Will PPP ever replace RTK? Over the past decade, we have seen the convergence of the PPP and RTK techniques towards “PPP-RTK,” where satellite orbit/clock/bias corrections are augmented by local atmospheric corrections to enable instantaneous convergence to cm-level accuracies. But how close are we to instantaneous cm-level PPP-AR, without local augmentation?
In the July 2018 edition of the GPS World Innovation column, Denis Laurichesse (CNES) and myself explained how single-epoch (or “snapshot”) PPP-AR could lead to cm- to dm-level positioning accuracies, thanks mainly to the Galileo E6 signal. We proved this concept practically using a small network of 6 stations in Australia to compute phase biases on all GPS and Galileo signals.
Now that more IGS stations are tracking all constellations and signals, Denis has pushed this concept further by implementing global phase-bias estimation on a routine basis. Using a global network of stations, he derived phase and code biases for all constellations (GPS, GLONASS, Galileo and BeiDou). These biases allow for multi-GNSS and multi-frequency ambiguity resolution, except for BeiDou (widelanes only) and GLONASS (no AR). Ambiguity resolution capabilities on these signals enable instantaneous horizontal accuracies between 1 cm (full ambiguity resolution) and 20 cm (widelane ambiguity resolution). Real-time PPP monitoring using these products is available on the PPP-WIZARD website.
Denis has also implemented a post-processed solution based on GFZ’s MGEX orbit and clock products. The estimated phase and code biases are available on his FTP server since 10 December 2018. New products are being made available in batches every month or so. Daily bias and attitude files are produced, the latter ensuring satellite orientation consistency (more details in a future post). The GFZ orbit and clock files are available on the CDDIS FTP server.
The following two plots, courtesy of Denis, illustrate the type of results that we can obtain from these products. Figure 1 shows epoch by epoch multi-GNSS solutions obtained from fixing only widelane ambiguities: already, a 10-cm horizontal accuracy is achievable *without* precise atmospheric corrections from a local or regional network.
Figure 1 – Horizontal estimates from multi-GNSS epoch-by-epoch solutions with widelane ambiguity resolution (courtesy of Denis Laurichesse, CNES)
The magic really happens when performing ambiguity resolution on the full set of ambiguities, as shown in Figure 2. At this point, instantaneous cm-level accuracies become possible!
Figure 2 – Horizontal estimates from multi-GNSS epoch-by-epoch solutions with full ambiguity resolution (courtesy of Denis Laurichesse, CNES)
So how close are we to instantaneous cm-level PPP-AR solutions? With these multi-GNSS, multi-frequency GNSS products, you can try it out and answer this question for yourself!
Thanks a lot to Denis Laurichesse who makes these products available to the GNSS community. This is truly AWESOME!
Laurichesse D, Banville S (2018) Instantaneous cm-level multi-frequency precise point positioning. GPS World, July 2018