| 1. |
- Bergstrand, Sten, 1971, et al.
(författare)
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Geometric Variations of a Geodetic Radio Telescope
- 2017
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Ingår i: Proceedings of the 23rd European VLBI Group for Geodesy and Astrometry Working Meeting. - 9789188041104 ; , s. 61-64
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 2. |
- 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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| 3. |
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| 4. |
- 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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| 5. |
- 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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