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Gyroscope biases may drift in long-term storage, and the drifts become the most important factors that affect the azimuth alignment accuracy of the inertial navigation systems (INSs). In this article, a three-position in- field calibration method is proposed, providing a completely new way to solve the gyroscope bias estimation problem without the turntable available. Based on the analysis of the azimuth errors of attitude tracking, the attitude tracking information at the current position is used to extract the azimuth error of self-alignment at the previous position. The observation equations are derived, with the input arguments of the differences between the attitude tracking results and the self-alignment results at the last two positions, and the solution vectors of the gyroscope biases. Condition number is used to analyze the ill-conditioned problem of the equations. Simulation and experiment results show that the gyroscope biases can be effectively calibrated by designing the rotation angles reasonably. [ABSTRACT FROM AUTHOR]