Browsing by Author "Saari, Timo"

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Geodetic SAR for Height System Unification and Sea Level Research—Results in the Baltic Sea Test Network
(2022-07-01) Gruber, Thomas; Ågren, Jonas; Angermann, Detlef; Ellmann, Artu; Engfeldt, Andreas; Gisinger, Christoph; Jaworski, Leszek; Kur, Tomasz; Marila, Simo; Nastula, Jolanta; Nilfouroushan, Faramarz; Nordman, Maaria; Poutanen, Markku; Saari, Timo; Schlaak, Marius; Świątek, Anna; Varbla, Sander; Zdunek, Ryszard
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Coastal sea level is observed at tide gauge stations, which usually also serve as height reference stations for national networks. One of the main issues with using tide gauge data for sea level research is that only a few stations are connected to permanent GNSS stations needed to correct for vertical land motion. As a new observation technique, absolute positioning by SAR using off the shelf active radar transponders can be installed instead. SAR data for the year 2020 are collected at 12 stations in the Baltic Sea area, which are co-located to tide gauges or permanent GNSS stations. From the SAR data, 3D coordinates are estimated and jointly analyzed with GNSS data, tide gauge records and regional geoid height estimates. The obtained results are promising but also exhibit some problems related to the electronic transponders and their performance. At co-located GNSS stations, the estimated ellipsoidal heights agree in a range between about 2 and 50 cm for both observation systems. From the results, it can be identified that, most likely, variable systematic electronic instrument delays are the main reason, and that each transponder instrument needs to be calibrated individually. Nevertheless, the project provides a valuable data set, which offers the pos-sibility of enhancing methods and procedures in order to develop a geodetic SAR positioning technique towards operability.
Valtakunnalliset korkeudenmääritysmenetelmät
(2013) Saari, Timo
School of Engineering | Master's thesis
The aim of this research was to study different height determination techniques that could be used for the creation of the next national height system of Finland. There have been three height systems in the history of Finnish surveying, which have been based on the traditional precise levelling. For over a century, precise levelling has been the most accurate and reliable height determination technique, although it has its downsides. Creating a national height system with precise levelling is an extremely slow process, as in Finland it has been taken approximately twenty-five years. Height determination techniques that were studied in this research were the mobile laser scanning (MLS), GNSS-levelling (static GPS-measurement and VRS-measurement) and the traditional precise levelling. In Finland GNSS-levelling has been studied for the national height system more than a decade ago. Since that time, satellite techniques have been greatly developed. Using MLS for this kind of concept is a quite new approach, even though laser scanning has been one of the fastest growing techniques in the field of surveying. The research was divided into two sections: theoretical and practical. The techniques were described in the theoretical section, where also their usability for height determination was studied. The practical section consists of field measurements where height differences between two height benchmarks were measured with all of the techniques in question. The resulted height differences were compared to the height difference from the present national height system of Finland, N2000. The results differed from the N2000 in the following way: precise levelling 0,17 mm (+-0,27 mm), static GPS-measurements -0,31 mm (+- 3.44 mm) and 1,29 mm (+- 3,73 mm), MLS -5,22 mm and 6,67 mm (+- 16,24 mm) and VRS-measurement 10,65 mm (+- 12,46 mm). The precise levelling was the most accurate and reliable technique as predicted. Static GPS-measurements were quite reasonable especially when we take into account, that the surroundings of the benchmarks were pretty challenging for satellite measurements and only GPS-satellites were observed. The average height difference from MLS differed only 0,73 mm from the N2000, but the uncertainty (+- 16,24 mm) and the scatter of the measurements were remarkably high. The results of this master's thesis can be held as a preliminary study for the techniques in question. In order to make decisions about the most optimal height determination technique for the next national height system of Finland, we should expand this study into a much larger test fields. Then we could get a better picture of the accuracies and the time efficiencies of the studied techniques and explore the special characteristics of the MLS.
Investigations of geoid models in Finland - Towards GNSS-related height system
(2022) Saari, Timo
School of Engineering | Doctoral dissertation (article-based)
In Fennoscandia, heights and their relations between each other are in constant change due to the post-glacial rebound, so the national height system needs to be updated occasionally. Traditionally, the update has been done with a method known as precise levelling, which is accurate but considered slow, laborious, and expensive. This dissertation studied the modern, mainly Global Navigation Satellite System (GNSS) -based, height determination techniques that could replace precise levelling as the method for the next national height system of Finland. GNSS-based techniques provide the height component relative to a reference ellipsoid, which is a mathematical surface and therefore lacks a physical connection to the Earth – i.e. have no information on the direction of water flow. Here, we need a (quasi-)geoid model to tie the ellipsoidal heights to the surface of the Earth and to national height systems. The (quasi-)geoid model's accuracy is therefore crucial with GNSS/geoid techniques. The presented case study gave us knowledge from several techniques. The static GNSS proved the most promising, as the result was close to the one from precise levelling. However, due to the closeness of the evaluation points, the relative error of the geoid between them is negligible in practice, which is not the case in a nationwide network. The national quasi-geoid model of Finland, FIN2005N00, was created nearly two decades ago. The gravity satellites, Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) and Gravity Recovery and Climate Experiment (GRACE), have since measured the Earth's gravitational field in unprecedented detail. At the time, we investigated all the published GOCE and GRACE global gravitational models (GGM) in Finland. We learned that the best models already performed at the same level or better than the pre-GOCE era high-resolution models. The most suitable model, DIR5, was chosen as a background model with the high-resolution EIGEN-6C4 in the quasi-geoid modelling of Finland. The new and present quasi-geoid models were evaluated on land and in sea areas. A significant improvement was achieved over the present models.A marine GNSS/gravity campaign was performed in the Gulf of Finland to improve the accuracy and validation of a quasi-geoid model in sea areas. Geoid height differences of up to 15 cm were found with the new gravity data included in the quasi-geoid modelling. The result was confirmed in the evaluation with marine GNSS measurements in combination with sea surface models. This dissertation's results will be important for producing the next national quasi-geoid model of Finland. Additionally, the knowledge obtained from the GNSS/geoid method will be beneficial for the decision making of the chosen method for the next national height system of Finland.
Validating Geoid Models with Marine GNSS Measurements, Sea Surface Models, and Additional Gravity Observations in the Gulf of Finland
(2021) Saari, Timo; Bilker-Koivula, Mirjam; Koivula, Hannu; Nordman, Maaria; Häkli, Pasi; Lahtinen, Sonja
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Traditionally, geoid models have been validated using GNSS-levelling benchmarks on land only. As such benchmarks cannot be established offshore, marine areas of geoid models must be evaluated in a different way. In this research, we present a marine GNSS/gravity campaign where existing geoid models were validated at sea areas by GNSS measurements in combination with sea surface models. Additionally, a new geoid model, calculated using the newly collected marine gravity data, was validated. The campaign was carried out with the marine geology research catamaran Geomari (operated by the Geological Survey of Finland), which sailed back and forth the eastern part of the Finnish territorial waters of the Gulf of Finland during the early summer of 2018. From the GNSS and sea surface data we were able to obtain geoid heights at sea areas with an accuracy of a few centimetres. When the GNSS derived geoid heights are compared with geoid heights from the geoid models differences between the respective models are seen in the most eastern and southern parts of the campaign area. The new gravity data changed the geoid model heights by up to 15 cm in areas of sparse/non-existing gravity data.