Single-frequency RTK GNSS positioning

Abstract

Real Time Kinematic (RTK) GNSS positioning is a carrier-phase differential positioning technique depending upon the fixing of the carrier phase integer ambiguities to ensure cm- level positioning accuracy. It is up to now the most viable technique to achieve cm-level accuracy for kinematic positioning in post-processing mode and particularly in real time mode. (Odijk 2014) discussed RTK and Precise Point Positioning (PPP) techniques for single-frequency case. It is suggested that instantaneous ambiguity fixing is achievable for single-frequency multi-constellation RTK, but single-frequency PPP integer ambiguity fixing is more challenging due to the need of additional information like satellite hardware phase biases and ionospheric corrections. Thus, single-frequency RTK GNSS positioning is the most feasible technique to achieve centimeter-level high precision positioning in real time using low-cost single- frequency GNSS antennas and receivers. It is a promising technique, which answers the growing high-precision navigation demand from industrial drones, self-driving cars and automated farming in which low-cost is crucial to democratize its application. Compared to the expensive geodetic GNSS receivers and antennas, low-cost single-frequency GNSS antennas and receivers do have some limitations in their performances like larger measurement variance and less suppression off multipath errors. In this thesis, the variances of code and carrier phase measurements of the single-frequency antenna and u-blox receiver are analyzed through computing the empirical standard deviation of code and carrier phase residuals in zero baseline tests and short-baseline tests. Multiplying the obtained variances with a proper weighting function of the measurements, a realistic stochastic model is constructed. The author proposes a mixed weighting function, where both C/N0 and satellite elevation angle are taken into account to better dilute the multipath error’s effect. The antenna C/N0 pattern is modeled using measurements of geostationary satellites and this pattern is used as the input to the proposed mixed weighting function. The results show that the proposed weighting function can well detect and down-weight the multipath contaminated carrier phase measurements and leads to better RTK positioning accuracy. The author has also estimated the phase center variations of the Trimble Bullet III antenna by processing the GNSS measurements collected from 4 sessions with the antenna pointing to 0°, 90°, 180° and 270°. Applying the estimated antenna phase center variations to RTK processing indeed reduces the systematic trend and biases/offsets in the carrier-phase residuals. Finally, by comparing the GPS-only RTK solution with GPS + BeiDou and GPS + GLONASS solution, the results indicate that using more satellites from additional constellations can significantly increase the ratio of ambiguity-fixed to ambiguity-float solutions in single-frequency RTK GNSS positioning.