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Polarization/Inertial/Celestial (PIC) integrated navigation system presents a promising solution for attitude information acquisition in unfamiliar and electromagnetic interference environments at night. However, the fusion strategy and the installation errors among polarization sensor (PS), star sensor (SS), and inertial measurement unit (IMU) can affect system accuracy. To improve the system performance, this paper proposes an enhanced nighttime attitude determination method for PIC integrated navigation system. In comparison with existing schemes, a novel system model is devised, wherein installation errors are augmented into the state variables, with the angle between moon vector provided by PS and star vector provided by SS being utilized to establish measurement models. By this means, simultaneous estimation of attitude misalignment angles and installation errors can be achieved. Furthermore, in order to enhance the nighttime environmental adaptability, a trimodal fusion strategy is presented, which could assess the effectiveness of the polarization sensor and star sensor, and selects and switches between Polarization/Inertial/Celestial (PIC) mode, Polarization/Inertial (PI) mode, and Inertial/Celestial (IC) mode. Finally, a series of simulation experiments validate the feasibility and effectiveness of the proposed approach.
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