Synchronized clock using GPS carrier phase with self-calibration in holdover mode

Synchronized clock using GPS (Global Positioning System) carrier phase double-difference with self-calibration in holdover mode is presented. The GPS receivers used in our system were elaborately modified in order to estimate the frequency offset of the remote Oven-Controlled Crystal Oscillator (OCXO) clock with respect to the primary cesium atomic clock in real time by performing the double-differences on the GPS carrier phase observables. The PD (Proportional-Derivative) controller is employed to implement the controller of our system. Through the D/A converter, the remote clock was then steered to synchronize with the primary clock. Moreover, in the holdover mode, the predicted corrections of the frequency offset and aging are applied to the remote clock to improve the frequency accuracy when the GPS signals are absent. For averaging times of one day under the configuration of about a 30-meter baseline, our experimental results show that the accuracy of the remote clock can be improved from about 3x10(-9) to about 3x10(-14), and the stability of the remote clock can be improved from about 3x10(-10) to about 2x10(-14). With the Self-calibration, the remote clock can be improved to obtain the frequency accuracy of about 1x10(-11) in holdover mode.