| 1. |
- Larson, Kristine, et al.
(författare)
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Coastal Sea Level Measurements Using a Single Geodetic GPS Receiver
- 2013
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Ingår i: Advances in Space Research. - : Elsevier BV. - 1879-1948 .- 0273-1177. ; 51:8, s. 1301-1310
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a method to derive local sea level variations using data from a single geodetic-quality Global Navigation Satellite System (GNSS) receiver using GPS (Global Positioning System) signals. This method is based on multipath theory for specular reflections and the use of Signal-to-Noise Ratio (SNR) data. The technique could be valuable for altimeter calibration and validation. Data from two test sites, a dedicated GPS tide gauge at the Onsala Space Observatory (OSO) in Sweden and the Friday Harbor GPS site of the EarthScope Plate Boundary Observatory (PBO) in USA, are analyzed. The sea level results are compared to independently observed sea level data from nearby and in situ tide gauges. For OSO, the Root-Mean-Square (RMS) agreement is better than 5 cm, while it is on the order of 10 cm for Friday Harbor. The correlation coefficients are better than 0.97 for both sites. For OSO, the SNR-based results are also compared with results from a geodetic analysis of GPS data of a two receivers/antennae tide gauge installation. The SNR-based analysis results in a slightly worse RMS agreement with respect to the independent tide gauge data than the geodetic analysis (4.8 cm and 4.0 cm, respectively). However, it provides results even for rough sea surface conditions when the two receivers/antennae installation no longer records the necessary data for a geodetic analysis.
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| 2. |
- Larson, Kristine, et al.
(författare)
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The GPS Tide Gauge Problem Revisited
- 2011
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Ingår i: American Geophysical Union Fall Meeting 2011, 5-9 December, San Francisco, USA.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- It is well-known that GPS instruments can be used to measure local sea level. In most experiments, two antennas are deployed at a coastal site. A geodetic antenna - optimized for RHCP signals - is used in the traditional orientation and tracks the direct signal. The second antenna is optimized for reflected signals - which are primarily LHCP - and is pointed towards the ocean. The sea surface can then be estimated by analyzing the carrier phase data. While the data from the "up" antenna are dominated by the direct signal, the effects of signals reflected from the ocean are also present in its data. Thus in principle, one might be able to estimate sea level using only data from the "up" antenna. This is similar in concept to recent multipath studies where geodetic GPS installations are being used to measure soil moisture variations and snow depth.We have analyzed GPS data for a three-month period from a GPS tide gauge installation at the Onsala Space Observatory. It is located on the western coast of Sweden. We used the SNR data from the "up" antenna only. The data were windowed by azimuth for ocean-reflections and elevation angles from 18-40 degrees. This provides hourly sea level measurements. Comparisons were made to an average for tide gauge records 18 km south and 33 km north of Onsala. The standard deviation of the residual between our solutions and the tide gauges is 4.9 cm. This is less precise than the combined up-down antenna system of 2.6 cm. These precision values include errors associated with real tidal motion at the GPS site. While the "down" antenna performs poorly in high-wind conditions (> 8 m/s), we found that the "up" antenna performs significantly better at these times.
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| 3. |
- Löfgren, Johan, 1983, et al.
(författare)
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High-rate local sea level monitoring with a GNSS-based tide gauge
- 2010
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Ingår i: Proceedings of the 2010 IEEE International Geoscience and Remote Sensing Symposium, Proceedings DVD-ROM. - 9781424495665 ; , s. 3616-3619
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We present first results from the analysis of high-rate observations with a GNSS-based tide gauge at the Onsala Space Observatory. The goal is to determine local sea level with high temporal resolution. The GNSS-based tide gauge makes use of right-hand circular polarized GNSS signals that are directly received and left-hand circular polarized GNSS signals that are reflected from the sea surface. An experimental setup of the GNSS-based tide gauge was operated in the spring of 2010 and data were recorded with a sampling rate of 20 Hz.We analyzed data decimated to 1 Hz using different temporal resolution between 5 and 240 seconds, and the resulting time series of local sea level were compared to each other and to results from two stilling well gauges. The comparison with the data from the stilling well gauges shows a common trend.The comparison of the results from analyses with different temporal resolution show consistent results. There is also an indication that the GNSS-based tide gauge might be able to give information on the sea surface state.
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| 4. |
- Löfgren, Johan, 1983, et al.
(författare)
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Monitoring coastal sea level using reflected GNSS signals
- 2011
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Ingår i: Advances in Space Research. - : Elsevier BV. - 1879-1948 .- 0273-1177. ; 47:2, s. 213-220
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Tidskriftsartikel (refereegranskat)abstract
- A continuous monitoring of coastal sea level changes is important for human society since it is predicted that up to 332 million people in coastal and low-lying areas will be directly affected by flooding from sea level rise by the end of the 21st century. The traditional way to observe sea level is using tide gauges that give measurements relative to the Earth’s crust. However, in order to improve the understanding of the sea level change processes it is necessary to separate the measurements into land surface height changes and sea surface height changes. These measurements should then be relative to a global reference frame. This can be done with satellite techniques, and thus a GNSS-based tide gauge is proposed. The GNSS-based tide gauge makes use of both GNSS signals that are directly received and GNSS signals that are reflected from the sea surface. An experimental installation at the Onsala Space Observatory (OSO) shows that the reflected GNSS signals have only about 3 dB less signal-to-noise-ratio than the directly received GNSS signals. Furthermore, a comparison of local sea level observations from the GNSS-based tide gauge with two stilling well gauges, located approximately 18 km and 33 km away from OSO, gives a pairwise root-mean-square agreement on the order of 4 cm. This indicates that the GNSS-based tide gauge gives valuable results for sea level monitoring.
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| 5. |
- Löfgren, Johan, 1983
(författare)
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Observing Sea Level Using Reflected Global Navigation Satellite System Signals
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Sea-level rise due to global warming is predicted to have a large impact on human society, especially for populations living in coastal regions and on islands. It is therefore of great importance to monitor the sea level and to increase the understanding of the local hydrodynamic and meteorological responses to a global sea-level rise.Presented in this thesis is a technique to measure local sea level using Global Navigation Satellite System (GNSS) signals. This GNSS-based tide gauge acquires both the directly received GNSS signals and the GNSS signals that are reflected off the sea surface, using standard geodetic GNSS receivers. With the directly received signals the installation measures land-surface height changes, whereas the reflected signals are used to measure sea-surface height changes. Both measurements are done with respect to the Earth's centre of mass. By combining these observations it is possible to estimate the local sea level, which is directly related to the volume of the ocean.Several GNSS-based tide gauge campaigns have been carried out at the Onsala Space Observatory (OSO) on the west coast of Sweden. Today the installation is still in place and continues to record GNSS data with a sampling frequency of 1 Hz. In this thesis, data from the campaigns are analysed in a post-processing mode with an in-house developed software. Sea-level estimates are produced with a temporal resolution from 5 s to 20 min and compared to independent sea-level observations from stilling well gauges located approximately 18 km south and 33 km north of OSO. The results for three months of GNSS-derived sea level show an agreement, with respect to the stilling well gauge sea level, with typical root-mean-square differences of better than 6 cm and correlation coefficients of higher than 0.95.Additionally, using an ocean-tide analysis of three months of sea-level observations from the GNSS-based tide gauge, it was possible to determine several tidal components, i.e., M2, S2, N2, O2, and M4. The amplitudes and phases show reasonable agreement with the ones derived from one year of stilling well gauge sea-level data.
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| 6. |
- Löfgren, Johan, 1983, et al.
(författare)
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Remote Sensing of the Coastal Ocean with Standard Geodetic GNSS-Equipment
- 2012
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Ingår i: European Geosciences Union General Assembly 2012, 22-27 April, Vienna, Austria.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We use standard geodetic Global Navigation Satellite System (GNSS) equipment to perform remote sensing measurements of the coastal ocean. This is done by a so-called GNSS-based tide gauge that uses both direct GNSS-signals and GNSS-signals that are reflected off the sea surface. Our installation is located at the Onsala Space Observatory (OSO) at the west coast of Sweden and consists of a zenith-looking Right Hand Circularly Polarized (RHCP) and a nadir-looking Left Hand Circularly Polarized (LHCP) antenna. Each antenna is connected to a standard geodetic-type GNSS-receiver.We applied two different analysis strategies to our GNSS data set. The first strategy is based on a traditional geodetic differential analysis [Löfgren et al., 2011] and makes use of the data from both receivers; connected to the zenith and the nadir looking antennae. This approach results in local sea level that is automatically corrected for land motion, meaning that the GNSS-based tide gauge can provide reliable sea-level estimates even in tectonic active regions. The second strategy focuses on the Signal-to-Noise Ratio (SNR) recorded with the receiver connected to the zenith-looking antenna [Larson et al., 2011]. The SNR is affected by multipath originating from the sea surface reflections. Analysis of the SNR data allows to determine the distance between the antenna and the reflecting surface, and thus to measure sea surface height. Results from both analysis strategies are compared to independently observed sea-level data from two stilling-well gauges operated by the Swedish Meteorological and Hydrological Institute (SMHI), which lie in a distance of several km from OSO. The root-mean-square agreement between the different time series of several month's length is on the order of 5 cm and better. These results indicate the large potential for using coastal GNSS-sites for the monitoring of the coastal ocean.References:Löfgren J.S., Haas R., Scherneck H-G., Bos M.S., (2011), Three months of local sea level derived from reflected GNSS signals, Radio Science, 46 (RS0C05).Larson K., Löfgren J.S., Haas R., (2011), The GPS tide gauge problem revisited, AGU Fall Meeting, 5-9 December, San Francisco, USA, Poster.
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| 7. |
- Löfgren, Johan, 1983, et al.
(författare)
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Sea-Level Analysis Using 100 Days of Reflected GNSS Signals
- 2011
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Ingår i: Proceedings of the 3rd International Colloquium - Scientific and Fundamental Aspects of the Galileo Programme, 31 August - 2 September 2011, Copenhagen, Denmark. ; :WPP 326, s. 5-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Global Navigation Satellite System (GNSS) signals reflected off the sea surface can be used for remote sensing of the sea level. We present results from a GNSS-based tide gauge using standard geodetic-type GNSS receivers for receiving both the reflected and the direct GNSS signals. The local sea level is then obtained using relative geodetic processing of the carrier phase delay.We show results from our analysis of 100 days of GNSS data from the Onsala Space Observatory (OSO). The GNSS-derived sea level is compared to a weighted average of sea level observations from two stilling well gauges located 18 km and 33 km away from OSO. The comparison shows a high level of agreement with a correlation coefficient of 0.96. Furthermore, the standard deviation (1σ) between the time series is 5.0 cm and the pairwise mean difference is 3.6 cm.Additionally, we present a tidal analysis of the three sea level datasets and compare the derived tidal constituents both to each other and to a Regional Tide Model (RTM). From the GNSS-derived sea level we find significant ocean tidal signals with reasonable amplitudes and with most phases in between those for the stilling well gauges sites. The comparison to the RTM shows limitations of the model for long-period tidal signals.
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| 8. |
- Löfgren, Johan, 1983, et al.
(författare)
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Sea Level Derived from Reflected GNSS Signals
- 2011
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Ingår i: American Geophysical Union Fall Meeting 2011, 5-9 December, San Francisco, USA.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The traditional way to observe sea level is to use tide gauges, resulting in measurements relative to the Earth’s crust. However, in order to measure the sea-level change due to changes in ocean water volume and/or other oceanographic phenomena, all types of crustal motion at the measurement site need to be known.We present a remote sensing technique for measuring local sea level using standard geodetic-type Global Navigation Satellite System (GNSS) receivers. The installation consists of a zenith-looking Right Hand Circular Polarized (RHCP) antenna, receiving the direct signals, and a nadir-looking Left Hand Circular Polarized antenna, receiving the signals reflected of the sea surface. Each antenna is connected to a receiver and the antenna pair is deployed back-to-back at a coastal site. Estimating the vertical baseline between thetwo antennas, using standard geodetic analysis, the local sea level and its temporal variations can be determined. The advantage of this technique is that it allows to measure both sea surface height changes with relative positioning and land surface height changes, e.g., by precise point positioning of the RHCP antenna. Furthermore, the combined measurements of local sea level are automatically corrected for land motion, meaning that this installation could provide continuously reliable sea-level estimates in tectonic active regions.This GNSS-based tide gauge has been operating continuously at the Onsala Space Observatory (OSO) on the west coast of Sweden since September 2010. We present results from several months of operations and compare them to sea-level measurements from two stilling well gauges about 18 km south and 33 km north of OSO. We find a high degree of agreement between the time series with correlation coefficients of larger than 0.95. The root-mean-square differences between the GNSS-derived sea level and the stilling well gauge measurements are 5.9 cm and 5.5 cm, which is lower than between the two stilling well (6.1 cm). Furthermore, we present a tidal analysis of the three independent sea level time series and compare the derived tidal constituents among each other and with respect to theoretical models.
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| 9. |
- Löfgren, Johan, 1983, et al.
(författare)
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Three months of local sea level derived from reflected GNSS signals
- 2011
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Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 46:RS0C05
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Tidskriftsartikel (refereegranskat)abstract
- By receiving Global Navigation Satellite System (GNSS) signals that are reflected off the sea surface, together with directly received GNSS signals (using standard geodetic‐type receivers), it is possible to monitor the sea level using regular single difference geodetic processing. We show results from our analysis of three months of data from the GNSS‐based tide gauge at the Onsala Space Observatory (OSO) on the west coast of Sweden. The GNSS-derived time series of local sea level is compared with independent data from two stilling well gauges at Ringhals and Gothenburg about 18 km south and 33 km north of OSO, respectively. A high degree of agreement is found in the time domain, with correlation coefficients of up to 0.96. The root‐mean‐square differences between the GNSS‐derived sea level and the stilling well gauge observations are 5.9 cm and 5.5 cm, which is lower than for the stilling well gauges together (6.1 cm). A frequency domain comparison reveals high coherence of the data sets up to 6 cycles per day, which corresponds well to the propagation of gravity waves in the shallow waters at the Kattegat coast. Amplitudes and phases of some major tides were determined by a tidal harmonic analysis and compared to model predictions. From the GNSS‐based tide gauge results we find significant ocean tidal signals at fortnightly, diurnal, semi‐diurnal, and quarter‐diurnal periods. As an example, the amplitudes of the semi‐diurnal M2 and the diurnal O1 tide are determined with 1σ uncertainties of 11 mm and 12 mm, respectively. The comparison to model calculations shows that global ocean tide models have limited accuracy in the Kattegat area.
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| 10. |
- Löfgren, Johan, 1983, et al.
(författare)
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Tropospheric correction for InSAR using interpolated ECMWF data and GPS zenith total delay from the Southern California Integrated GPS Network
- 2010
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Ingår i: Proceedings of the 2010 IEEE International Geoscience and Remote Sensing Symposium, Proceedings DVD-ROM. - 9781424495665 ; , s. 4503-4506
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- A tropospheric correction method for Interferometric Synthetic Aperture Radar (InSAR) was developed using profiles from the European Centre for Medium-Range Weather Forecasts (ECMWF) and Zenith Total Delay (ZTD) from the Global Positioning System (GPS). The ECMWF data were interpolated into a finer grid with the Stretched Boundary Layer Model (SBLM) using a Digital Elevation Model (DEM) with a horizontal resolution of 1 arcsecond. The output were converted into ZTD and combined with the GPS ZTD in order to achieve tropospheric correction maps utilizing both the high spatial resolution of the SBLM and the high accuracy of the GPS. These maps were evaluated for three InSAR images, with short temporal baselines (implying no surface deformation), from Envisat during 2006 on an area stretching northeast from the Los Angeles basin towards Death Valley.The RMS in the InSAR images was greatly reduced, up to 32%, when using the tropospheric corrections. Two of the residuals showed a constant gradient over the area, suggesting a remaining orbit error. This error was reduced by reprocessing the troposphere corrected InSAR images with the result of an overall RMS reduction of 15 − 68%.
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