| 11. |
- Hobiger, Thomas, 1978, et al.
(författare)
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Combining GPS and VLBI for inter-continental frequency transfer
- 2015
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Ingår i: Metrologia. - : IOP Publishing. - 0026-1394 .- 1681-7575. ; 52:2, s. 251-261
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Tidskriftsartikel (refereegranskat)abstract
- For decades the global positioning system (GPS) has been the only space geodetic technique routinely used for inter-continental frequency transfer applications. In the past very long baseline interferometry (VLBI) has also been considered for this purpose and the method's capabilities were studied several times. However, compared to GPS current VLBI technology only provides few observations per hour, thus limiting its potential to improve frequency comparisons. We therefore investigate the effect of combining GPS and VLBI on the observation level in order to draw the maximum benefit from the strength of each individual technique. As a test-bed for our study we use the CONT11 campaign observed in 2011. First we review the frequency transfer performance that can be achieved with independent technique-specific analyses, both with individual software packages and with the multi-technique software c5++. With this analysis approach both techniques, GPS and VLBI, show similar frequency link instabilities at the level of 10−14 to 10−15 (MDEV) on inter-continental baselines for averaging times of one day. Then we use the c5++ software for a combined analysis of GPS and VLBI data on the observation level. We demonstrate that our combination approach leads to small but consistent improvements for frequency transfer of up to 10%, in particular for averaging periods longer than 3000 s.
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| 12. |
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| 13. |
- Hobiger, Thomas, 1978, et al.
(författare)
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Combining VLBI and GPS for inter-continental frequency transfer
- 2015
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Ingår i: 22nd European VLBI for Geodesy and Astrometry (EVGA) Working Meeting.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- For decades the Global Positioning System (GPS) has been the only space geodetic technique routinely used for inter-continental frequency transfer applications. In the past VLB) has also been considered for this purpose and the method's capabilities were studied several times. However, compared to GPS current VLBI technology only provides few observations per hour, thus limiting its potential to improve frequency comparisons. We therefore investigate the effect of combining VLBI and GPS on the observation level in order to draw the maximum benefit from the strength of each individual technique. As a test-bed for our study we use the CONT11 campaign observed in 2011. First we review the frequency transfer performance that can be achieved with independent technique-specific analyses. With this analysis approach both techniques, GPS and VLBI, show similar frequency link instabilities at the level of 1e-14 to 1e-15 (MDEV) on inter-continental baselines for averaging times of one day. We also perform a combined analysis of VLBI and GPS data on the observation level and demonstrate that our combination approach leads to small but consistent improvements for frequency transfer of up to 10%, in particular for averaging periods longer than 3000 s. We discuss the implications of these findings and present our ideas about how VLBI can contribute to international frequency transfer tasks.
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| 14. |
- Jaldehag, R. T. Kenneth, 1962, et al.
(författare)
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A GPS Carrier-Phase Aided Clock Transport for the Calibration of a Regional Distributed Time Scale
- 2009
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Ingår i: Proceedings EFTF-IFCS2009 joint conference 20-24 April 2009, IEEE catalog number:CFP09FRE-CDR. - 1075-6787. - 9781424435104 ; , s. 659-663
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Konferensbidrag (refereegranskat)abstract
- Clock transportation is a historically proven time transfer method for the calibration of time links and time scales. With the establishment of satellite-based time transfer methods, however, clock transportation has become less attractive especially on long baselines. In order to match for instance the GPS common view time transfer method with calibration uncertainties of a few nanoseconds, it is necessary to transport high quality, expensive clocks such as caesium beam frequency standards. The stability of the clock during transportation and the duration of the transport set the limit of the prediction uncertainty. Being able to measure the clock during transportation instead of predicting it would yield some major advantages: (a) the use of less expensive and small clocks such as rubidium or quartz oscillators for transportation, (b) no need for environmental conditioning of the transported clock, and (c) the duration of the transport is not critical as long as the clock can continuously be measured. One solution to the clock measurement problem during transport is the use of GPS carrier-phase observations as described and evaluated in this paper. It is shown that a calibration uncertainty of less than one nanosecond is potentially achievable.
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| 15. |
- Jaldehag, R. T. Kenneth, 1962, et al.
(författare)
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Time and frequency activities at SP in Sweden
- 2009
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Ingår i: 41st Precise Time and Time Interval (PTTI) Systems and Applications Meeting, Santa Ana Pueblo. NM, November 16-19. - 9781617386541 ; , s. 231-251
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The national time and frequency laboratory of Sweden has since 1995 been a part of the measurement technology department at the SP Technical Research Institute of Sweden. The laboratory is responsible for maintaining a realization of Swedish standard time and the dissemination of this time scale in Sweden. The objective of the laboratory is to support and supply Swedish industry and authorities with accurate measures of time and frequency by instrument calibration, knowledge transfer, time dissemination, and research and development. Swedish standard time is connected by law to UTC as maintained by the BIPM. UTC (SP) is the realization of UTC in Sweden and is traceable to UTC via BIPM and time transfer using the GPS and TWSTFT techniques. This paper describes the generation and maintenance of UTC (SP) and the equipment, including clocks and time transfer equipment, needed for this task as well as the concept of a "Distributed Time Scale" using alternate versions of UTC (SP) maintained at sister laboratories in Sweden. The paper presents also activities related to the dissemination of Swedish standard time including GPS time transfer, Network Time Protocol (NTP), telephone time code, and a speaking clock. Finally, research activities including time transfer in optical fiber networks, continuous GNSS carrier-phase processing, and Kalman-filter-based ensemble clock generation are briefly presented.
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| 16. |
- Jaldehag, R. T. Kenneth, 1962, et al.
(författare)
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Time and frequency transfer using asynchronous fiber optical networks: progress report
- 2009
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Ingår i: 41st Precise Time and Time Interval (PTTI) Systems and Applications Meeting, Santa Ana Pueblo. NM, November 16-19.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- SP Technical Research Institute of Sweden has since 2004 been running a project withthe aim of performing time and frequency transfer using commercial asynchronous fiberopticalnetworks. The project is motivated by the need for an alternative and complementarytime transfer method on a national basis with the goal of reaching accuracy and stabilitycomparable to satellite-based methods. Previous results using an OC-192/STM-64 10-Gb/spacket over SONET/SDH network, show that time transfer accuracy of the order of a fewnanoseconds is possible on baselines exceeding 500 km [1]. The method is based on passivelistening on existing data traffic and the detection of certain bit sequences in the SDH frameheaders continuously transmitted by the network routers. By using two-way time transfer, itis possible to estimate and compensate for symmetric delays in the optical fibers. The methodrelies on that time dependent residual delays are small or can be can be compensated forand constant residual delays can be calibrated.This paper briefly revises the method and presents new results in comparison with theGPS carrier-phase technique, with focus on residual effects due to temperature variationswhich have shown to have significant impact on the stability and accuracy. It also discusseshardware miniaturizations as well as new ideas for active time transfer using bit-sequencegenerators/transmitters in dedicated wavelength slots of the optical network. Finally, the useof a subset of the IEEE standard 1588-2008 (Precise Time Protocol, PTP) for data transportis briefly discussed.
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| 17. |
- Jaldehag, R. T. Kenneth, 1962, et al.
(författare)
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Time Transfer Using Frame Detection in Fiber-Optical Communication Networks: New Hardware
- 2011
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Ingår i: Frequency Control and the European Frequency and Time Forum (FCS), Joint Conference of the IEEE International.
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Konferensbidrag (refereegranskat)abstract
- In this paper, a new, recently developed hardware for time transfer using passive listening and detection of SDH frame headers in fiber-optical networks is described. The method has been presented earlier, and results, using prototype equipment and an experimental fiber-link, have shown that time transfer with a precision of the order of a few nanoseconds is possible over links with network distances exceeding 500 km. In order to further develop the method and make it available to regular users of time keeping equipment, it has been essential to minimize the space requirements of needed hardware and to make the implementation and installation more easily and straight forward. The new hardware is in this paper described in detail.
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| 18. |
- Rieck, Carsten, 1972
(författare)
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Real-Time Time Metrology Using Space Geodetic Methods
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Two main objectives in time and frequency metrology are the realization of the SI unit of time, and the construction and dissemination of atomic time scales based on the SI second. This thesis is mainly concerned with the latter of the two and has investigated the characteristics of space geodetic time and frequency comparison methods. Furthermore, strategies have been developed that increase the accuracy and redundancy of time comparisons and reduce the latency between the time measurement process and the application of the results. International time keeping is a global effort resulting from the cooperation of many national partners. National Metrology Institutes use different space based systems to compare their local realizations of UTC, UTC(k) with each other and contribute with clock measurements to the formation of TAI. Time measurements with Global Navigation Satellite Systems, GNSS, are used with robust and powerful methods that are currently the dominant tool for time comparisons over long distances. The use of carrier phase observation allows users to exploit the full capabilities of GNSS. The nature of carrier phase ambiguities was studied and its effect on precise time comparisons was determined. This has resulted in the development of real-time methods that allow to determine relative time differences between receiver clocks with link instabilities in the order of one part in 1e15 for time intervals of one day.Distributed time scales are a means for increasing the redundancy in timekeeping by establishing a grid of national nodes with similar capabilities. For the accuracy of the necessary remote time comparisons the repeated calibration of the interconnecting time links is an essential exercise. Carrier phase observations are utilized in a novel method of a GNSS aided clock transport, which allows the calibration of time links with sub-nanosecond accuracy. When several link techniques are implemented, the combination of the links increases the accuracy and redundancy in time comparisons between the national nodes. Kalman filtering is used in a method that allows to combine multiple time links in real-time. Diversity of time and frequency transfer methods is important in order to avoid a dependence on a single technique. The capabilities of geodetic Very Long Baseline Interferometry, VLBI, have been studied as a possible alternative for comparisons over long distances. VLBI performs on a similar level as GNSS carrier phase, and together they can be used to improve the precision of the intercontinental frequency comparisons. In the future continuously operated geodetic VLBI has the potential to contribute to international time metrology.
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| 19. |
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| 20. |
- Rieck, Carsten, 1972, et al.
(författare)
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VLBI time-transfer using CONT08 data
- 2010
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Ingår i: Proceedings of the 24th European Frequency and Time Forum ESA/ESTEC Noordwijk (NL) 13-16 April 2010. - 9781467359702
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- One important prerequisite for geodetic Very Long Baseline Interferometry (VLBI) is the use of frequency standards with excellent short term stability, i.e. hydrogen masers. This makes VLBI stations, which are often co-located with Global Navigation Satellite System (GNSS) receiving stations, interesting for studies of time- and frequency-transfer techniques. In this paper we present an assessment of VLBI time-transfer based on the data of the two week long consecutive IVS Cont08 VLBI-campaign by using GPS Carrier Phase (GPSCP). Cont08 was a 15 days long campaign in August 2008 that involved eleven VLBI stations on five continents. For Cont08 we estimated the worst case VLBI time link stability between the station clocks of ONSALA and WETTZELL to about 1.5e-15 at one day. Comparisons with clock differences estimated with GPSCP confirm the VLBI results. The paper also indicates time-transfer related problems of the VLBI technique as used today. © 2010 IEEE.
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