A Feasible Clock Control and Synchronization Method Used in CAPS Master Station

W. Jing, X. Lu, J. Wang, D. Zhao, C. He, Y. Rao

Abstract:	In the Navigation Satellite Timing and Ranging system, the task of base-band circuit is to compute the distance between satellites and users by measuring the propagation time-delay. The measurement of the propagation time-delay is closely related to the local clock accuracy. That is to say the local clock performance plays an important role in determining the accuracy of the propagation time-delay, and determining the ranging accuracy. This paper studies the influence of the local clock drift on the measurement accuracy of ranging base-band. The Oven-Controlled Crystal Oscillator (OCXO) is used in ranging base-band in order to keep the oscillator frequency at a constant temperature. The temperature of the oven is constantly checked by an electronic circuit to shield the oscillator from temperature changes in the environment. The need exists for further controlling the frequency of an OCXO to provide enhanced stability over greater temperature range. The method is presented based on microcomputer controlling technique. As is known to all, an OCXO comprises: a crystal oscillator, a heater, a frequency compensator, a sensor. The microcomputer is connected to the resistor in series with the heater by A/D converter, and the task is monitoring a power consumption of the heater by measuring a voltage drop across the resistor. We study a relationship between the power consumption of the heater and a required frequency compensating signal. According to a predetermined relationship, the microcomputer provides a frequency compensating signal periodically. Observational data of the voltage drop across the resistor include not only the long Period Trend but also the short period trend, so the smoothing algorithm is presented based on Kernel smoother. In real control system, it is impossible to sample the voltage value nonstop, and adjust the frequency compensator very quickly. The sampling period and adjusting period is studied by MATLAB simulation. Then apply the simulation result to the control of real system, we can obtain the optimal value of sampling period and adjusting period by trial and error. In this system, the clock signal is divided to four channels. Each channel clock is amplified and filtered. In this process, the phase difference may be induced. In order to ensure Zero-phase accordance very strictly for four channels clock signal, we need measure phase difference for four channels clock signal very precisely. The measuring algorithm is presented based on the recurrence characterization of the frequency signal. During half period between –pi/2 and pi/2 of the frequency signal, one phase corresponds to one voltage. If the frequency stability is very well, these voltage values should have repeatability. If we sample the signal at k epoch during the same period for clock1 and clock2 channel signal, we can get two voltage values: V1 and V2. If V2 is greater than V1, it means that the clock2’s phase is ahead of clock1’s phase. On the contrary, if V2 is less than V1, it means that the clock2’s phase lags behind clock1’s phase. The formula between phase difference and voltage difference is derived in this paper. For the above mentioned reason, the measuring accuracy of voltage value is the key factor. In order to get the voltage very precisely, a precision Analog-to-Digital (A/D) converters with 24-bit resolution is used in the microcomputer system. A/D converter resolution is 24 bits can ensure that the voltage resolution can be reached microvolt magnitude. This is helpful to improve the phase resolution, and Zero-phase difference among four channel clock signal can measure very delicately. The practical results indicated that the control algorithm can adjust the frequency correction in real time. The method is simple and effective, and reaches the desired goal. The research of this paper makes contributions to the control technology of local clock and the measurement of the Zero-Phase inconsistency for different channel clock signal. Readers may refer to it according to their demands.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	3516 - 3520
Cite this article:	Jing, W., Lu, X., Wang, J., Zhao, D., He, C., Rao, Y., "A Feasible Clock Control and Synchronization Method Used in CAPS Master Station," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 3516-3520.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In