDMA) satellite signal structure, which introduces inter-frequency channel biases (ICBs) and other system biases. The effects of these biases are visible in the pseudorange and carrier-phase residuals, which affect GNSS Precise Point Positioning (PPP) convergence period, un-differenced ambiguity resolution and overall positioning accuracy. Current research has shown the correlation between receiver stations from heterogeneous networks, such as the International GNSS Service (IGS), in PPP processing and the increase in magnitude of the pseudorange and carrier-phase ICBs in GLONASS-only PPP solutions. Discounting other system biases which may be present, the correlation is due to mixed receiver and antenna hardware types, differences in firmware versions, and irregularities in the updates of the receiver equipment at the stations. With new and expanding satellite constellations, it is expected that PPP convergence period will decrease due to improved geometry, more observations and stronger signals. However, the inclusion of GLONASS has introduced additional biases that need to be accounted for in the data processing or else this relationship will not hold. So, does the current performance of GLONASS PPP reflect the limits of the processing technique or by accurate modelling of GLONASS biases, can there be improvements in the solution accuracy and reliability? And can the behaviour of the ICBs help mitigate the effect of these biases that compromise the solution integrity of GLONASS PPP? With an increasing number of receiver and antenna hardware types available, the error modelling for the pseudorange and carrier-phase biases becomes more complex. In the GNSS community, there is only a limited understanding of these equipment biases, which introduce varying magnitudes of observable error due to each receiver-antenna combination. A strong correlation between the pseudorange and carrier-phase inter-channel biases and receiver firmware and antennas exists, which relates to the differences that exist in the estimated inter-channel biases and similar firmware of the same receiver types. Currently, there is no standard correction format for PPP users in relation to these biases given a specific receiver firmware or antenna type. This research proposes a possible GLONASS inter-frequency channel bias correction using 350 IGS stations, based on 32 receiver types and 8 antenna types, by observing the unique trends observed in the bias estimates in relation to the GLONASS satellites. Given the unique trends of the inter-channel frequency biases with respect to frequency channels, receiver and antenna types, modelling the pseudorange ICBs show an improvement in the initial convergence period of 20 minutes with a reduction of 13% for GLONASS PPP. While these pseudorange and carrier-phase equipment biases do not cause significant long-term errors on GLONASS PPP positioning results, and have almost no effect on GPS PPP results, they impact initial float ambiguity and associated float covariance estimates. By improving these estimates, more accurate fixed PPP solutions can be produced and more quickly. Further analysis will be done to evaluate the realism of the associated float covariances with bias modelling, and the impact on PPP fixed solutions.