| 1. |
- Löfgren, Johan, 1983, et al.
(författare)
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Sea Level Monitoring Using a GNSS-Based Tide Gauge
- 2009
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Ingår i: 2nd International Colloquium - Scientific and Fundamental Aspects of the Galileo Programme, 14 - 16 October 2009, Padua, Italy.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Global climate change is believed to result in the melting of large masses of ice in Polar Regions, bringing freshwater into the ocean, and changing the sea level. The traditional way to measure the sea level, by tide gauges, results in measurements relative to the Earth’s crust. However, in order to fully understand the sea level changes, absolute measurements (change in sea level in relation to the Earth’s center of gravity) are necessary, in particular in regions affected by post-glacial uplift, e.g., Fennoscandia. Satellite techniques, e.g., GNSS can be used to determine the motion of the Earth’s crust in relation to the center of gravity. By measuring reflected GNSS-signals from the sea surface, information of the sea level change can be obtained. Therefore a GNSS-based tide gauge is proposed.The proposed GNSS-based tide gauge installation consists of two antennas, one zenith looking right hand circular polarized (RHCP) and one nadir looking left hand circular polarized (LHCP), mounted back-to-back on a beam over the ocean. The RHCP antenna receives the GNSS-signals directly, whereas the LHCP antenna receives the signals reflected from the sea surface. Because of the additional path delay of the reflected signal, the LHCP antenna will appear to be a virtual GNSS-antenna located below the sea surface. When the sea level changes, the path delay of the reflected signal changes, thus the LHCP antenna will appear to be in a new position. The vertical position change corresponds to twice the sea level change, and therefore monitors sea level changes.Multiple satellites with different elevation and azimuth angles are observed each epoch and will give rise to reflected signals with different incidence angles from different directions. This means that the estimated sea level change can not be considered to originate from one specific point on the surface, but rather represents the change of an average surface formed by the reflection points.An experimental setup was installed in December 2008 over the ocean at Onsala Space Observatory (OSO) at the west coast of Sweden. Data was collected during three days using two Leica GRX1200+ receivers (one for the direct and one for the reflected signal). The receivers recorded 40 hours of continuous 20Hz data. The signal-to-noise ratio (SNR) as determined by the two receivers was used as a first data quality check. On average the SNR difference between the directly received and the reflected signals was less than 3dB.The data was analyzed using an in-house developed software in MATLAB. Solutions were made using L1 phase delays for relative positioning. Two approaches to estimate the vertical difference between the RHCP and the LHCP antenna were tested: hourly estimates of the vertical difference, and high-rate estimates of the vertical difference. For the hourly estimates 40 hours of continuous 1Hz data (reduced for faster processing using the TEQC software) were used. Each solution was made using 20 minutes of data every full hour, solving for differences in the local vertical components together with receiver clock and phase ambiguities differences for each epoch.The solution for the high-rate vertical component was made in two steps. First, the phase ambiguity differences were determined. This was done using equally distributed short intervals of ~1 second (21 epochs) from ~20 minutes of 20Hz data, solving for difference in phase ambiguities and receiver clocks every epoch together with differences in vertical coordinate for each short interval. The processing was done based on the assumptions that the sea surface does not change significantly during ~1 second and that the satellite geometry changes considerably in ~20 minutes. Second, the differences in phase ambiguities were rounded to the nearest integer and inserted as known values for a reprocessing of the 20Hz data. In this reprocessing the receiver clock parameters were estimated every epoch and the vertical coordinate difference with different time resolutions (e.g. 0.05s, 1s, 30s).The resulting time-series for the sea level change from the hourly solutions were compared to data from two traditional tide gauges operated by the Swedish Meteorological and Hydrological Institute at Ringhals and Göteborg, about 18 km south of and 33 km north of OSO, respectively. The GNSS-derived sea level change resembles reasonably well the independently observed sea level change. This indicates that the GNSS-tide gauge gives valuable results for sea level monitoring. Furthermore, the use of the high-rate GNSS-receivers additionally allows a flexible time resolution for sea level monitoring.
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| 2. |
- Hobiger, Thomas, 1978, et al.
(författare)
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Software-Defined Radio Direct Correlation GNSS Reflectometry by Means of GLONASS
- 2016
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Ingår i: IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing. - 2151-1535 .- 1939-1404. ; 9:10, s. 4834-4842
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Tidskriftsartikel (refereegranskat)abstract
- Ground-based GNSS reflectometry (GNSS-R)systems can be realized by different means. The concept ofcorrelation between direct and reflected GNSS signals is basicallypossible with all GNSS systems. However, using signals fromthe Russian GLONASS system simplifies the signal processingso that software-defined radio (SDR) components can be usedat replace expensive hardware solutions. This paper discusseshow such a solution, called GLONASS-R, can be realized usingentirely off-the-shelf components. Field tests with such a systemdemonstrate the capability to monitor sea surface heights with aprecision of 3 cm or better even with a sampling rate of 1.5 Hz.The flexibility of a SDR and the simple concept of GLONASS-Rallow build such a system with low costs and adapt it to the needsof any ground-based GNSS-R problem.
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| 3. |
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| 4. |
- Löfgren, Johan, 1983, et al.
(författare)
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Sea level measurements using multi-frequency GPS and GLONASS observations
- 2014
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Ingår i: Eurasip Journal on Advances in Signal Processing. - : Springer Science and Business Media LLC. - 1687-6172 .- 1687-6180. ; 2014:1
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Tidskriftsartikel (refereegranskat)abstract
- Global Positioning System (GPS) tide gauges have been realized in different configurations, e.g., with one zenith-looking antenna, using the multipath interference pattern for signal-to-noise ratio (SNR) analysis, or with one zenith- and one nadir-looking antenna, analyzing the difference in phase delay, to estimate the sea level height. In this study, for the first time, we use a true Global Navigation Satellite System (GNSS) tide gauge, installed at the Onsala Space Observatory. This GNSS tide gauge is recording both GPS and Globalnaya Navigatsionnaya Sputnikovaya Sistema (GLONASS) signals and makes it possible to use both the one- and two-antenna analysis approach. Both the SNR analysis and the phase delay analysis were evaluated using dual-frequency GPS and GLONASS signals, i.e., frequencies in the L-band, during a 1-month-long campaign. The GNSS-derived sea level results were compared to independent sea level observations from a co-located pressure tide gauge and show a high correlation for both systems and frequency bands, with correlation coefficients of 0.86 to 0.97. The phase delay results show a better agreement with the tide gauge sea level than the SNR results with root-mean-square differences of 3.5 cm (GPS L1 and L2) and 3.3/3.2 cm (GLONASS L1/L2 bands) compared to 4.0/9.0 cm (GPS L1/L2 ) and 4.7/8.9 cm (GLONASS L1/L2 bands). GPS and GLONASS show similar performance in the comparison, and the results show that for the phase delay analysis, it is possible to use both frequencies, whereas for the SNR analysis, the L2 band should be avoided if other signals are available. Note that standard geodetic receivers using code-based tracking, i.e., tracking the un-encrypted C/A-code on L1 and using the manufacturers’ proprietary tracking method for L2 , were used. Signals with the new C/A-code on L2 , the so-called L2C, were not tracked.Using wind speed as an indicator for sea surface roughness, we find that the SNR analysis performs better in rough sea surface conditions than the phase delay analysis. The SNR analysis is possible even during the highest wind speed observed during this campaign (17.5 m/s), while the phase delay analysis becomes difficult for wind speeds above 6 m/s.
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| 5. |
- Haas, Rüdiger, 1966, et al.
(författare)
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Observing UT1‑UTC with VGOS
- 2021
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Ingår i: Earth, Planets and Space. - : Springer Science and Business Media LLC. - 1880-5981 .- 1343-8832. ; 73:1
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Tidskriftsartikel (refereegranskat)abstract
- We present first results for the determination of UT1-UTC using the VLBI Global Observing System (VGOS). During December 2019 through February 2020, a series of 1 h long observing sessions were performed using the VGOS stations at Ishioka in Japan and the Onsala twin telescopes in Sweden. These VGOS-B sessions were observed simultaneously to standard legacy S/X-band Intensive sessions. The VGOS-B data were correlated, post-correlation processed, and analysed at the Onsala Space Observatory. The derived UT1-UTC results were compared to corresponding results from standard legacy S/X-band Intensive sessions (INT1/INT2), as well as to the final values of the International Earth Rotation and Reference Frame Service (IERS), provided in IERS Bulletin B. The VGOS-B series achieves 3–4 times lower formal uncertainties for the UT1-UTC results than standard legacy S/X-band INT series. The RMS agreement w.r.t. to IERS Bulletin B is slightly better for the VGOS-B results than for the simultaneously observed legacy S/X-band INT1 results, and the VGOS-B results have a small bias only with the smallest remaining standard deviation.
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| 6. |
- Hobiger, Thomas, 1978, et al.
(författare)
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Hard- and software tools for the education of Geodetic VLBI
- 2016
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Ingår i: 9th IVS General Meeting, March 2016, Johannesburg, South Africa.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Onsala Space Observatory hosts two 2.3 m radio telescopes called SALSA ("Such a lovely small antenna") which are utilised to bring front-line interactive astronomy to the classroom. Until now SALSA has been used for astronomical educational purposes solely, in particular demonstrating the concept of single dish measurements. However, it is possible to combine both SALSAs to an interferometer by making use of hardware which has been developed for software-defined radio. In doing so, one can utilise the SALSA antenna pair as a student demonstrator for geodetic Very Long Baseline Interferometry. We will discuss the COTS hardware components that are necessary to turn the SALSA installation into an interferometer. Moreover, we will show how a simple correlator has been used to detect fringes and provide single-band delays. Such delays were then processed with our analysis software c5++. We are going to discuss how it is possible to mimic the complete processing chain of geodetic VLBI and how this can be used for training of students and other interested parties.
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| 7. |
- Hobiger, Thomas, 1978, et al.
(författare)
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Hard and Software Tools for the Education of Geodetic VLBI
- 2016
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Ingår i: IVS 2016 General Meeting Proceedings "New Horizons with VGOS". ; :NASA/CP-2016-219016, s. 234-238
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The Onsala Space Observatory hosts two 2.3 m radio telescopes called SALSA (”Such a lovely small antenna”) which are utilised to bring front-line interactive astronomy to the classroom. Until now SALSA has been used for astronomical educational purposes solely, in particular demonstrating the concept of single dish measurements. However, it is possible to combine both SALSAs to an interferometer by making use of hardware which has been developedfor software-defined radio. In doing so, one can utilise the SALSA antenna pair as a student demonstrator for geodetic Very Long Baseline Interferometry. Itis discussed which COTS hardware components are necessary to turn the SALSA installation into an interferometer. A simple Octave-based correlator hasbeen written in order to process SALSA data. Results from a test run during which the Sun was tracked are presented and discussed here.
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| 8. |
- Haas, Rüdiger, 1966, et al.
(författare)
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Onsala Space Observatory – IVS Network Station
- 2013
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Ingår i: International VLBI Service for Geodesy and Astrometry 2012 Annual Report. ; , s. 130-133
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- During 2012 we participated in 40 IVS sessions. As in the previous four years, we used the majority of the sessions that involved both Onsala and Tsukuba to do ultra-rapid dUT1 observations together with our colleagues in Tsukuba. We observed one four-station ultra-rapid EOP session together with Tsukuba, Hobart, and HartRAO. We also observed the RadioAstron satellite and several GLONASSsatellites using the Onsala 25 m telescope.The highlight in 2012 was that our proposal to the Knut and Alice Wallenberg Foundation to establish a twin-telescope system at Onsala in accordance with the VLBI2010 recommendations was accepted.
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| 9. |
- Strandberg, Joakim, 1991, et al.
(författare)
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Improving GNSS-R sea level determination through inverse modeling of SNR data
- 2016
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Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 51:8, s. 1286-1296
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a new method for retrieving sea surface heights from Global Navigation Satellite Systems reflectometry (GNSS-R) data by inverse modeling of SNR observations from a single geodetic receiver. The method relies on a B-spline representation of the temporal sea level variations in order to account for its continuity. The corresponding B-spline coefficients are determined through a nonlinear least squares fit to the SNR data, and a consistent choice of model parameters enables the combination of multiple GNSS in a single inversion process. This leads to a clear increase in precision of the sea level retrievals which can be attributed to a better spatial and temporal sampling of the reflecting surface. Tests with data from two different coastal GNSS sites and comparison with colocated tide gauges show a significant increase in precision when compared to previously used methods, reaching standard deviations of 1.4 cm at Onsala, Sweden, and 3.1 cm at Spring Bay, Tasmania.
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| 10. |
- Hobiger, Thomas, 1978, et al.
(författare)
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Ground-based GNSS-R solutions by means of software defined radio
- 2016
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Ingår i: Proceedings of the 2016 IEEE International Geoscience and Remote Sensing Symposium (IGARSS 2016). Beijing; China; 10-15 July 2016. - 9781509033324 ; 2016-November, s. Art no 7730472, Pages 5635-5637
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Konferensbidrag (refereegranskat)abstract
- Usually ground-based GNSS-R installations are either existing geodetic GNSS stations or they are built with dedicated components that enable the deduction and monitoring of physical and geometrical properties of the reflecting area around that particular site. In both cases, hardware components usually enable real-time operation of such instruments. However, as software-defined radio (SDR) technology has advanced in the recent years it is now possible to carry out signal processing in real-time, which makes it an ideal candidate for the realization of a flexible GNSS-R system. It is shown how SDR can help to realize GNSS-R solutions for sea-level monitoring at the Onsala Space Observatory, Sweden. Moreover, such SDR solutions can be mounted on an unmanned aerial vehicle (UAV) in order to collect data from higher altitudes and even provide Delay-Doppler information for extended GNSS-R studies.
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