• Journal of Positioning, Navigation, and Timing
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• 제5권2호
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• pp.67-74
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• 2016

In this paper, time comparison is performed with standardization institution in Japan using a Two-Way Satellite Time and Frequency Transfer (TWSTFT) technique as one of the methods for high precision time comparison. To analyze the performance of time comparison in the TWSTFT method, time comparison results via the Global Positioning System (GPS) code and carrier wave are analyzed. Through the time comparison performance, frequency stability is analyzed using modified Allan deviation and by this result, characteristics of time comparison of the TWSTFT that is utilized in international time comparison are presented.

• Journal of Positioning, Navigation, and Timing
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• 제12권2호
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• pp.201-209
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• 2023

Time comparison techniques are required for generating and keeping Coordinated Universal Time (UTC) and to distribute standard clocks. These techniques play an important role in various fields, including science, finance, military, and communication. Among these techniques, Two-Way Satellite Time and Frequency Transfer (TWSTFT) ensures a relatively high accuracy, with a time comparison accuracy at a nanosecond level. However, TWSTFT systems have some limitations, such as the dependency on extra network links. In this paper, we propose an improved method for TWSTFT system operation and design a message structure for the suggestion. Additionally, we estimate the data rate and redundancy for the new TWSTFT signal with the designed message structure.

• Journal of Positioning, Navigation, and Timing
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• 제12권3호
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• pp.245-255
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• 2023

Time comparison techniques are necessary for generating and keeping Coordinated Universal Time (UTC) and distributing standard time clocks. Global Navigation Satellite System (GNSS) Common View, GNSS All-in-View, Two-Way Satellite Time and Frequency Transfer (TWSTFT), Very Long Baseline Interferometry (VLBI), optical fiber, and Network Time Protocol (NTP) based methods have been used for time comparison. In these methods, GNSS based time comparison techniques are widely used for time synchronization in critical national infrastructures and in common areas of application such as finance, military, and wireless communication. However, GNSS-based time comparison techniques are vulnerable to jamming or interference environments and it is difficult to respond to GNSS signal disconnection according to the international situation. In response, in this paper, Code-Division Multiple Access (CDMA) based All-to-All TWSTFT operation method is proposed. A software-based simulation platform also was designed for performance analysis in multi-TWSTFT signal environments. Furthermore, code and carrier measurement jitters were calculated in multi-signal environments using the designed simulation platform. By using the technique proposed in this paper, it is anticipated that the TWSTFT-based time comparison method will be used in various fields and satisfy high-performance requirements such as those of a GNSS master station and power plant network reference station.

• 제어로봇시스템학회논문지
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• 제15권3호
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• pp.346-352
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• 2009

UTC (Coordinated Universal Time) has been made by the comparison results and the statistical analysis of primary clocks maintained by national standard institutes. Some kinds of technique have been used for international time transfer; since 1980s the study on methods and development of time transfer has conducted with activation of GPS application. And the more accurate and easier method made it use the official time transfer method for the generation of UTC. But recently TWSTFT (Two-Way Satellite Time and Frequency Transfers) as well as GPS time transfer are increasing in number because the TWSTFT is able to improve the accuracy and precision of time comparison owing to the elimination of the ionospheric and tropospheric delay errors thanks to the reciprocal propagation path. In this paper, we introduce the TWSTFT results by a multi-channel modem comparing with GPS P3-code.

• 한국항해항만학회:학술대회논문집
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• 한국항해항만학회 2006년도 International Symposium on GPS/GNSS Vol.2
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• pp.479-481
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• 2006

1967년 시간의 기본 단위인 초가 세슘원시계에 의해 정의된 이후, 각 국의 표준기관에서는 보다 정확한 원자시계개발 연구와 평가를 수행하고 있다. 이러한 연구는 정확한 시간척도(Time Scale)와 국제원자시를 생성하여 산업과 과학분야에 기여하기 위한 것이며, 이에 필연적으로 국제간 시각비교 개선을 위한 연구도 함께 발전되어 왔다. 전파를 이용한 시각비교는 지상파를 이용하던 70년대 초부터 시작되었는고 1981년부터 GPS에 의한 시각비교 방법이 소개된 이후, 80년대 후반에 들어서면서 GPS 활성화에 따라 급격히 시각비교 정확도가 향상되었다. 그러나 GPS와 더불어 통신위성을 이용한 양방향 시각비교(TWSTFT)의 필요성에 따라 세계 선진 표준기관들은 이 방법을 수행하고 있다. 한국표준과학연구원(KRISS)에서도 통신위성을 이용한 양방향시각비교를 구축하여 운용하고 있으며 아시아, 오세아니아, 유럽지역과 비교를 할 수 있는 시스템을 구축하여 운용하고 있다. 본 논문에서는 PAS-8위성을 이용한 KRISS와 NMIA(호주)와 양방향시각비교를 수행함에 있어 필수적으로 계산 또는 측정하여야하는 오프셋값을 결정방법으로 GPS와 Circular-T를 이용하여 산출한 결과를 제시한다.

• Journal of Positioning, Navigation, and Timing
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• 제6권3호
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• pp.125-133
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• 2017

Time comparison is necessary for the verification and synchronization of the clock. Two-way satellite time and frequency (TWSTFT) is a method for time comparison over long distances. This method includes errors such as atmospheric effects, satellite motion, and environmental conditions. Ionospheric delay is one of the significant time comparison error in case of the carrier-phase TWSTFT (TWCP). Global Ionosphere Map (GIM) from Center for Orbit Determination in Europe (CODE) is used to compare with Bernese. Thin shell model of the ionosphere is used for the calculation of the Ionosphere Pierce Point (IPP) between stations and a GEO satellite. Korea Research Institute of Standards and Science (KRISS) and Koganei (KGNI) stations are used, and the analysis is conducted at 29 January 2017. Vertical Total Electron Content (VTEC) which is generated by Bernese at the latitude and longitude of the receiver by processing a Receiver Independent Exchange (RINEX) observation file that is generated from the receiver has demonstrated adequacy by showing similar variation trends with the CODE GIM. Bernese also has showed the capability to produce high resolution IONosphere map EXchange (IONEX) data compared to the CODE GIM. At each station IPP, VTEC difference in two stations showed absolute maximum 3.3 and 2.3 Total Electron Content Unit (TECU) in Bernese and GIM, respectively. The ionospheric delay of the TWCP has showed maximum 5.69 and 2.54 ps from Bernese and CODE GIM, respectively. Bernese could correct up to 6.29 ps in ionospheric delay rather than using CODE GIM. The peak-to-peak value of the ionospheric delay for TWCP in Bernese is about 10 ps, and this has to be eliminated to get high precision TWCP results. The $10^{-16}$ level uncertainty of atomic clock corresponds to 10 ps for 1 day averaging time, so time synchronization performance needs less than 10 ps. Current time synchronization of a satellite and ground station is about 2 ns level, but the smaller required performance, like less than 1 ns, the better. In this perspective, since the ionospheric delay could exceed over 100 ps in a long baseline different from this short baseline case, the elimination of the ionospheric delay is thought to be important for more high precision time synchronization of a satellite and ground station. This paper showed detailed method how to eliminate ionospheric delay for TWCP, and a specific case is applied by using this technique. Anyone could apply this method to establish high precision TWCP capability, and it is possible to use other software such as GIPSYOASIS and GPSTk. This TWCP could be applied in the high precision atomic clocks and used in the ground stations of the future domestic satellite navigation system.