| 1. |
- 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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| 2. |
- Ebenhag, Sven-Christian, 1976, et al.
(författare)
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Time Transfer between UTC(SP) and UTC(MIKE) Using Frame Detection in Fiber-Optical Communication networks
- 2011
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Ingår i: 43rd Precise Time and Time Interval (PTTI) Systems and Applications Meeting. - 9781622767953 ; , s. 431-441
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Konferensbidrag (refereegranskat)abstract
- This paper presents recent results from a time transfer method using passive listening and detection of SDH frame headers in fiber-optical networks. The results are based on an experimental fiber-link that is implemented between the national time and frequency laboratories at SP in Borås, Sweden and at MIKES in Espoo, Finland with an intermediate connection at STUPI time and frequency facility in Stockholm, Sweden. The total fiber length exceeds 1129 km and is implemented in SUNET (Swedish University Network) and FUNET (Finnish University and Research Network). The two networks are connected via NORDUnet (Nordic Infrastructure for Research & Education) and the links are DWDM-based (Dense Wavelength Division Multiplexing).Both SP and MIKES maintains local representations of UTC and contributes with clock data to TAI, which gives the opportunity to compare the fiber-based method with those independent methods that are used regularly by the laboratories for the links to UTC. Preliminary results show that a time transfer stability of less than 10 picoseconds is obtained for averaging times of a few hundred seconds. The results also show that the method suffers from daily variations of a few nanoseconds, presumable due to temperature sensitive network equipment and asymmetric fiber paths. Nevertheless, a comparison to GPS carrier phase time transfer over three months shows an rms-agreement of less than 1 nanosecond.
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| 3. |
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| 4. |
- Tortora, P., et al.
(författare)
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AWARDS: Advanced microwave radiometers for deep space stations
- 2013
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Ingår i: Space Communications. - 0924-8625 .- 1875-9211. ; 22:2-4, s. 159-170
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Tidskriftsartikel (refereegranskat)abstract
- The objective of this study, named AWARDS (Advanced microWAve Radiometers in Deep space Stations), is the preliminary design of a transmission Media Calibration System (MCS) to be located at an ESA Deep Space Antenna (DSA) site. The crucial aspect is the capability to accurately retrieve the tropospheric path delay along the line-of-sight of the deep space probe in order to allow precise tropospheric calibration of deep space observables (range and range-rate) with particular reference to the BepiColombo spacecraft and its primary DSA at Cebreros (ES). The study focuses on two main aspects which lead to the preliminary design of the Mercury Orbiter Radioscience Experiment (MORE) MCS: the characterization of current microwave radiometers (MWRs) available at ESA/ESTEC and the atmospheric fluctuation effects on the MCS error budget, in terms of the Allan standard deviation (ASD). In the course of the study, further critical aspects have been identified (effects of Sun contamination, effects of ground noise emission), and mitigation strategies have been proposed. The final outcome is a preliminary design of the MWR (and the entire MCS) to be deployed at the ESA/ESTRACK (ESA Tracking station network) sites and being compliant with MORE requirements.
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| 5. |
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| 6. |
- Graziani, A., et al.
(författare)
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Assessment of Ground-Based Microwave Radiometry for Calibration of Atmospheric Variability in Spacecraft Tracking
- 2014
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Ingår i: IEEE Transactions on Antennas and Propagation. - 0018-926X .- 1558-2221. ; 62:5, s. 2634-2641
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Tidskriftsartikel (refereegranskat)abstract
- In a suggested radio propagation experiment using a deep space antenna, accurate calibration of the propagation delay through the Earth’s atmosphere is essential. One or two microwave radiometers can be used for this purpose. Differences in precise locations of the radiometer(s) and antenna to be calibrated leave a residual wet path delay value. We computed the Allan Standard Deviation (ASD) of this residual, as well as the one resulting from different pointing positions in the plane of the sky, by simulations.Pointing offsets, e.g., to avoid solar radiation into the radiometer beam, lead in general to an increased ASD. However, for many observation geometries a deliberate pointing offset can compensate for the location differences. In the case studied we found a reduction of the ASD with up to 45% compared to the ASD obtained for a zero pointing offset. The size of the calculated ASD depends strongly on the model parameters used, e.g., the turbulence strength parameter C_n^2, which has a significant natural variation over a year.
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| 7. |
- 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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| 8. |
- 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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| 9. |
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