Wide-Area Between-Satellite Single-Difference VTEC Ionospheric Model and Its Assessment for Undifferenced and Uncombined PPP

  Objectives  High-precision ionospheric correction is an important prerequisite for undifferenced and uncombined precise point positioning (PPP) with fast convergence capability. Different from the traditional use of pseudoranges to extract ionospheric information, the ionospheric accuracy obtained by PPP with fixed ambiguity is higher.

  Methods  A wide-area vertical total electron content(VTEC) ionospheric model is developed based on a reference network, where the VTEC of each station is extracted from the undifferenced and uncombined PPP approach. The proposed VTEC model is expressed in the between-satellite single-differenced polynomial function of the sites' latitude and solar hour angle. A data processing procedure and software system are developed for both processing server and user PPP client. In the server part, the VTEC ionospheric delay of the whole network is estimated, the VTEC polynomial function is established and the model parameters are broadcasted to the PPP clients. In the user PPP client, real-time positioning using the broadcasted wide-area VTEC parameters are performed.

  Results  Experiments using the GNSS network in the European region are performed, and results show that the internal RMSE(root mean squared error)and external RMSE of the derived VTEC model is around 3 TECus and 1 TECu for the GLONASS(global navigation satellite system) and GPS, respectively. User kinematic PPP test results using the new VTEC ionospheric model show that, 78 of the whole 136 (57.35%) kinematic undifferenced and uncombined PPP hourly samples converge in 5 minutes, where for 97 samples (71.32%) converge in 10 minutes, 122 samples(89.7%)converge in 15 minutes and 132 samples(97.06%)converge without 25 minutes. In the unconstrained PPP experiment, the results in the above convergence time are 15 (11.03%), 64 (47.06%), 91 (66.91%), and 110 (80.88%), respectively.

  Conclusions  Experiments show that single-difference VTEC ionospheric model has a huge improvement in the accuracy of extracting the ionosphere and shortening the real-time PPP convergence time in a wide-area.