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18respectively (Löfgren et al., 2010). Theresults show that the pair wise rootmean-squareagreement between thethree independent time series wasbetter than 4 cm, indicating that theGNSS-based tide gauge gives valuableresults for sea level monitoring.Figure 3.3: Gravity signal recorded withthe super-conducting gravimeter at OnsalaSpace Observatory.3.4 GNSS3.4.1 Measuring sea surface heightusing GNSS signalsDuring 2008 we started a project tomeasure local sea level and its variationusing GNSS signals. The measurementsare done using a dual GNSS antennaassembly, that we call a GNSS basedtide gauge, at the coast at the OnsalaSpace Observatory, see Figure 3.4. Oneantenna is directed upward, receivingthe direct GNSS signals, and measuringthe land surface height, whereas theother antenna is directed downward,receiving the GNSS signals reflectedfrom the sea surface, and measuring thesea surface height.The analysis of phase measurementsperformed with the correspondingGNSS receivers allows to estimate thelocal sea surface height and itsvariation. Results from hourly solutionsof the local sea level at Onsala werecompared with data from two stillingwell gauges, operated by SMHI 39atRinghals and Göteborg about 18 kmsouth and 33 km north of Onsala,39SMHI = Swedish Meteorological and HydrologicalInstituteFigure 3.4: GNSS-based tide-gauge at theOnsala Space Observatory.3.4.2 Using GNSS signals to measurethe long term change of the Earth’satmospheric water vapourWater vapour is a key element in ourclimate system. It takes part in thehydrological cycle by transportingwater in the atmosphere andredistributing energy throughevaporation and condensation and itaffects the precipitation and soilmoisture. Water vapour is also the mostimportant green-house gas, absorbingand trapping radiations emitted fromthe Earth’s surface. Therefore,Knowledge of the concentration andlong-term changes of water vapour inthe Earth’s atmosphere is of crucial