The Performance of Virtual Reference Stations in Active Geodetic GPS-networks under Solar Maximum Conditions

In order to use fast static or real-time kinematic (RTK) cm-accurate positioning over larger distances, active GPS reference receiver networks have been established in parts of Germany with station distances of 30 to 50 km. Within these networks, the distance dependent error sources, like ionospheric and tropospheric refraction and broadcast or-bit errors, are modelled satellite-by-satellite and with high temporal resolution. This procedure leads to the concept of virtual reference stations whose observations are com-puted from the data of the surrounding real reference sta-tions and which are located at the user’s approximate po-sition. The modelling algorithm for distance dependent errors in GPS-networks assumes that these errors can be linearly interpolated, i.e. that their spatial wavelength is much larger than the distances between reference stations. This, however, is not the case for Medium-Scale Travelling Io-nospheric Disturbances (MSTIDs), which have horizontal wavelengths of 100 to several hundred km. Whereas the number of observed MSTIDs over Europe was small in the solar minimum years (1994 - 1998), the situation dramatically changed with the commencement of the MSTID winter season 1998-1999. From October 1998 to March 1999 their occurrence produced adverse effects on classical baseline positioning and also on the quality of virtual reference stations. It is found that even in the presence of MSTIDs most of the relative ionospheric errors can be modelled and cor-rected for in regional GPS reference networks. Neverthe-less some ionospheric effects remain in the short baseline between virtual reference station and rover receiver. Am-biguity resolution and coordinate estimation of this base-line have to take these ionospheric effects into account.