The accuracy of a gyroscope free INS is deeply affected by the bias,
scale factor and misalignment. However, these error parameters can be
obtained by calibration. This paper presents two research issues. The first is an improvement to the original six-accelerometer model to achieve more realistic performance. The second is a novel calibration method for the proposed six-accelerometer INS. In navigation applications the improved model will determine the rotational and translational acceleration of a vehicle from the six accelerometer outputs. The proposed calibration method will decrease the system error to enhance the navigation performance.