Accelerometer bias and gyro drift determines the long term precision of the strapdown inertial navigation system (SINS) which is the primary and critical component of the strapdown airborne gravimeter (SAG). Making use of the complementary characteristic of DGPS and SINS has been widely and successfully used in many practical applications to prohibit the long term drift on condition that they are consistent in time and space, which stands for time synchronization and lever arm effect respectively. The paper extends kalman filter with lever arm as one of its states and gives the observability analysis in a global perspective. Observability shows that at least two segments of the trajectory with linearly independent angle rate make the lever arm observable. Simulation demonstrates that 10 centimeters error in three quantities of the lever arm vector has an impact at the level of 3 mGal in the horizontal accelerometer bias but little effect on the vertical quantity. Different simulations prove the analysis and highlight the pitch maneuvers during the climbing of the flight test and emphasize the importance of the angle rate to the estimation of the lever arm rather the magnitude of the angle itself.