基於卡爾曼濾波器之合成時間尺度建立

Abstract

時間，如影隨形地出現在你我生活之間。於日常生活中，所有與時間相關之應用皆需計時。一個相當精確的時鐘可作為時間尺度(Time Scale)，其如同一把尺，吾人可藉由比較以得到待測時鐘與其之相對時間差。本論文採用一時鐘集合，其內部皆為同樣等級之振盪器，藉由演算生成一虛擬時鐘，並將其作為時間尺度用以提供各時鐘參照與計時。 精確時間協定(Precision Time Protocol, PTP)為IEEE 1588標準所定義，其適用於區域網路系統各節點之即時時間同步。時間伺服器(Time Servo)為PTP之核心，其將透過網路訊息交換所獲取的時間戳記資料，加以運算得到從時鐘(Slave Clock)相對於主時鐘(Master Clock)之時間偏移量，並可進一步藉由控制法則讓從時鐘頻率不斷地微幅調整。 本論文於自行建立之區域網路下，採用精確時間協定常駐程式(Precision Time Protocol daemon, PTPd)開放原始碼，量測各從時鐘節點與主時鐘節點之時間偏移量，並透過卡爾曼濾波器估測其相對於主時鐘節點之實際相位，再藉由加權平均的演算，得知於過去運行量測相位程序時所對應之合成時間尺度與主時鐘節點的相位關係。在此系統架構下，透過修正型亞倫方差(Modified Allan Deviation)進行從時鐘節點及合成時間尺度之頻率穩定度分析，並模擬各從時鐘節點與此合成時鐘之同步行為。相較於時鐘集合內任一實體時鐘，透過演算所得之合成時間尺度具有長期穩定度及可靠度較佳之優點。

In our life, time follows us wherever we go. All time-related applications require a timing mechanism. A fairly accurate clock can be used as a time scale. It’s like a ruler, by comparison, we can obtain the relative time difference between it and any other clock. In this paper, we use a set of clocks to generate a virtual composite clock, and set this composite clock as a time scale for timing. Precision Time Protocol (PTP) is defined in the IEEE 1588 Standard. It applies to a local-area network system for real-time time synchronization between two nodes. The time servo is the heart of PTP. Through calculation , we can get the time offsets between the slave clocks and the master clock from the timestamp data, which is obtained by message exchanging over the network. Furthermore, we can use these offset data as the input of the controller to adjust the slave clock frequency such that the frequency of the slave clocks and that of the master clock are the same. In this paper, the Precision Time Protocol daemon (PTPd) is used to measure the time offsets between the master clock and each slave clock over a network we established. Then we utilize the Kalman filter to estimate the time offset of each slave clock and generate the composite time scale. Within a four-node network, Modified Allan Deviation is used to analyze the frequency stability of each clock node and the composite time scale. Also, we simulate the time-synchronization behavior between each clock node and the composite time scale. We find that the reliability and long-term frequency stability of the composite time scale is superior to those of any individual clock in the ensemble.