| 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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| 11. |
- Emardson, Ragne, 1969
(författare)
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Studies of Atmospheric Water Vapor Using the Global Positioning System
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Atmospheric water vapor is of fundamental interest in the Global Positioning System (GPS) and Very Long Baseline Interferometry (VLBI) techniques. These techniques are based on the timing of radio waves propagating through the atmosphere. The received signals are delayed by the water vapor mainly located in the lower part of the atmosphere called the troposphere. In order to achieve accurate positioning, such effects have to be corrected for. This can be done by estimating the delay from the signals themselves. From these results it is possible to derive the amount of water vapor in the atmosphere. The GPS and VLBI techniques can thus be used both for positioning and for determination of the amount of atmospheric water vapor. This thesis deals mainly with the technique to estimate the amount of water vapor in the atmosphere with GPS. Modeling of the atmosphere is treated as well as how to convert the signal delay estimates from GPS to the water vapor amount. The results obtained are assessed through comparisons with independent techniques, such as radiosondes and a microwave radiometer. The results in this thesis show that it is possible to measure the atmospheric water vapor with GPS with an accuracy comparable to techniques presently in use for that purpose. GPS gives, however, a better temporal and spatial resolution as well as being more cost-effective than the present radiosonde launches. This may prove to be useful for the meteorological community, as water vapor is an important factor in numerical weather prediction, and in climate studies since it is one of the most important greenhouse gases. Improved modeling of the water vapor and assessment of the results will also benefit positioning with GPS, which is useful, for example, in the determination of the land uplift after the last glaciation period.
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| 12. |
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| 13. |
- Emardson, Ragne, 1969, et al.
(författare)
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Time and Frequency Transfer in an Asynchronous TCP/IP over SDH-network Utilizing Passive Listening
- 2005
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Ingår i: IEEE Topical meeting on Precise Time and Time Interval 2005.
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Konferensbidrag (refereegranskat)abstract
- SP, Swedish National Testing and Research Institute together with STUPI have developed a technique for time and frequency transfer over an asynchronous TCP/IP network. The method is based on passive listening to existing data traffic in the network. We use the frame alignment bytes of the SONET/SDH protocol used between routers as references in order to compare standards located at the routers. As a test bed for the experiment, we will use the Swedish university Computer Network (SUNET). A preliminary assessment of the technique in a lab environment will be performed in late 2005.
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| 14. |
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| 15. |
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| 16. |
- Hedekvist, Per Olof E, 1967, et al.
(författare)
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Accurate time transfer utilizing the synchronization in an SDH-network
- 2006
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Ingår i: 2006 Optical Fiber Communication Conference, and the 2006 National Fiber Optic Engineers Conference; Anaheim, CA; United States; 5 March 2006 through 10 March 2006. - 9781557528032 ; 2006
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Konferensbidrag (refereegranskat)abstract
- A nationwide system for accurate time distribution is being developed, utilizing synchronization in an SDH-network. The first experimental results based on this technique are presented, performed on, but not limited to, STM-64.
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| 17. |
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| 18. |
- Jarlemark, Per O. J., 1962, et al.
(författare)
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Ground-Based GPS for Validation of Climate Models: The Impact of Satellite Antenna Phase Center Variations
- 2010
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Ingår i: IEEE Transactions on Geoscience and Remote Sensing. - 0196-2892 .- 1558-0644. ; 48:10, s. 3847-3854
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Tidskriftsartikel (refereegranskat)abstract
- The amount of water vapor in the atmosphere is an important indicator for climate change. Using the Global Positioning System (GPS), it is possible to estimate the integrated water vapor (IWV) above the ground-based GPS receiver. In order to optimally determine the IWV, a correct model of the received signal phase is essential. We have studied the effect of the satellite antenna phase center variations (PCVs) on the IWV estimates by simulating the effect and by studying the estimates of the IWV based on the observed GPS signals. During a period of five years, from 2003 to 2008, a new satellite type was introduced, and it steadily grew in numbers. The antenna PCVs for these satellites deviate from the earlier satellite types and contribute to excess IWV estimates. We find that ignoring satellite antenna phase variations for this time period can lead to an additional IWV trend of about 0.15 kg/m2/year for regular GPS processing.
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| 19. |
- Merlone, A., et al.
(författare)
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The MeteoMet project - metrology for meteorology: challenges and results
- 2015
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Ingår i: Meteorological Applications. - : Wiley. - 1350-4827 .- 1469-8080. ; 22, s. 820-829
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Tidskriftsartikel (refereegranskat)abstract
- The study describes significant outcomes of the Metrology for Meteorology' project, MeteoMet, which is an attempt to bridge the meteorological and metrological communities. The concept of traceability, an idea used in both fields but with a subtle difference in meaning, is at the heart of the project. For meteorology, a traceable measurement is the one that can be traced back to a particular instrument, time and location. From a metrological perspective, traceability further implies that the measurement can be traced back to a primary realization of the quantity being measured in terms of the base units of the International System of Units, the SI. These two perspectives reflect long-standing differences in culture and practice and this project - and this study - represents only the first step towards better communication between the two communities. The 3 year MeteoMet project was funded by the European Metrology Research Program (EMRP) and involved 18 European National Metrological Institutes, 3 universities and 35 collaborating stakeholders including national meteorology organizations, research institutes, universities, associations and instrument companies. The project brought a metrological perspective to several long-standing measurement problems in meteorology and climatology, varying from conventional ground-based measurements to those made in the upper atmosphere. It included development and testing of novel instrumentation as well as improved calibration procedures and facilities, instrument intercomparison under realistic conditions and best practice dissemination. Additionally, the validation of historical temperature data series with respect to measurement uncertainties and a methodology for recalculation of the values were included.
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| 20. |
- Ning, Tong, 1975, et al.
(författare)
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High-Rate GNSS Techniques for the Detection of Large Seismic Displacements
- 2009
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Ingår i: the IEEE International Geoscience and Remote Sensing Symposium (IGARSS). ; , s. V 359-362
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Detection of the seismic strong motion is of great importancefor earthquake studies. We have investigated the usageof high-rate sampled Global Navigation Satellite System(GNSS) data to measure seismic motion by implementingan industrial robot simulating the displacements close to anearthquake. The motion of the robot is tracked by GlobalPositioning System (GPS) signals. Two baselines-400 m and60 km-from the robot to reference stations are used to processthe observed GPS data. Both methods give similar (within0.5 mm) Root Mean Square (RMS) differences between theestimated and the commanded coordinates. The RMS differencesare approximately 3.5 mm in the east component, 5 mmin the north component, and 7 mm in the vertical component.
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| 21. |
- Steinmetz, Erik M, 1984, et al.
(författare)
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Assessment of GPS derived speed for verification of speed measuring devices
- 2014
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Ingår i: Int. J. of Instrumentation Technology. - 2043-7862. ; 1, s. 212-227
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Tidskriftsartikel (refereegranskat)abstract
- Speed information from GPS is increasingly used and provides an alternative to conventional methods such as wheel speed sensors. We investigate the possibility to use GPS derived speed as a reference when verifying laser and radar-based speed measuring devices used in traffic enforcement. We have set up a realistic test scenario where a GPS equipped vehicle was driven at three different speeds (40, 90 and 130 km/h) through a pre-defined measurement zone. An independent and traceable reference speed was calculated by accurately measuring the length of the measurement zone (approximately 15 metres), and the time it took to pass through it. The reference speed was compared to the average GPS speed for each passage. This comparisons show that the standard uncertainty of such GPS speed measurements is less than 0.05 km/h. Hence, GPS derived speed meets the accuracy requirements for verification of laser and radar based speed measuring devices.
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