| 1. |
- Ebenhag, Sven-Christian, 1976, et al.
(författare)
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A fiber based frequency distribution system with enchanced output phase stability
- 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. 1061-1064
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Konferensbidrag (refereegranskat)abstract
- Experimental results on the stability of the output phase of a frequency distribution system from several days of measurement is presented, in addition to a discussion regarding the influence of control loop parameters. The setup handles the issue that the output phase stability of a system depends on perturbations along the transmission length. This is especially critical if the signal is transmitted through optical fiber, at lengths of a few 100 m. An experimental evaluation using a laser based transmitter at a wavelength of 850 nm, and 625 m of multimode fiber where 575 m where placed outdoor, a temperature dependence of 100 ps/°C was detected. Tocompensate for these slow variations in real time, a setup using two-way transmission, in conjunction with an adjustable optical delay, was constructed. This device is adjusted to induce a delay variation of equal magnitude but opposite direction, in comparison to the delay change of the fiber. Calculating the modified Allan deviation of the transmitted signal, it is apparent that without active compensation, the deviation at τ below 1000 s is comparable to the values from the measurement system without transmission. At longer integration times, however, the slow variations in the fiber transmission will deteriorate the modified ADEV substantially. When activating the dynamic adjustment of pre-delay in the system, the deviation at shorter times will increase with a few dB, however, the modified ADEV decreases continuously with τ, eventually below the values for the uncompensated system. In conclusion, activating a dynamically controlled pre-delay in a fiber based frequency transmission system will induce a small penalty on fast variations of the output phase, however giving a remarkable improvement on slower variations. The usefulness of this added functionality must therefore be determined by the application of the signal.
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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 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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| 4. |
- 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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| 5. |
- 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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| 6. |
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| 7. |
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| 8. |
- 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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| 9. |
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| 10. |
- 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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| 11. |
- 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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| 12. |
- Dutta, Uttama, 1979, et al.
(författare)
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Ionospheric Effects on GNSS RTK
- 2022
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Ingår i: 35th International Technical Meeting of the Satellite Division of the Institute of Navigation, ION GNSS+ 2022. - : Institute of Navigation. ; 3, s. 1557-1566
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Konferensbidrag (refereegranskat)abstract
- GNSS signals are influenced by free electrons as they propagate through the ionosphere. Studies have shown how the spatial variations of electron density in the ionosphere, affects measurements with network-Real Time Kinematic (NRTK) (Emardson et al 2011). This paper aims to predict what can be expected from measurements during the next solar maximum that is expected to occur around 2025 and discusses how it would affect RTK for automated transport applications. The ionospheric activity and its impact on positioning in the coming solar cycle maximum is discussed. This study focuses on data from Kiruna in northern Sweden (67.8N), mainly captured in January 2014 - in the middle of the most active time during the last cycle - which has been analyzed to predict the coming solar cycle. Based on the data, it was concluded that there is a risk of occasions with simultaneous signal slips on several satellites caused by the ionosphere which could cause temporary (minutes) loss of positioning ability for the RTK equipment. It is expected to occur a couple of times per month during the most active months of the solar cycle.
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| 13. |
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| 14. |
- Elgered, Gunnar, 1955, et al.
(författare)
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Ground-based microwave radiometry and long-term observations of atmospheric water vapor
- 1998
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Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 33:3, s. 707-717
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Tidskriftsartikel (refereegranskat)abstract
- Microwave radiometer data and radiosonde data from the time period 1981-1995 have been used to study long-term trends in the integrated precipitable water vapor (IPWV). The two instruments have operated 37 km apart on the Swedish west coast. Model parameters are estimated for the entire data sets as well as for subsets of the data. The IPWV model parameters are a mean value, a linear drift with time, and the amplitude and phase of an annual component. The radiosonde data, which are uniformly sampled in time, show an increase in the IPWV of 0.03 mm/yr with a statistical standard deviation of 0.01 mm. The microwave radiometer data, which are not at all uniformly sampled in time, show -0.02+/-0.01 mm/yr. We show that the disagreement is caused by the different sampling of the data for the two instruments. When the two data sets are reduced to include only data that are sampled simultaneously, we find an agreement between all estimated model parameters, given their statistical uncertainties. This suggests that if the microwave radiometer had also been operating continuously over the 15-year period, its data would have implied a linear trend similar to the result obtained from the radiosonde data. The general quality of the data, in terms of the short time scatter, has been improved over the time period. The root mean square (RMS) difference between the IPWV measured by the radiometer and by the radiosondes was 2.1 mm during the first 5 years and was reduced to 1.6 mm during the last 4 years. These values include the real difference in the IPWV between the two sites. The bias, radiometer-radiosonde, was 0.1 mm for the whole data set and varied between -0.2 and 0.9 mm for smaller data sets of a few years.
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| 15. |
- 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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| 16. |
- Jarlemark, Per O. J., 1962
(författare)
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Analysis of Temporal and Spatial Variations in Atmospheric Water Vapor Using Microwave Radiometry
- 1997
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The amount of atmospheric water vapor is highly variable in both time and space. In this thesis some aspects of the variations in the atmospheric water vapor are treated. Short term variations of up to approximately a day in the radio propagation delay due to water vapor has an important role as one of the limiting factors in space geodesy and radio astronomical interferometry. Since the atmospheric water vapor is a greenhouse gas, its change over many years is of interest in climatology. A microwave radiometer at the Onsala Space Observatory was used for data acquisition. Its data are used for characterizations of the variations in the amount of water vapor. The accuracy of the amount of water vapor determined using microwave radiometry is evaluated, as well as methods that utilize the knowledge of short-term atmospheric variability in order to improve the accuracy of the water vapor estimates. The role of the water vapor in space geodetic techniques such as Very-Long-Baseline Interferometry (VLBI) and the Global Positioning System (GPS) is also discussed. In these techniques the radio wave propagation is used for precise positioning. It is shown that an increased statistical knowledge of the variations in the amount of water vapor can improve the estimation of the contribution of water vapor to the propagation path length. The result is better positioning, while at the same time the amount of water vapor can be derived from the geodetic data, which is of interest for the meteorological community.
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| 17. |
- 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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| 18. |
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| 19. |
- 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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| 20. |
- 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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| 21. |
- Stoew, Borys, 1970, et al.
(författare)
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Temporal correlations of atmospheric mapping function errors in GPS data analysis
- 2007
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Ingår i: Journal of Geodesy. - : Springer Science and Business Media LLC. - 0949-7714 .- 1432-1394. ; 81:5, s. 311-323
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Tidskriftsartikel (refereegranskat)abstract
- The developments in global satellite navigation using GPS, GLONASS, and Galileo will yield more observations at various elevation angles. The inclusion of data acquired at low elevation angles allows for geometrically stronger solutions. The vertical coordinate estimate of a GPS site is one of the parameters affected by the elevation-dependent error sources, especially the atmospheric corrections, whose proper description becomes necessary. In this work, we derive time-series of normalized propagation delays in the neutral atmosphere using ray tracing of radiosonde data, and compare these to the widely used new mapping functions (NMF) and improved mapping functions (IMF). Performance analysis of mapping functions is carried out in terms of bias and uncertainty introduced in the vertical coordinate. Simulation runs show that time-correlated mapping errors introduce vertical coordinate RMS errors as large as 4 mm for an elevation cut-off angle of 5°. When simulation results are compared with a geodetic GPS solution, the variations in the vertical coordinate due to mapping errors for an elevation cut-off of 5° are similar in magnitude to those caused by all error sources combined at 15° cut-off. This is significant for the calculation of the error budget in geodetic GPS applications. The results presented here are valid for a limited area in North Europe, but the technique is applicable to any region provided that radiosonde data are available. © Springer-Verlag 2006.
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| 22. |
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