This paper deals with determining errors in gimballed inertial navigation systems. One of the aims of the study is to obtain expressions for transformations between different coordinate systems and expressions describing the system kinematics. The appropriate matrices of directing cosines have been obtained. The expressions for correction of reading of accelerometers taking into consideration rotation of the Earth, motion of the moving vehicle, and Coriolis acceleration have been obtained. Features of the considered system lie in the usage of the trajectory coordinate system as a basic navigation system contrary to the geographical reference frame and the usage of the two-axis gimballed platform. The structure chart and the necessary instrumentation of the researched system are represented. The equations for the determination of methodical errors for the inertial stabilization and inertial system have been obtained. The simulation of the basic errors of the studied system has been carried out. The obtained results of the simulation show the high accuracy of the researched system.