Evaluation of Precise Wide Area Differential GPS Positioning

1. Evaluation of Precise Wide Area Differential GPS Positioning Hyunho Rho and Richard B. Langley Department of Geodesy and Geomatics Engineering University of New Brunswick, Fredericton, N.B. Canada PPP Workshop 2013, 12-14 June 2012, Ottawa, Canada

2. Introductions • A number of wide area differential GPS (WADGPS) satellite-based augmentation system (SBAS) services (e.g., WAAS, EGNOS, GAGAN and MSAS) are now in operation and more are planned for the future. • These free services will enhance the availability of real-time corrections across countries and continents by providing a quality geo-referencing capability especially for single frequency L1 GPS users. • However, the satellite orbit and clock SBAS corrections can also be used to improve the GPS positioning accuracy for dual-frequency users. • The goal of the research described in this presentation is to evaluate the overall performance of the developed algorithm using (real-time) WAAS corrections with respect to various criteria including position convergence time, positioning accuracy and reliability.

3. Review of Some Issues • Issue I: Low Resolution of WAAS fast clock correction: • The SBAS satellite orbit and clock corrections are optimized for use with GPS pseudorange measurements which have about a meter level of measurement noise, i.e., the resolution of WAAS satellite clock corrections is 0.125 m [RTCA, 1999]. • Issue II: Residual orbit and clock errors: • The accuracy of the WAAS satellite orbit and clock corrections is about 30 cm to 50 cm in terms of user range errors and revealed as long term residual variations in the precise point positioning (PPP) process [Rho and Langley, 2007]. • Issue III: Residual clock reference issue (P1-C1 bias issue): • SBAS services typically provide clock corrections that are referenced to the GPS C1 (L1 C/A-code pseudorange) observable. There exist satellite clock referencing issues for the proper use of the corrections for dual-frequency observations [Rho and Langley, 2005].

4. Observation Model with Precise Orbit and Clock where:

5. Observation Model with SBAS Corrections where: where: (1) Side effects from variations of residual orbit and clocks. (2) Varying ambiguity method. (3) Residual satellite and receiver inter-frequency bias.

6. Review for the Issues I Issue I:Low Resolution of WAAS Fast Clock Corrections Correction domain smoothing: Weighted Moving Average Filter : degree of filtering;

7. Review of the Issues I • Issue 1: Low resolution of WAAS fast clock corrections. • With this method, a few cm level of improvements in the horizontal and more than 10 cm level of improvements in the vertical components of errors have been observed. • This factor was identified as a significant factor for using carrier-phase measurements for the PPP process with WAAS corrections. Ref: Rho and Langley (2013). “Precise Point Positioning with GPS Dual-Frequency Carrier-Phase Measurements using WADGPS Corrections”, ION GNSS 2013.

8. Review of the Issues 3 • Issue 3: Residual clock reference issue: • The computed P1C1 biases have been used for the corrections. • When the correction is applied less than 2 cm level of positioning improvements have been observed.

9. Varying Ambiguity Method Residual Orbit and Clock Errors • System noise for varying ambiguity: • Static process: 0.01 • Kinematic process: 0.015

10. Evaluation of Precise Wide Area Differential GPS Positioning in the Static Mode 1) Generating PPP solutions with (real-time) WAAS corrections. Generating PPP results with the IGS Precise Orbit and Clocks 2) ) Generating PPP solutions by using varying ambiguity methodwith WAAS corrections. 3) ) Generating PPP solutions with IGS real-time orbit and clock corrections. Evaluations in terms of Position convergence time Reliability Positioning Accuracy

11. Criteria for the Position Convergence Continuous 7 days of UNBJ data have been used (May 15 to May 21, 2013). • Condition 1: • The convergence time has been taken to be the time when the positioning errors reach the 20 cm level. • Condition 2: • The positioning errors should remain at or below that threshold for at least 50 epochs.

12. PPP Solutions with WAAS Corrections

13. PPP Solutions with WAAS Corrections(Varying Ambiguity Method)

14. PPP Solutions with IGS Real-time Orbit and Clock Corrections

15. PPP Solutions with IGS Precise Orbit and Clocks

16. Position Convergence Time( 7 continuous days at UNB)

17. Positioning Accuracy and Reliability (7 continuous days at UNB )

18. Conclusion I • WAAS orbit and clock corrections can be used for PPP processing with the carrier-phase observable, however, it will be limited by slower parameter convergence compared to using IGS products. • It was shown that the WAAS correction with varying ambiguity method can make improvements in terms of positioning convergence time, accuracy and reliability compared with the WAAS PPP solutions without using varying ambiguity method. • With varying ambiguity method, WAAS solutions are comparable to the solutions using IGS real-time orbit and clock corrections.

19. Conclusion II • Varying ambiguity method could be considered as a possible approach for the PPP process when given satellite orbit and clock corrections are less precise than precise products. • The evaluation results indicate that although a few- centimeter level of positioning accuracy with real-time WAAS orbit and clock corrections in static mode are slightly less accurate than those obtained when using precise orbit and clock data, the technique gives more flexibility and chances for choosing an appropriate positioning technique, which can meet the majority of user expectations for their applications. • And the algorithm developed in this research could be used for seamless PPP solutions with future SBAS corrections when all the planned SBASs are in operation.

20. Thank you very much for your attention!!