On the Feasibility of Instantaneous Multi-GNSS Multi-Frequency PPP-RTK

Ambiguity resolution enabled precise point positioning (PPP-RTK) immediately provides centimeter-level positioning information, once the carrier phase ambiguities are correctly resolved. With the ongoing GNSS development and modernization, a steadily growing number of satellites can be observed on multiple signal bands. The joint processing of multi-frequency multi-GNSS data leads to much stronger positioning models with shorter convergence times. However, we show that even when combining GPS, Galileo, BDS-2/3, and QZSS on up to five frequencies, instantaneous reliable ambiguity resolution is not feasible. Instead of fixing the ambiguities, we make use of the MSE-optimal best integer-equivariant estimator to investigate the limits of the positioning performance that can be achieved with the current constellations. A simulation-based analysis shows that instantaneous centimeter-level horizontal PPP-RTK results can indeed be expected when combining all four systems with three or more frequencies. Real-data experiments are used to confirm these findings. For an exemplary day, empirical instantaneous horizontal RMS positioning errors of 7–8 mm are observed when combining four systems and three or five frequencies.