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

19importance for the operational weatherand climate forecasting. However, dueto it is variable both spatially andtemporally, measuring the content ofthe water vapour, especially for longtime series is difficult. Based on thetiming of radio waves propagatingthrough the atmosphere, GNSS can beused to determine the amount ofatmospheric water vapor abovereceivers on the ground. Along withdensification and extension ofpermanent GNSS station networksglobally, using GNSS measurement toestimate atmospheric water vapourcontent is a promising application.Motivated by the purpose to validateand improve climate models, whichnormally are used for forecasting andalso to assess our future climate,independent measurements of thecontent of the Earth’s atmosphericwater vapour from continuouslyoperating GNSS networks are used. Inthe project, longer time seriesobservations (some up to 15 years)from more than 100 GNSS sites (most ofthem in Europe, and some global) areanalyzed, see Figure 3.5.Figure 3.5: The distribution of the GNSSstations used in the project withindependent measurements of the content ofthe Earth’s atmospheric water vapour.Systematic effects will be studied inorder to obtain realistic trends, withsmall uncertainties. The GNSSestimates are compared to the watervapour content derived from twoclimate models. The results are not onlyinteresting for the climate applications,but also can be used to improve theperformance of GNSS techniques.3.4.3 Investigations on theelectromagnetic environment of theGNSS ground-based antennasAfter decades of continuousdevelopment, data from GNSS havebeen used successfully in manyapplications. For example, continuouslyoperating GPS stations have significantadvantages for determining the Earth’satmospheric water vapour content. Theformal uncertainty is in the order of 0.5kg/m2 and Root-Mean-Square (RMS)difference seen in comparisons to otherinstruments, such as radiosondes andmicrowave radiometers. Based on thehighly precise orbit information andconsistent Earth orientation parameters,the accuracy of horizontal positionestimates from the GPS data are at themillimetre level. However, to get thesame accuracy in the verticalcomponent of the coordinate estimatesfrom GNSS, the impact of theelectromagnetic environment of theGNSS antennas, i.e. scattering andmultipath reflection, should beinvestigated and mitigated. Thereforethe influence on the measurementsfrom attaching anti-reflection material,i.e. microwave absorbers to the GNSSantenna is of great interest to study.Additionally, many antennas ofgeodetic stations are protected byradomes from extreme weatherconditions. The effects from theinstallation of radomes are alsoimportant to know.