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Precise carrier phase positioning is a widely used technique in vehicle navigation, localization, and mapping, which utilizes pseudorange and carrier phase measurements with the extended Kalman filter (EKF) to estimate position, velocity, and ambiguities. For highly accurate vehicle positioning requirements, propagation errors of measurements need to be considered, especially the ionospheric delay, which is the most significant error source. In EKF, a priori information of different error characteristics can be fully utilized as constraints to obtain more precise state estimation results, even when no corrections are available. This paper aims to improve the positioning accuracy for vehicle navigation and proposes a spherical harmonic function (SHF)-based undifferenced and uncombined (UDUC) carrier phase positioning method for vehicles to fit the ionospheric delays of multiple satellites in a single formula and reduce the marginal effect caused by polynomials. Better positioning results are expected compared to traditional UDUC and polynomial fitting approaches. The feasibility of the proposed method is tested on a kinematic dataset from a real-world car test. The experimental results show that the proposed method outperforms the traditional UDUC and polynomial fitting methods, respectively, with a root mean square error of 0.860m and a standard deviation of 0.579m when using broadcast ephemeris. In summary, the results demonstrate that the proposed method is highly beneficial for precise vehicle navigation.INDEX TERMS Carrier phase positioning, state estimation, spherical harmonic function, undifferenced and uncombined, vehicle navigation.