Methods and apparatus are present for determining position of a rover from observations of GNSS signals. Observations of GNSS signals are obtained at a rover location, and observations of the GNSS signals are obtained at a plurality of reference stations, each reference station defining a respective baseline between the rover location and a reference station location. For each reference station, a respective differentially-corrected rover position is determined, wherein at least one of the differentially-corrected rover positions is based on one of (i) a multiple-frequency ionosphere-free observable combination, (ii) a multiple-frequency code-phase observable combination, and (iii) a single-frequency carrier-phase and code-plus-carrier-phase combination. A weighted combination of the differentially-corrected rover positions is prepared.