GPS tropospheric zenith delay is correlated with the site co-ordinates, especially with the vertical one. For meteorological application there is no need to estimate them when processing GPS data, but, in order to derive the ‘best’ possible ZTD estimates, there is the need to know site co-ordinates with a certain level of accuracy. Generally they are obtained averaging over a longer period of time daily station estimates. So even for pure meteorological application there is the need of station co-ordinates monitoring. Of course, they are related to the terrestrial reference frame (TRF) in which they have been computed, and the changing of TRF could introduce biases into the GPS ZTD IWV products. Furthermore constrains to the reference frame are also induced by fixing the GPS orbits (IGS orbits are given in a TRF) during the data reduction, how is commonly done when regional network are considered. Therefore it is an interesting question to understand how to deal with the biases related to reference frame, even for climate investigations. Furthermore, the geodetic reference frame is always being improved, there are occasionally slight changes which can lead to offsets in the long-term trend of GPS ZTD.
· The influence different reference frames have on GPS ZTD estimates will be evaluated.
· ASI will establish a methodology for dealing with updates to the reference frame
· ASI will verify that differences between processing centres estimates for the reference IGS stations are not due to orbit errors, or co-ordinate errors or reference frame errors.
· ASI will establish guidelines for verifying the quality of GPS ZTD IWV data by examining repeatability of co-ordinates and lead the implementation of these guidelines in the GPS ZTD IWV processing.