The impact of the electromagnetic environment of the antenna on GPS

Water vapour is an important green-house gas. Accurate measurements of water vapour in the atmosphere are in general difficult and costly to carry out with high temporal and spatial resolution over long time. The ability of the Global Positioning System (GPS) to provide estimates of the atmospheric water vapour content above receivers on the ground is therefore an interesting application. Based on the timing of radio waves propagating through the atmosphere, the GPS can be used to determine the amount of atmospheric water vapour. Currently in Sweden there are 24 GPS stations under the operation of the Swedish Nationwide Multipurpose Network (SWEPOS) used for meteorology and climatology purposes. The end users of the SWEPOS data are the climate researchers at the Rossby Centre at the Swedish Meteorological and Hydrological Institute (SMHI).The effects introduced by the geometry of the installation of the geodetic GPS antennas on the estimated time series of atmospheric water vapour is of fundamental importance. This is especially true when the application of the estimated times series of water vapour is climate monitoring. A special GPS station has been designed at the Onsala Space Observatory, south of G\ua8oteborg, where the antenna is mounted on an x-y-z positioning platform in order to investigate these effects. This means that the antenna can be moved in different directions with respect to the radome as well as relative to the concrete pillar. Additionally, an arrangement has been implemented in order to move the radome along the vertical coordinate relative to the pillar. The data from the permanent SWEPOS GPS station at Onsala is used to provide a reference time series of water vapour to be compared to the corresponding estimates from the experimental station.By implementation of an analysis software, the GIPSY-OASIS II, the Zenith Total Delay (ZTD) can be obtained from the GPS data. The “wet” part of the propagation delay is often referred to as the Zenith Wet Delay (ZWD) and can be calculated from the ZTD by removing a component which can be estimated with high accuracy from observations or model estimates of the total ground pressure. Finally, by making an additional conversion we can derive the integrated water vapour content(IWV) from the ZWD.We present a first assessment of the impacts from a changing geometry of the nearby antenna environment. The long term goal is to be able to quantify the specifications on the geometric stability needed in order to detect variations in the water vapour content of the order of 0.1 mm/decade