一種用於雙向衛星時頻傳遞之軟體定義無線電接收機

Abstract

國際度量衡局(Bureau international des poids et mesures, BIPM)根據各國家計量機構(National Metrology Institute, NMI)所維持原子鐘的穩定性，權衡產生國際協調時(Coordinated Universal Time, UTC)，是時間的國際標準。雙向衛星時頻傳遞(Two-Way Satellite Time and Frequency Transfer, TWSTFT)能夠測量國際之間兩個NMI標準時間差，是國家標準時間同步UTC的方法之一。周日效應是TWSTFT主要的測量誤差，會導致測量值以日為週期有數百皮秒的起伏變化，在文獻上，其原因可能來自衛星地面站設備延遲的變化、信號雙向傳播時間延遲的變化、以及TWSTFT接收機的測量誤差等，然而確切的原因並未明朗。 TWSTFT的運作為，兩個NMI藉由衛星通訊發射擬似亂(Pesudo-Random Noise, PRN)碼信號，並測量對方信號的抵達時間(Time of Arrival, TOA)。鎖延遲迴路(Delay-Locked Loop, DLL)是現有接收機的測量TOA的方法，可能造成穩態誤差和拉回誤差，另外，多路徑干擾和多重接取干擾也可能造成測量誤差。為了避免這些誤差，本論文使用軟體無線電(Software-Defined Radio, SDR)搭配GPU平行運算單元設計PRN碼接收機，使用開路測量取代DLL；另外，藉由SDR接收機更高的取樣速率，實現了窄距相關器以抑制多路徑干擾；再者，藉由SDR接收機豐富的資源，實現即時繼次干擾消除(Successive Interference Cancellation, SIC)以抑制多重接取干擾。 為了測試SDR接收機是否能降低TWSTFT測量誤差，在中華電信研究院(Telecommunication Laboratories, TL)測量兩台原子鐘的時間差，結果現有接收機呈現周日效應，而SDR接收機則無；接著測試SDR是否能提升國際時頻傳遞穩定性，在日本情報通信研究機構(National Institute of Information and Communications Technology, NICT)以及韓國標準科學研究院(Korea Research Institute of Standards and Science, KRISS)現有的TWSTFT系統上附加SDR接收機，測量NICT-TL和KRISS-TL的時間差，現有接收機呈現了明顯的周日效應，而SDR接收機則無，另外，KRISS-TL啟動了SIC抑制多重接取干擾進而提升了短期穩定性。有鑒於SDR接收機能改善周日效應，BIPM與時頻諮議會(Consultative Committee for Time and Frequency, CCTF)為了提升UTC的準確性，成立了先鋒研究團隊 (Pilot Study Group, PSG)，在國際上十餘個NMI的現有TWSTFT系統上附加SDR接收機，根據數個月的測量結果，使用SDR接收機都能提升短期穩定性，特別在幾個時頻傳遞結果上，明顯看出SDR接收機改善了周日效應。

The Bureau international des poids et mesures (BIPM) generates Coordinated Universal Time (UTC) by weighting the atomic clocks in several National Metrology Institutes (NMIs). This relies on time transfer techniques to obtain the time differences between atomic clocks in different places. Two-way satellite time and frequency transfer (TWSTFT) is a time transfer technique that has been used for several NMIs to synchronize their local realizations with UTC. However, as atomic clocks become more stable, the stability of TWSTFT must improve. The time-difference fluctuations with a period of one day, which are known as the diurnal effect, make it difficult to increase this stability. Previous studies have shown that this effect may be caused by unstable radio-frequency (RF) components, unequal propagation delays between two paths, or inaccurate measurement in TWSTFT receivers. However, the dominant factor has not been identified. This dissertation focuses on the measurement algorithm for TWSTFT receivers. During TWSTFT, each NMI receives pseudo-random noise codes that are transmitted from the others via a satellite link and then measures the times of arrival (TOAs) using a receiver. The conventional algorithm uses two correlators and a delay-locked loop (DLL). Using the DLL, the dynamic behavior of the TOA can produce steady-state and pull-in errors that make the measurement inaccurate. The multipath error and multiple access interference (MAI) are difficult to mitigate due to that limitations of hardware resources. Therefore, a TWSTFT receiver that uses a software-defined radio (SDR) technique is proposed. The receiver comprises an analog-to-digital conversion sampler and a computer with a graphic processing unit (GPU) board. The SDR receiver implements the algorithm, including the multiple correlator and the open-loop scheme. The high-resolution correlator and the successive interference cancellation (SIC) respectively mitigate multipath effect and MAI use parallel computation that is provided by the GPU board. For evaluation purposes the SDR receivers are installed in three Asia-Pacific NMIs – TL in Taiwan, NICT in Japan, and KRISS in South Korea – for comparison with currently used receivers. There is clear suppression of the diurnal effect in the TWSTFT between TL and KRISS. The multichannel property of the SDR receiver is enabled a two-by-two TWSTFT for TL and NICT and an unstable amplifier is identified. When the amplifier is removed, the stability of time transfer between TL and NICT is improved. Because of this improvement, the BIPM and the Consultative Committee for Time and Frequency (CCTF) working group on TWSTFT launched a project to install the SDR receivers in NMIs in Asia, Europe and the USA to generate UTC. Twelve NMIs, which have a total weight of about 40% in UTC, are operating the receiver. Most TWSTFT results show an improvement in short-term stability. In particular, some exhibit clear mitigation of the diurnal effect.