National Report of Sweden to the NKG General ... - Lantmäteriet

17comparison and evaluation offrequency transfer with VLBI andGNSS. Our results show that geodeticVLBI can reach frequency transferstability of 1e-15 during one day, andthat this is in good agreement withGNSS-based techniques.We use the global geodetic VLBI dataset to derive long time series oftropospheric zenith wet delay (ZWD)and atmospheric gradient values. ForOnsala, these time series cover morethan 25 years. The ZWD can beconverted into information on theintegrated water vapour content(IPWV) and compared toindependently derived IPWV resultsfrom a ground based microwaveradiometer operated at Onsala andradiosonde observations from theLandvetter-Gothenburg airport. Figure3.2 shows the corresponding timeseries.Figure 3.2: Time series of integratedprecipitable water vapor (IPWV) asdetermined from geodetic VLBI dataobserved at Onsala (VLBI, top) microwaveradiometry at Onsala (WVR, middle) andradiosondes at Gothenburg-Landvetter (RS,bottom).the order of 0.4 to 0.6 kg/m2 perdecade. However, the agreement is notperfect. A major problem in thecomparison is the different sampling ofthe three data sets and the individualdata gaps. Synchronization of the datasets results in small biases on the orderof 1 kg/m2 and root-mean-square(RMS) differences of less than 2 kg/m2,but does not improve the agreement ofthe trends.3.3 Super-conducting gravimetryOn June 10 2009, a super-conductinggravimeter (SCG, series number GWR-054) was taken into operation at theOnsala Space Observatory. The mainuse of the new facility is providing acalibrated gravity station for visitinggroups within absolute gravity projectsaiming e.g. at determining gravitychanges in the Nordic countries inconnection with GIA. The newgravimeter station provides us with the"third pillar" of geodesy, i.e. gravityand geopotential measurement,complementing our contributions theother two pillars, earth rotation andearth deformation. This developmenthas now raised Onsala to the status of aFundamental Geodetic Station, a corestations for the maintenance of theInternational Reference System. Figure3.3. presents the gravimeter record forslightly more than one year. Severalexternal research groups with absolutegravimeters have been visiting Onsalasince mid 2009 to do parallelmeasurements with the newinstrument.The IPWV data derived from theindividual techniques show highcorrelation with correlation coefficientsof 0.95 and better. All three techniquesshow positive trends for the IPWV on