Applying Time-Differenced Carrier Phase in Nondifferential GPS/IMU Tightly Coupled Navigation Systems to Improve the Positioning Performance

This paper presents how the time-differenced carrier phase (TDCP) can be implemented in nondifferential global positioning system/inertial measurement unit (GPS/IMU) tightly coupled navigation systems to improve positioning performance. The TDCP is expressed as a conventional TDCP, representing the carrier phase difference between two successive GPS epochs, and a modified TDCP, which is defined in this paper as the carrier phase difference between current and reference GPS epochs. Both of these two TDCP expressions are implemented in a GPS/IMU tightly coupled navigation Kalman filter as observations: The conventional TDCP with a more accurate approximation of the velocity integration is applied to estimate velocity, whereas the modified TDCP is used for position estimation. When initialized with a high-accuracy reference position, the proposed navigation strategy can achieve submeter positioning accuracy in the nondifferential mode without estimating ambiguities. Compared with the conventional TDCP-only method, the inherent position drift problem of the TDCP observations can also be avoided. The absolute positioning accuracy of the proposed conventional+modified TDCP measurement update method is determined by the reference position, since the TDCP, including conventional and modified TDCP, is a relative measurement. However, even when initialized with a low-accuracy reference position, the relative positioning accuracy (or precision) of the proposed method is still very high, and the reference positioning accuracy can be maintained, owing to the proposed method. The proposed method can greatly improve the positioning accuracy, as compared with traditional pseudorange+Doppler GPS/IMU tightly coupled navigation systems.