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.