| 1. |
- Bergstrand, Sten, 1971, et al.
(författare)
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Ionospheric Corrections for Accurate Positioning in Real Time
- 2005
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Ingår i: in Proc. Radio Science and Communication (RVK), eds. O. Gustafsson, P. Löwenborg, Linköping, Sweden, June 14-16, 2005. - 9170561222 ; , s. 391-394
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 2. |
- Ebenhag, Sven-Christian, 1976, et al.
(författare)
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Measurements and Error Sources in Time Transfer Using Asynchronous Fiber Network
- 2010
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Ingår i: IEEE Transactions on Instrumentation and Measurement. - 1557-9662 .- 0018-9456. ; 59:7, s. 1918-1924
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Tidskriftsartikel (refereegranskat)abstract
- We have performed time transfer experiments basedon passive listening in fiber optical networks using Packet over synchronous optical networking (SONET)/synchronous digital hierarchy(SDH). The experiments have been performed with differentcomplexity and over different distances. For assessmentof the results, we have used a GPS link based on carrier-phase observations. On a 560-km link, precision that is relative to the GPS link of
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| 3. |
- Ebenhag, Sven-Christian, 1976, et al.
(författare)
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Time Transfer over a 560 km Fiber Link
- 2008
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Ingår i: EFTF 2008 - 22nd European Frequency and Time Forum. ; , s. Paper E5A04 -
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Konferensbidrag (refereegranskat)
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| 4. |
- Ebenhag, Sven-Christian, 1976, et al.
(författare)
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Time transfer using an asynchronous computer network: An analysis of error sources
- 2007
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Ingår i: European Frequency and Time Forum, 29/5 - 1/6, Geneva, CH.
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Konferensbidrag (refereegranskat)abstract
- We have performed a time transfer over a distance ofapproximately 75 km using an asynchronous computer networkbased on optical fibers. In order to validate the results from thisfiber-link, we have compared the results with a GPS-link, whichconsists of carrier phase observations. All electronic cabinetswere equipped with temperature and humidity sensors. Here wepresent experiments where the temperature and humidity of thedelay in the electrical components were investigated. Allcomponents showed some temperature dependence, but nosignificant humidity dependence was found. By using thederived temperature coefficient for the components the standarddeviation of the difference between the fiber link and GPS linkdecreased from 243ps to184ps.
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| 5. |
- Ebenhag, Sven-Christian, 1976, et al.
(författare)
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TIME TRANSFER USING AN ASYNCHRONOUS COMPUTER NETWORK: RESULTS FROM A 500 KM BASELINE EXPERIMENT
- 2007
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Ingår i: Topical Meeting on Precise Time and Time Interval, 27-30/11, Long Beach, CA.
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Konferensbidrag (refereegranskat)abstract
- SP Technical Research Institute of Sweden and STUPI have performed a time transferexperiment over a 500km long baseline between Borås and Stockholm. The time transfertechnique passively utilizes the data bit stream generated in an optical fiber computer networkbased on the packet over SONET/SDH technique. A small fraction of the optical signal ismonitored both at the transmitter and at the receiver. When an occurrence of a unique bitsequence of the SDH frames is detected, an electrical pulse is generated and compared with aresolution of 100 ps to a local clock. With data from all four positions of an optical bidirectionallink, two-way time-transfer can be achieved and any symmetrical variations in delay canpotentially be cancelled. The results presented here have been obtained over OptoSUNET, thenew Swedish University Network. In the experiment, 10 Gbit/s traffic from SP over OptoSUNETis extended in Stockholm to STUPI, a clock laboratory which is the second node in this setup.This reconnection enables that a communication channel is established between two nodes,with no intermediate jump. The time-transfer experiment includes more than 500 km of fibertransmission, of which several km is via air-lines. By comparing the results from a GPS carrierphaselink, a precision better than ± 1 ns is achieved over several months of measurementsbetween two Hydrogen-masers.
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| 6. |
- Ebenhag, Sven-Christian, 1976, et al.
(författare)
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Time transfer using an asynchronous computer network: Results from three weeks of measurements
- 2007
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Ingår i: European Frequency and Time Forum, 29/5 - 1/6, Geneva, CH.
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Konferensbidrag (refereegranskat)abstract
- We have performed a time transfer experimentbetween two atomic clocks, over a distance of approximately 75km using an 10 Gbit/s asynchronous fiber-optic computernetwork. The time transfer was accomplished through passivelistening on existing data traffic and a pilot sequence in the SDHbit stream. In order to assess the fiber-link clock comparison, wesimultaneously compared the clocks using a GPS carrier phaselink. The standard deviation of the difference between the twotime transfer links over the three-week time period was 243 ps.
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| 7. |
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| 8. |
- Elgered, Gunnar, 1955, et al.
(författare)
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Validation of climate models using European ground-based GNSS observations
- 2009
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Ingår i: Proc. of 2nd Colloquium Scientific and Fundamental Aspects of the Galileo Programme, European Space Agency, 15-19 October, 2009, Padua, Italy. ; CD ROM
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We summarize an ongoing research project where we assess the quality of time series of the Integrated Water Vapour in the atmosphere estimated from ground-based GNSS data for the application of validating and possibly improving climate models. The focus is on the factors limiting the accuracy and especially the long-term stability of the GNSS technique.Higher order ionospheric corrections have been studied, using realistic values for the Total Electron Content (TEC) close to the solar maximum in 2002. Averaged over ten days we find that the impact in the mean IWV is less than 0.1 kg/m^2. Another factor is the model used for antenna phase centre variations. We have studied this effect on the IWV estimates by simulations and by studying estimates of the IWV based on observed GPS signals. We find that ignoring satellite antenna phase variations, when processing GPS data from 2003-2008, can significantly influence the values of the estimated linear trends. The value depends on the latitude of the site as well as on the elevation cut-off angle used in the data analysis. Finally, we show a significant correlation between estimated linear trends in the IWV and the corresponding linear trends in the independently observed ground temperature.
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| 9. |
- Emardson, Ragne, 1969, et al.
(författare)
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GROUND- AND SATELLITE- BASED GNSS MEASUREMENTS OF WATER VAPOUR WITH FOCUS ON CLIMATE APPLICATIONS
- 2014
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Ingår i: Proceedings of Metrology for Meteorology and Climate 2014 conference.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Water vapour is one of the most important green-house gases and the amount of water vapor in the atmosphere is an important indicator for climate change. Due to its high variability, irregular height distribution and poor mixing with other constituents, accurate measurements of water vapour in the atmosphere are difficult and costly to carry out with high temporal and spatial resolution over long time. The delay of Global Navigation Satellite Systems (GNSS) signals when propagating through the atmosphere due to the presence of water vapour is nearly proportional to the quantity of water vapour integrated along the signal path. Hence, using ground-based GNSS receiver networks it is possible to measure the signal delay in the atmosphere, which in turn can be used to derive the atmospheric water vapour content, the Integrated Water Vapour (IWV). Since time interval measurements can be traceable to standards, it is a promising method for providing an observational system for climate monitoring. However, in order to make a proper uncertainty assessment of the measured water vapour, the different error sources affecting the measurements need to be controlled. GNSS occultation measurements are obtained by deploying GNSS receivers onboard Low Earth Orbit (LEO) satellites. The main observable in GNSS occultation measurements is the Doppler shift of the received signal phase. Based on the Doppler shift the vertical profile of the refractive index can be inferred. From the refractive index measurements, we can estimate temperature, pressure, and the amount of water vapour. A key issue in using occultation measurement for climate purposes lies in the methodology of making the measured parameters traceable to references. Here, we present new methods of using booth ground-based and satellite-based GNSS measurements for climate studies. We have used 6 months of GNSS measurements collected both from ground-based networks and from receivers deployed on satellites.
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| 10. |
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