Measuring and modelling variations in the distribution of atmospheric water vapour using GPS

Doctoral thesis, 2008

Turbulence introduce variations in the refractive index of air, which in turn result in fluctuations in the phase and amplitude of radio signals propagating through the atmosphere. Most important are the variations due to fluctuations in the water vapour field. These fluctuations limit the accuracy of space geodetic techniques such as VLBI (Very Long Baseline Interferometry) and GNSS (Global Navigation Satellite Systems). In order to obtain the highest possible accuracy it is important to understand these effects. In this thesis several different techniques for measurements of turbulence induced refractive index variations are presented. Using data from high resolution radiosondes vertical profiles of the variations above the radiosonde station can be obtained. Vertical average of the magnitude of the variations can be studied by using a radiometer to investigate the variations in the slant wet delays as function of direction. Similarly, GNSS data from several GNSS stations in a network could be used to investigate the variations in the zenith delay as function of location. One area where it is important to have a good modelling of the refractive index variations is in GNSS tropospheric tomography. This technique uses GNSS data from several stations in a dense local network in order to obtain the 3D structure of the atmospheric water vapour. The problem with this technique is that the geometry of the tomographic problem is poor, especially for flat networks, making it very difficult to obtain the vertical water vapour profile. This makes it necessary to use additional information or constraints. One way to obtain such constraints is using information of the refractive index variation from turbulence theory. It is shown in this thesis that if information on the vertical profile of the refractive index variations is available, better estimations of the water vapour profile could be achieved. If this is not the case the water vapour profile cannot be retrieved very accurately. However, it is shown that when retrieving the horizontal variations in the water vapour field, a higher accuracy is achievable.