In this paper, the onboard computer software program was developed to predict the full trajectory of the missile in order to improve the accuracy at impact. For system in which time is not critical, the alignment process can be performed prior to launch by utilizing navigation data from the launch platform and calibrating the missile's INS to this reference. However, for systems that require rapid reaction time, significant pre-launch delays for alignment are not tolerable. In addition, due to low quality of the INS and its life cycle is short; hence every aborted mission will bring the INS closer to its mean time between failures (MTBF). These problems can be minimized by in-flight alignment (IFA). While IFA may seem to be less accurate and more complicated than alignment in the rest, it turns out that the ability of the carrier to maneuver during the IFA phase is a blessing in disguise since it enables the excitation of latent modes and thus, enhances their observability of the whole INS. The effects that maneuvers have on the estimability of the INS states during IFA were investigated in the past. But until now, there are many questions such as: what is the degree of observability? What limits the estimability of the system? What are the minimal additional measurements needed to turn the system into a completely observable one? And what are the useful numbers of maneuvers in IFA? , were not completely addressed. These questions are answered through this paper. We introduce the error model, which appropriately describes the INS behavior during IFA. We then substantiate its adequacy for consideration as a piece-wise constant system (PWCS) and then the simpler stripped observability matrix (SOM) can be used in the system observability analysis.