Precise Point Positioning (PPP) is a typical positioning technique based on GNSS pseudorange, carrier phase observations and precise satellite orbit and clock products. It has been widely used on geological researches and navigation applications by virtue of centimeter-level positioning accuracy without any reference stations. However, PPP has been constantly suffering from long convergence time of up to a few tens of minutes to get reliable centimeter-level positions owing to poor satellite geometry and pseudorange precision. This defect greatly hindered the further use of PPP on some real-time applications such as disaster early warning and autonomous driving. On the other hand, for geologists, PPP positioning precision is better to be as high as possible (millimeter-level) to reflect more geological signals. We have thus been dedicating to PPP rapid convergence and high-precision positioning since 2009 and focusing on two aspects.
GNSS Carrier-phase measurements typically have very low noise (millimeter-level) compared to pseudorange counterparts (decimeter-level), while they have any number of integer cycles. If the value of integer ambiguities were obtained exactly, unambiguous carrier phase observables would be recovered and enable a reliable centimeter-level positioning without any seconds of convergence time. Ambiguity Resolution (AR) is thus a breakthrough of the slow convergence of PPP. Since PPP-AR was firstly proposed by Ge in 2008, we had carried out some innovative and effective researches towards rapid PPP-AR. We demonstrated the equivalence relation of ambiguity resolution with fractional-cycle biases (FCB) and integer-recovery clocks (IRC) in 2010; in 2012, we proposed the improved narrow-lane FCBs derived from an ambiguity-ﬁxed GPS network solution for a high-precision FCB estimation; we proposed a triple-frequency PPP-AR approach for the rapid convergence in 2013; we achieved the estimation of GLONASS phase biases across inhomogeneous receivers in 2016; we also enabled inter-system ambiguity resolution among GPS and BeiDou for a better ambiguity resolution performance in 2018; we proposed an global instant decimeter-level positioning approach called PPP-WAR in 2019, aiming at time- and safe-critical navigation applications. Now we are chasing for the reduction of PPP-AR convergence time and improvement of positioning accuracy using multi-GNSS and multi-frequency data.
Integer ambiguity resolution at a single station can be achieved by introducing predetermined phase biases into the float ambiguity estimates of PPP. This integer resolution technique has the potential of leading to a PPP-RTK (Real-Time Kinematic) model where PPP provides rapid convergence (a few seconds) to a reliable centimeter-level positioning accuracy based on an RTK reference network. Using the precise ionosphere delay information estimated from a local reference network, real-time PPP is able to resolve undifferenced ambiguities successfully in several seconds. So PPP-RTK is another potential technique to extend PPP application scenarios.
We had firstly applied the local ionosphere delay corrections on PPP for the rapid re-convergences to ambiguity-fixed solutions in 2010, after which we proposed a PPP-RTK model for the rapid ambiguity resolution in 2011. In 2017, we performed GPS and GLONASS ambiguity resolution simultaneously by introducing ionosphere corrections estimated from a dense reference network and demonstrated that real-time PPP solutions could be initialized successfully within 5 min. We will still take efforts on regionally augmented PPP to meet the requirement of real-time GNSS positioning applications.
1. Integer ambiguity resolution in precise point positioning: Method comparison. J. Geod (2010)
2. Rapid re-convergences to ambiguity-ﬁxed solutions in precise point positioning. J. Geod (2010)
3. Improving the estimation of fractional-cycle biases for ambiguity resolution in precise point positioning. J. Geod (2012)
4. Triple-frequency GPS precise point positioning with rapid ambiguity resolution. J. Geod (2013)
5. GLONASS fractional-cycle bias estimation across inhomogeneous receivers for PPP ambiguity resolution. J. Geod (2016)
6. Rapid initialization of real-time PPP by resolving undifferenced GPS and GLONASS ambiguities simultaneously. J. Geod (2017)
7. Inter-system PPP ambiguity resolution between GPS and BeiDou for rapid initialization. J. Geod (2018)
8. Toward global instantaneous decimeter-level positioning using tightly coupled multi-constellation and multi-frequency GNSS. J. Geod (2019)
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