Abstract: A navigation system includes an inertial measurement unit, a navigation computer, a GPS receiver, and a clock controller. The inertial measurement unit has a first clock and a first switch, the navigation computer has a second clock and a second switch, and the GPS receiver has a third clock. The clock controller controls the first and second switches. Accordingly, the inertial measurement unit, the navigation computer, and the GPS receiver may use their own clocks, or the inertial measurement unit and the navigation computer may use the second clock, or the inertial measurement unit, the navigation computer, and the GPS receiver may use the third clock.