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- Bock, O., et al.
(author)
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Use of GNSS Tropospheric Products for Climate Monitoring (Working Group 3)
- 2020
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In: Advanced GNSS Tropospheric Products for Monitoring Severe Weather Events and Climate. - Cham : Springer International Publishing. - 9783030139001 ; , s. 267-402
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Book chapter (other academic/artistic)abstract
- There has been growing interest in recent years in the use of homogeneously reprocessed ground-based GNSS, VLBI, and DORIS measurements for climate applications. Existing datasets are reviewed and the sensitivity of tropospheric estimates to the processing details is discussed. The uncertainty in the derived IWV estimates and linear trends is around 1 kg m^2 RMS and ± 0.3 kg m^2 per decade, respectively. Standardized methods for ZTD outlier detection and IWV conversion are proposed. The homogeneity of final time series is limited however by changes in the stations equipment and environment. Various homogenization algorithms have been evaluated based on a synthetic benchmark dataset. The uncertainty of trends estimated from the homogenized times series is estimated to ±0.5 kg m^2 per decade. Reprocessed GNSS IWV data are analysed along with satellites data, reanalyses and global and regional climate model simulations. A selection of global and regional reprocessed GNSS datasets and ERA-interim reanalysis are made available through the GOP-TropDB tropospheric database and online service. A new tropo SINEX format, providing new features and simplifications, was developed and it is going to be adopted by all the IAG services.
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| 3. |
- Elgered, Gunnar, 1955, et al.
(author)
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Assessing the Quality of Water Vapor Radiometer Data from Onsala during the CONT11 Geodetic VLBI Campaign
- 2012
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In: International VLBI Service for Geodesy and Astrometry 2012 General Meeting Proceedings: "Launching the Next-Generation IVS Network", eds. Dirk Behrend and Karen D. Baver. ; NASA/CP-2012-217504, s. 410-414
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Conference paper (other academic/artistic)abstract
- Two water vapour radiometers (WVRs), called Astrid and Konrad, were operated during the CONT11 campaign at the Onsala Space Observatory. A well known feature of WVRs is that their algorithm brakes down when there are large drops of water affecting the observations. Without any independent information one has to rely on the WVR data themselves to detect rain and remove the corresponding low quality results. In order to assess this technique we operated different types of rain sensors during CONT11: a zenith looking Doppler rain radar, three optical, and three capacitive sensors. The first 8.25 days had frequent rain events whereas the last 7.75 days where significantly drier. We summarize the data set acquired by the WVRs and the rain detectors. The inferred time series of the equivalent zenith wet delay (ZWD) and linear horizontal gradients are compared to the corresponding estimates from GPS data (acquired at the IGS station ONSA) and the VLBI data themselves.
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| 4. |
- Elgered, Gunnar, 1955, et al.
(author)
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Assessment of GNSS stations using atmospheric horizontal gradients and microwave radiometry
- 2023
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In: Advances in Space Research. - 1879-1948 .- 0273-1177. ; In Press
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Journal article (peer-reviewed)abstract
- We have assessed the quality of four co-located GNSS stations by studying time series of estimated linear horizontal gradients in the signal delay. The stations have different electromagnetic environments. We also examine the consistency of the results by using two different GNSS softwares, GipsyX and c5++, and applying three different elevation cutoff angles: 5°, 10°, and 20°. The estimated gradients are compared with the corresponding ones estimated from microwave radiometer observations acquired during six months (April–September 2021). For all four stations and using both softwares we find that is is possible to track gradient variations over time scales from less than one hour using GPS observations only. We have indications that it is an advantage to equip the area below the GNSS antenna with microwave absorbing material. However, the differences are small, a reduction in rms differences in the gradients compared to those from the microwave radiometer of less than 2 %. More studies are needed to decide if such an investment is reasonable in terms of cost and maintenance.
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| 6. |
- Elgered, Gunnar, 1955, et al.
(author)
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Comparison of Atmospheric Gradients Estimated From Ground-Based GNSS Observations and Microwave Radiometry
- 2019
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In: The proceedings to the 7th Galileo Science Colloquium.
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Conference paper (peer-reviewed)abstract
- Observations over four years from two nearby groundbased Global Navigation Satellite System (GNSS) stations and one microwave radiometer have been used to estimate linear horizontal gradients in the atmosphere. We find that gradients estimated by the radiometer have larger amplitudes than those estimated using data from the Global Positioning System (GPS). One reason for this is that they are estimated, every 15 min, independently of previous estimates, whereas the gradients from GPS are estimated every 5 min using constraints on their variability. We also find that the elevation cutoff angle has a significant impact on the estimated GPS gradients. Decreasing the cutoff angle results in smaller gradient amplitudes. The estimated gradients are not homogeneously distributed in all directions. When studying the largest gradients they all occur during the warmer period of the year, beginning in April and ending in October. Specifically, for the 25 events with the largest gradient amplitudes from the GPS data, we find that the vast majority of them are associated with the passage of weather fronts.
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| 7. |
- Elgered, Gunnar, 1955, et al.
(author)
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Comparison of four different GNSS antenna installations using estimated atmospheric gradients and microwave radiometry
- 2022
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Conference paper (other academic/artistic)abstract
- We have assessed the quality of four GNSS antenna installations by studying time series of estimated linear horizontal gradients in the signal delay. The stations have different electromagnetic environments. We also examine the consistency of the results by using two different GNSS softwares, GipsyX and c5++, and applying three different elevation cutoff angles: 5◦, 10◦, and 20◦. The estimated gradients are compared to the corresponding ones estimated from microwave radiometer observations acquired during six months (April–September 2021). We have indications that it is an advantage to equip the area below the GNSS antenna with microwave absorbing material. However, the differences are small and more studies are needed to decide if such an investment is reasonable in terms of cost and maintenance.
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| 9. |
- Elgered, Gunnar, 1955, et al.
(author)
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On the information content in linear horizontal delay gradients estimated from space geodesy observations
- 2019
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In: Atmospheric Measurement Techniques. - : Copernicus GmbH. - 1867-1381 .- 1867-8548. ; 12:7, s. 3805-3823
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Journal article (peer-reviewed)abstract
- We have studied linear horizontal gradients in the atmospheric propagation delay above ground-based stations receiving signals from the Global Positioning System (GPS). Gradients were estimated from 11 years of observations from five sites in Sweden. Comparing these gradients with the corresponding ones from the European Centre for Medium- Range Weather Forecasts (ECMWF) analyses shows that GPS gradients detect effects over different timescales caused by the hydrostatic and the wet components. The two stations equipped with microwave-absorbing material below the antenna, in general, show higher correlation coefficients with the ECMWF gradients compared to the other three stations. We also estimated gradients using 4 years of GPS data from two co-located antenna installations at the Onsala Space Observatory. Correlation coefficients for the east and the north wet gradients, estimated with a temporal resolution of 15 min from GPS data, can reach up to 0.8 for specific months when compared to simultaneously estimated wet gradients from microwave radiometry. The best agreement is obtained when an elevation cut-off angle of 3° is applied in the GPS data processing, in spite of the fact that the radiometer does not observe below 20°. We also note a strong seasonal dependence in the correlation coefficients, from 0.3 during months with smaller gradients to 0.8 during months with larger gradients, typically during the warmer and more humid part of the year. Finally, a case study using a 15 d long continuous verylong- baseline interferometry (VLBI) campaign was carried out. The comparison of the gradients estimated from VLBI and GPS data indicates that a homogeneous and frequent sampling of the sky is a critical parameter.
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| 10. |
- Elgered, Gunnar, 1955, et al.
(author)
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On the use of water vapour radiometry for assessment of wet delay estimates from space geodetic techniques
- 2023
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Conference paper (other academic/artistic)abstract
- During the development of the Mark III VLBI system in the seventies, water vapour radiometers (WVR) were envisaged to provide independent observations of the signal propagation delay due to water vapour along the line of sight. The standard design of the WVR is to measure the atmospheric emission at two frequencies, close to and further away from the centre of the water vapour emission line at 22.2 GHz. These measurements are used to estimate two unknowns, the amount of water vapour, or the wet delay, and the amount of liquid water, along the line of sight. The main drawback of using a WVR is that the retrieval algorithm requires that any drops of liquid water in the sensed volume of air are much smaller than the wavelength observed by the WVR, i.e. approximately 1 cm. The algorithm therefore more or less breaks down during rain, meaning that the instrument cannot be relied on for 100 % of time, unless it never rains on, or close to, the site. The method generally used to avoid using WVR data with poor accuracy is to ignore observations obtained during rain and when the inferred liquid water content is above a specific threshold. However, there are a couple of difficulties with these procedures. (i) There may be rain drops in the sensed atmospheric volume in spite of the fact that no drops are detected at the ground on the site; (ii) there may still be drops of water on the WVR instrument, such as on the protective covers of the horn antennas and the mirrors many minutes after the rain has stopped; (iii) a low density of large drops may result in a smaller liquid water content than many small drops. We have used WVR data from 2022 together, with rain observations, to study the retrieval accuracy by comparing them to wet delay estimates from the GNSS station ONSA. We search for general rules of thumb searching for periods when WVR and GNSS data offer the best agreement in the equivalent zenith wet delay, given the rain observations and the inferred liquid water content.
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