ION GNSS 2012
Session E6: Precise Point Positioning 2

Title: Cycle-Slip Correction for Single-Frequency PPP
Author(s): S. Banville, Natural Resources Canada/University of New Brunswick, Canada; R.B. Langley, University of New Brunswick, Canada
Room: 209/210 (NCC)

Recent developments in GNSS positioning include broadcasting state-space corrections to users to reduce the contribution of various error sources. This approach was mainly popularized by the precise point positioning (PPP) technique, using a global network of reference stations to compute such corrections. Lately, this principle was also applied to local and regional networks, defining what is now termed PPP-RTK. This state-space approach allows users to process undifferenced observations and still obtain cm-level accuracies, usually after a convergence period of a few minutes. For single-frequency users, this convergence period is often much longer if precise ionospheric corrections are not available. Properly handling interruptions in carrier-phase tracking and the resulting cycle slips is thus crucial in maintaining a continuous and precise solution. This paper proposes a general approach to mitigate the impacts of cycle slips, which is applicable to any number of frequencies and to all GNSS, but the results presented herein focus on single-frequency applications.

Several functional models can be used to process single-frequency GNSS data, which are often based on filtered code observations or on the group and phase ionospheric combination (GRAPHIC). It is shown that, for the purpose of cycle-slip correction, including both carrier-phase and code measurements into a single system without forming any combination is greatly beneficial. A PPP solution is then carried out, which includes the estimation of receiver position and clock offset, stochastic ionospheric parameters and carrier-phase ambiguities. When discontinuities are detected in carrier-phase observations, their size is estimated by adding cycle-slip parameters to the PPP filter. An attempt is then made to fix those parameters to integers, which allows constraining the change in position between epochs and maintaining a continuous solution.

While the performance of single-frequency cycle-slip correction is typically affected by code noise, integrating this process within the PPP solution greatly mitigates this error source since all observations are used simultaneously in a weighted least-squares adjustment. As opposed to computing an explicit time-differenced solution, the PPP solution allows averaging the noise over several epochs, acting as a filtering procedure. This approach also explicitly models the receiver and clock dynamics, which alleviates another shortcoming of cycle-slip correction techniques using Doppler measurements or high-order carrier-phase differences. The benefits of our method are demonstrated through various scenarios, from static receivers with a 30-sec interval rate to high-rate data from kinematic platforms.

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