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An improved calibration method for a strap-down inertial navigation system (SDINS) is presented here. Calibration accuracy in conventional method is vulnerable to gyro uncertain errors such as random walk, Markov noise and dead band, etc. Earth-rate injection technique is suggested to reduce the calibration errors induced by gyro errors by replacing gyro output measurements with true earth-rate projections in static positions of calibration sequences. The true earth-rate projections to the IMU body frame are calculated with DCM (Directional Cosine Matrix) which is estimated using the information of accelerometer measurement. Simulation results show that calibration accuracy is significantly improved by incorporating the earth-rate injection technique. [ABSTRACT FROM PUBLISHER]