ION GNSS 2012
Session E6: Precise Point Positioning 2

Title: Precise Point Positioning with GPS Dual-Frequency Carrier-Phase Measurements using WADGPS Corrections
Author(s): H. Rho and R.B. Langley, University of New Brunswick, Canada
Date/Time: Friday, September 21, 2012, 2:35 p.m.
Room: 209/210 (NCC)

Satellite orbit and clock corrections from the Wide Area Differential GPS (WADGPS) (e.g., MSAS in Japan, EGNOS in Europe and WAAS in U.S.A) services can be used to improve GPS positioning accuracy for dual-frequency users by carefully take into account the satellite clock referencing issue as well as properly handling the increased noise level of the ionosphere-free dual-frequency combination [Rho and Langley, 2005]. However, since a primary purpose of WADGPS is the provision of real-time DGPS corrections across countries and continents, providing a quality geo-referencing capability especially for single frequency L1 (C/A) GPS users, the WADGPS satellite orbit and clock corrections are optimized to use for GPS pseudorange measurements which have about a meter level of measurement noise, i.e., the resolution of WADGPS satellite clock corrections is 0.125 m [RTCA, 1999] and 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 a long term residual variations in the precise point positioning (PPP) process [Rho and Langley, 2007]. However, to achieve high accuracy PPP results with GPS carrier-phase measurements using WADGPS corrections, these special issues should be carefully taken into account in the positioning process.

The goal of the research described in this paper is the design of a GPS dual-frequency carrier-phase data processing technique capable of producing high-accuracy positioning results with WADGPS corrections. For this purpose, first, a correction-domain weighted-moving-average filtering method has been investigated to mitigate the low resolution of WADGPS correction issue to improve the final positioning accuracy. Since the resolution of the WADGPS corrections gives rise to a noise-like peak-to-peak variation within a specific range and the low resolution of the clock corrections is the main source of the increased noise in the corrections, the WAAS clock corrections have been precisely analyzed and the proper weight of the filter has been determined. Secondly, to take into account the long term variations of the WADGPS satellite orbit and clock corrections in the PPP process, the varying ambiguity concept has been applied rather than assuming the ambiguity is constant over time. The main purpose of this assumption is to make an ambiguity parameter to absorb the satellite dependent long-term variations of the WADGPS satellite orbit and clock corrections by use of a stochastic approach in the sequential least squares method. To determine the optimal process noise for ambiguity parameter, the searching process has been conducted in all over the possible ranges of system noise, from 0 to infinity. And the optimal process noise has been chosen at a value of the minimum positioning errors.

For analyses conducted using the developed method, a worth of data set from six stations in the IGS network with data spanning eight days from January 1 to January 8, 2011 has been used. The processed results showed that the effect of the noisy WAAS fast corrections is an important factor for the developed algorithm and the significant improvement of about 16 cm levels has been observed in the daily r.m.s. of height component errors when the correction domain smoothing process has been applied in the kinematic mode. The results also showed the correction domain smoothing filter for WAAS orbit and clock corrections is effective and can make more reliable and better point positioning solutions than the compiled positioning results using original corrections. In terms of the varying ambiguity approach, the main advantage of this method has been observed as an improvement in the physical representations of other parameter estimates. For example, the estimated tropospheric zenith path delays (ZPDs) at the station(s) showed a better agreement with respect to the IGS final ZPDs when the varying ambiguity method has been applied. The level of agreement was about 3 cm in terms of r.m.s. errors when the fixed ambiguity method has been applied, however it was about 1 cm level with the varying ambiguity method in the static mode. For the positioning accuracy with the developed algorithm, a few cm levels of horizontal accuracies and less than 10 cm level of height component errors could be achievable in the static process. In the kinematic case, the results showed that the overall accuracy could be achievable in about 20 cm level.

This paper presents the detailed description of observation model and methodologies to properly handle the issues as well as the data processing and precisely analyzed results which have been conducted with the developed algorithm. Furthermore, the analyses of different positioning performances between the developed WADGPS PPP method and PPP with the precise orbit and clock products (such as those from IGS) are also presented. The presented results in this paper could serve as a baseline for further improvement in PPP with WADGPS corrections and the developed algorithm could be used to extend the benefits of WADGPS corrections into more precise applications.

Reference:
Rho, H. and R.B. Langley (2005). "Dual-frequency GPS Precise Point Positioning with WADGPS Corrections." Proceedings of ION GNSS 2005, 18th International Technical Meeting of the Satellite Division of The Institute of Navigation, 13-16 September 2005, Long Beach, CA; pp. 1470-1482.

Rho. H and R.B. Langley (2007), "The Usefulness of WADGPS Satellite Orbit and Clock Corrections for Dual-Frequency Precise Point Positioning." Proceedings of ION GNSS 2007, 20th International Technical Meeting of the Satellite Division of The Institute of Navigation, 25-28 September 2007, Fort Worth, TX; pp. 939-949.

RTCA Special Committee-159 (RTCA) (1999), Working Group 2, Minimum Operational Performance Standards for Global Positioning System/Wide Area Augmentation System Airborne Equipment, RTCA Documentation Number DO-229B, 6 October 1999, 255 pp.

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