The value of Inertial Navigation Systems (INS) is reduced for long time-duration flights because uncertainties produce increased position errors with time. A reduction in this position error buildup is obtained from a mechanization concept described in this .paper. This mechanization comprises an Electrically Suspended Gyroscope (ESG) mounted on the stabilized platform of a local-level INS. Position is estimated by tracking the local vertical while accounting for the Earth's rotation with an accurate clock. Subsystems are integrated by means of a Kalman-Bucy filter. An accuracy of 0.2 n. mi./hr for 12 hr of flight was obtained in a simulation study using an ESG and a 1 n. mi./hr INS.