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TitleA real-time clock combination supporting precise point positioning with ambiguity resolution
AuthorBanville, S; Laurichesse, D
Source 2017.
Alt SeriesNatural Resources Canada, Contribution Series 20170082
MeetingIGS Workshop; Paris; FR; July 3-7, 2017
Subjectsremote sensing; satellite geodesy; satellites; RTCM-SSR phase-bias message; IGS real-time clock combinations; ESOC; BKG; precise point positioning (PPP); ambiguity resolution (AR)
Illustrationsgraphs; diagrams
ProgramCanadian Spatial Reference System, Geodetic Survey
AbstractThe real-time service of the International GNSS Service (IGS) provides, among other products, global GNSS satellite orbit and clock corrections to support sub-decimeter precise point positioning (PPP) solutions. This information is transmitted via messages defined by the Radio Technical Commission for Maritime Services Special Committee 104 (RTCM-SC104), in a format referred to as state-space representation (SSR). A proposal for a RTCM-SSR message supporting phase biases for PPP with ambiguity resolution (PPP-AR) has been put forward, although it has not yet been adopted by the committee. Still, two analysis centers are currently transmitting these proposed messages as a proof of concept, namely the Centre National d¿Études Spatiales (CNES) and Natural Resources Canada (NRCan). At the user end, phase-bias messages can enable faster converge of PPP solutions due to the integer nature of the carrier-phase ambiguities.

The clock combination products generated by the IGS currently ignore phase-bias messages transmitted by analysis centers. Real-time PPP users are therefore forced to choose between the reliability of the IGS combination and the PPP-AR possibilities offered by individual analysis centers. For this reason, the present investigation looks into the feasibility of combining not only satellite orbit and clock corrections but also phase and code biases. Proper methods for combining these biases lead to a combined clock product enabling PPP-AR at the user end. This study has been conducted jointly by CNES and NRCan in an effort to show the inter-operability of RTCM-SSR products for PPP-AR and led to the development of an orbit and clock combination software.

An important characteristic of satellite clock corrections allowing for PPP-AR is that they can be aligned very precisely by determining, for each satellite, an integer number of cycles between two analysis centers. This concept is similar to carrier-phase ambiguity resolution in a network solution. Once these offsets are determined, the continuity of phase biases can be maintained even if one analysis center experiences issues such as resets in their clock estimates. This characteristic leads to an improved robustness of the combination while providing users with PPP-AR capabilities. Analysis centers that do not provide phase biases can also be included in the combination, although only a floating-point offset can be determined for aligning satellite clocks and, therefore, they do not contribute to the definition of the phase biases.

The proposed definition of the RTCM-SSR phase-bias messages also includes information regarding the attitude of the satellite, called the yaw angle. This value is also of critical importance for maintaining the highest consistency between analysis centers and to assure an integer property of the estimate clock offsets. For this reason, the combination software includes a special wind-up correction accounting for a rotation of the satellite during eclipsing periods. Testing has demonstrated, however, that such a correction is not always adequate for large orientation discrepancies of satellites with significant satellite antenna offsets. Therefore, the software is capable of determining eclipsing periods and use an operator-defined weighting scheme for each analysis centers during yaw maneuvers.

Initial results from the combination of the CNES and NRCan RTCM-SSR feeds are presented, along with PPP performance and convergence results with respect to the current IGS real-time clock combination.
Summary(Plain Language Summary, not published)
NRCan provides GPS solutions that enable clients to obtain consistent cm-level positioning in the Canadian Spatial Reference System. For the key product provided to NRCan¿s clients (Precise Point Positioning - PPP), ambiguity resolution plays a critical role in achieving fast convergence to expected accuracies. This presentation describes a collaboration between NRCan and the Centre National d'Études Spatiales (CNES) in France on the development of a real-time GPS clock combination software. The purpose of this project is to improve the reliability of real-time clock products supporting PPP with ambiguity resolution.