| 1. |
- Geremia-Nievinski, F., et al.
(författare)
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SNR-based GNSS reflectometry for coastal sea-level altimetry: results from the first IAG inter-comparison campaign
- 2020
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Ingår i: Journal of Geodesy. - : Springer Science and Business Media LLC. - 0949-7714 .- 1432-1394. ; 94:8
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Tidskriftsartikel (refereegranskat)abstract
- Ground-based Global Navigation Satellite System Reflectometry (GNSS-R) is quickly maturing toward the objective of becoming a viable alternative for operational coastal sea-level (SL) altimetry in a geocentric reference frame. SL has immense societal implications related to climate change. Of particular interest is the exploitation of existing coastal GNSS sites for reflectometry by means of signal-to-noise ratio (SNR) observables. We report results from the first inter-comparison campaign on SNR-based GNSS-R. The goal was to cross-validate retrieval solutions from independent research groups under comparable conditions. This action was an initiative of the International Association of Geodesy working group 4.3.9 (2015–2019 term). Data collected at the Onsala Space Observatory for a 1-year period (2015–2016) were compared to a co-located tide gauge (TG). SNR data for the GPS L1-C/A signal were processed by four groups, in Sweden, Luxembourg/Brazil, Germany, and the UK. Semidiurnal tidal constituents showed good agreement between TG and all GNSS-R groups. SL variations at diurnal and longer periods were also well captured by all series. Most GNSS-R solutions exhibited spurious tones at integer fractions of one sidereal day, the satellite revisit time of the particular GNSS constellation employed (GPS). Band-pass filtering between 3 h and 30 h confirmed that the dominant tidal components were well captured by most GNSS-R solutions. Higher-frequency SL variations (periods < 3 h) are poorly represented by GNSS-R as a consequence of its low temporal resolution. The solution with the worst agreement neglects a correction associated with the rate of change in sea level and uses narrower satellite elevation ranges per retrieval. Overall, there was excellent agreement, with correlation coefficients exceeding 0.9 and RMSE smaller than 5 cm.
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| 3. |
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| 4. |
- 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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| 5. |
- Hobiger, Thomas, 1978, et al.
(författare)
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Inverse modeling of ground-based GNSS-r - Results and new possibilities
- 2017
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Ingår i: International Geoscience and Remote Sensing Symposium (IGARSS). ; 2017-July, s. 2671-2681
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Konferensbidrag (refereegranskat)abstract
- Inverse modeling of ground-based GNSS reflections has the potential to improve existing GNSS-R applications and lead to novel concepts for retrieving geophysical parameters around existing or deliberately built GNSS sites. As we will show in this paper, consistent, accurate and precise determination of sea-surface heights can be realized. Moreover, we demonstrate that it is possible to detect sea-ice freeze states at coastal GNSS sites. In addition, it will be shown how the inverse modeling approach can be used for self-calibration of troposphere delays, which would otherwise bias the target parameters and need to be corrected for by external measurements or data-sets.
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| 6. |
- Hobiger, Thomas, 1978, et al.
(författare)
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Software defined radio for ground and airborne GNSS reflectometry
- 2018
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Ingår i: Proceedings of the European Navigation Conference 2018 (ENC 2018) Abstracts and Technical Papers. - 9789188041142 ; , s. 49-50
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Konferensbidrag (refereegranskat)abstract
- Software defined radio (SDR) appears as a suitable solution for dedicated GNSS reflectometry (GNSS-R) applications. Not only does the flexibility of SDR allow for easy and rapid prototyping, but also do recent technological developments of SDR front-ends support real-time operation of GNSS-R. Our presentation includes a discussion about the technical aspects of SDR for GNSS-R and we show results from a ground-based GNSS-R SDR receiver which was operated continuously over a more than a month at the Onsala Space Observatory. A summary of our current activities in relation to airborne GNSS-R solutions and initial results in the form of Delay-Doppler Maps (DDMs) will conclude the presentation.
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| 7. |
- Hoseini, Mostafa, et al.
(författare)
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On the Response of Polarimetric GNSS-Reflectometry to Sea Surface Roughness
- 2021
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Ingår i: IEEE Transactions on Geoscience and Remote Sensing. - 0196-2892 .- 1558-0644. ; 59:9, s. 7945-7956
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Tidskriftsartikel (refereegranskat)abstract
- Reflectometry of Global Navigation Satellite Systems (GNSS) signals from the ocean surface has provided a new source of observations to study the ocean-atmosphere interaction. We investigate the sensitivity and performance of GNSS-Reflectometry (GNSS-R) data to retrieve sea surface roughness (SSR) as an indicator of sea state. A data set of one-year observations in 2016 is acquired from a coastal GNSS-R experiment in Onsala, Sweden. The experiment exploits two sea-looking antennas with right- and left-hand circular polarizations (RHCP and LHCP). The interference of the direct and reflected signals captured by the antennas is used by a GNSS-R receiver to generate complex interferometric fringes. We process the interferometric observations to estimate the contributions of direct signals and reflections to the total power. The power estimates are inverted to the SSR using the state-of-the-art model. The roughness measurements from the RHCP and LHCP links are evaluated against match-up wind measurements obtained from the nearest meteorological station. The results report on successful roughness retrieval with overall correlations of 0.76 for both links. However, the roughness effect in LHCP observations is more pronounced. The influence of surrounding complex coastlines and the wind direction dependence are discussed. The analysis reveals that the winds blowing from land have minimal impact on the roughness due to limited fetch. A clear improvement of roughness estimates with an overall correlation of 0.82 is observed for combined polarimetric observations from the RHCP and LHCP links. The combined observations can also improve the sensitivity of GNSS-R measurements to the change of sea state.
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| 8. |
- Purnell, David, et al.
(författare)
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Quantifying the Uncertainty in Ground-Based GNSS-Reflectometry Sea Level Measurements
- 2020
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Ingår i: IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing. - 2151-1535 .- 1939-1404. ; 13, s. 4419-4428
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Tidskriftsartikel (refereegranskat)abstract
- Global Navigation Satellite System reflectometry (GNSS-R) tide gauges are a promising alternative to traditional tide gauges. However, the precision of GNSS-R sea-level measurements when compared to measurements from a colocated tide gauge is highly variable, with no clear indication of what causes the variability. Here, we present a modeling technique to estimate the precision of GNSS-R sea-level measurements that relies on creating and analyzing synthetic signal-to-noise-ratio (SNR) data. The modeled value obtained from the synthetic SNR data is compared to observed root mean square error between GNSS-R measurements and a colocated tide gauge at five sites and using two retrieval methods: spectral analysis and inverse modeling. We find that the inverse method is more precise than the spectral analysis method by up to 60 for individual measurements but the two methods perform similarly for daily and monthly means. We quantify the contribution of dominant effects to the variations in precision and find that noise is the dominant source of uncertainty for spectral analysis whereas the effect of the dynamic sea surface is the dominant source of uncertainty for the inverse method. Additionally, we test the sensitivity of sea-level measurements to the choice of elevation angle interval and find that the spectral analysis method is more sensitive to the choice of elevation angle interval than the inverse method due to the effect of noise, which is greater at larger elevation angle intervals. Conversely, the effect of tropospheric delay increases for lower elevation angle intervals but is generally a minor contribution.
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| 9. |
- Strandberg, Joakim, 1991, et al.
(författare)
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Can We Measure Sea Level With a Tablet Computer?
- 2020
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Ingår i: IEEE Geoscience and Remote Sensing Letters. - 1558-0571 .- 1545-598X. ; 17:11, s. 1876-1878
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Tidskriftsartikel (refereegranskat)abstract
- Modern mobile phones and tablet computers can have the capacity to store raw Global Navigation Satellite System (GNSS) data for further processing. With a short proof-of-concept campaign, we show that such data, recorded with a tablet computer, can be used to measure the sea level using GNSS reflectometry (GNSS-R). The results suggest that the tablet computer performs on a similar level as more high-end, geodetic-quality equipment.
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| 10. |
- Strandberg, Joakim, 1991, et al.
(författare)
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Coastal Sea Ice Detection Using Ground-Based GNSS-R
- 2017
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Ingår i: IEEE Geoscience and Remote Sensing Letters. - 1558-0571 .- 1545-598X. ; 14:9, s. 1552-1556
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Tidskriftsartikel (refereegranskat)abstract
- Determination of sea ice extent is important both for climate modeling and transportation planning. Detection and monitoring of ice are often done by synthetic aperture radar imagery, but mostly without any ground truth. For the latter purpose, robust and continuously operating sensors are required. We demonstrate that signals recorded by ground-based Global Navigation Satellite System (GNSS) receivers can detect coastal ice coverage on nearby water surfaces. Beside a description of the retrieval approach, we discuss why GNSS reflectometry is sensitive to the presence of sea ice. It is shown that during winter seasons with freezing periods, the GNSS-R analysis of data recorded with a coastal GNSS installation clearly shows the occurrence of ice in the bay where this installation is located. Thus, coastal GNSS installations could be promising sources of ground truth for sea ice extent measurements.
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| 11. |
- 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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| 12. |
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| 13. |
- Strandberg, Joakim, 1991, et al.
(författare)
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Inverse modelling of GNSS multipath for sea level measurements - initial results
- 2016
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Ingår i: Proceedings of the International Geoscience and Remote Sensing Symposium (IGARSS) Volume 2016-November, 1 November 2016, Article number 7729479, Pages 1867-1869 36th IEEE International Geoscience and Remote Sensing Symposium, IGARSS 2016; Beijing; China; 10 - 15 July 2016. - 9781509033324 ; 2016-November, s. 1867-1869
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Konferensbidrag (refereegranskat)abstract
- We present a new method to retrieve sea level from GNSS SNRdata that relies upon inverse modelling of the detrended SNR. This method can simultaneously use data from both GPS and GLONASS, and both L1 and L2 frequencies, to improve thesolution with respect to prior studies. Results from the GNSS-R installation at Onsala Space Observatory are presented and the retrieved sea level heights are compared with a co-located pressure mareograph. The method is found to give an RMS error of 1.8 cm. The results are also compared against previous implementations of GNSS tide gauges and found to have lower RMS than both the earlier SNR algorithm and also the dual receiver, phase delay method.
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| 14. |
- Strandberg, Joakim, 1991
(författare)
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Inverse modelling of GNSS multipath signals
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Measuring the world around us is necessary to observe and understand the changes that occur in our environment. A widely distributed network of measurement stations can help us to understand ongoing and predict future climate change. GNSS reflectometry has the capacity of providing data from all over the world, as there are already many GNSS stations established and operated for navigational and meteorological purposes. This thesis presents a new way of retrieving environmental data from GNSS signal-to-noise ratio measurements which has the capability to provide new types of measurements. The method is based on inverse modelling of the signal-to-noise ratio in order to retrieve physical parameters of reflecting surfaces around GNSS installations. It is successfully demonstrated that the method improves the precision of the GNSS reflectometry derived sea surface height measurements significantly. By using the signal-to-noise ratio pattern, it is also — for the first time — demonstrated that it is possible to use GNSS reflectometry to detect coastal sea ice.
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| 15. |
- Strandberg, Joakim, 1991
(författare)
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New methods and applications for interferometric GNSS reflectometry
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The GNSS reflectometry technique has been proven to be usable for measuring several environmental properties, such as soil moisture, snow depth, vegetation, and sea level. As numerous GNSS installations are already installed around the world for geodetic purposes, the technique opens up a large data set for new analyses, complementing other environmental measurement campaigns. However, a main drawback of the technique is that its precision generally is worse than more specialised equipment, and while this is in part compensated for its low cost and maintenance requirements, improved precision is still a main goal of research in the field of GNSS reflectometry. The first topic of this thesis concerns the development of new methods for analysing GNSS-R data to retrieve precise measurements, especially in the case of sea level. As GNSS-R measurements are usually done over time spans of around half an hour, the dynamic sea surface has proven to be a challenge to measure. However, using inverse modelling with least squares adjustment, we prove that we can significantly improve the retrieval precision. Developing on the inverse modelling approach, we also prove that high-precision real-time GNSS reflectometry is also feasible using Kalman filtering. The other main topic of this thesis is finding new applications for the GNSS-R technique. Firstly, we show that when a GNSS-R installation is mounted close to a body of water, it is possible to determine whether the surface is frozen or not. Secondly, while GNSS reflectometry is traditionally performed with high-precision geodetic instruments, we show that everyday devices, such as a mobile phone, can be used instead. We find that the precision of the mobile devices is on a similar level as for geodetic equipment. Finally, this thesis explores and highlights one of the challenges that are still left in GNSS-R research: absolute referencing of sea level measurements. Past research has mostly focused on precision, leaving out accuracy, and we show that there are unknown effects that cause an offset between GNSS-R measurements and co-located tide gauges.
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| 16. |
- Strandberg, Joakim, 1991, et al.
(författare)
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Real-time sea-level monitoring using Kalman filtering of GNSS-R data
- 2019
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Ingår i: GPS Solutions. - : Springer Science and Business Media LLC. - 1080-5370 .- 1521-1886. ; 23:3
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Tidskriftsartikel (refereegranskat)abstract
- Current GNSS-R (GNSS reflectometry) techniques for sea surface measurements require data collection over longer periods, limiting their usability for real-time applications. In this work, we present a new, alternative GNSS-R approach based on the unscented Kalman filter and the so-called inverse modeling approach. The new method makes use of a mathematical description that relates SNR (signal-to-noise ratio) variations to multipath effects and uses a B-spline formalism to obtain time series of reflector height. The presented algorithm can provide results in real time with a precision that is significantly better than spectral inversion methods and almost comparable to results from inverse modeling in post-processing mode. To verify the performance, the method has been tested at station GTGU at the Onsala Space Observatory, Sweden, and at the station SPBY in Spring Bay, Australia. The RMS (root mean square) error with respect to nearby tide gauge data was found to be 2.0 cm at GTGU and 4.8 cm at SPBY when evaluating the output corresponding to real-time analysis. The method can also be applied in post-processing, resulting in RMS errors of 1.5 cm and 3.3 cm for GTGU and SPBY, respectively. Finally, based on SNR data from GTGU, it is also shown that the Kalman filter approach is able to detect the presence of sea ice with a higher temporal resolution than the previous methods and traditional remote sensing techniques which monitor ice in coastal regions.
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| 17. |
- Strandberg, Joakim, 1991, et al.
(författare)
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Towards Real-Time GNSS Reflectometry Using Kalman Filtering
- 2018
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Ingår i: International Geoscience and Remote Sensing Symposium (IGARSS). ; 2018-July, s. 2043-2046
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Konferensbidrag (refereegranskat)abstract
- GNSS-R has emerged as an attractive way of using a signal of opportunity that is collected by GNSS stations all around the world to measure a wide variety of properties of the surroundings of the stations. Current state-of-the-art algorithms based on the inversion of SNR values rely on off-line processing, causing a significant delay before results are available. We present a new approach for ground-based GNSS-R that uses Kalman filtering with a realistic physical model that allows close to real-time inversion of SNR oscillations into sea-surface height with high precision. From the analysis of test measurements from the GTGU GNSS installation at the Onsala Space Observatory, Sweden, we conclude that the new method provides better estimates than single-arc retrievals from spectral analysis and that the final precision is close to that of post-processing inversion algorithms.
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