Strapdown ins sensors' calibration by celestial observations and distinctiveness of free phase trajectory

Major constituent of the trajectory of a ballistic vehicle is its flight in the free space where the aerodynamic forces are negligible due to insignificant air density. After the burnout point the vehicle falls freely under the influence of earth gravity. Under this special condition the accelerometer output comprises sensor's noises only. Navigation error during the free phase evolves as a function of sensors errors and alignment inaccuracy at the burnout point. The error growth rate sensitivity during the free phase to the prevailing imprecision at the burnout point is analyzed. The simulation results reveal that accelerometer's bias and velocity error at the burnout point constitute the major fraction of error accumulated during the free phase. Making use of the free fall condition for estimating the accelerometer bias and observation of distant stars for estimation of gyro drift, a technique is proposed to minimize the navigation error after the burnout point.