GPS data from a dense network of receivers can be an important source of information on Travelling Ionospheric Disturbances (TIDs), especially for smaller scale fluctuations (small and medium scale TIDs). Since absolute TEC ``calibration'' is not necessary it is sufficient to work with GPS differential phase data. We are using data from a dense network of GPS receivers distributed over Austria to assess TID activities on a bi-hourly basis. The raw data are stored in the well-known RINEX format. The phase differences are extracted and after correction of ``cycle slips'' the data are subjected to band-pass filtering. Cycle slip correction is a tricky but necessary process. The use of ``cycle slip free'' time intervals would leave only a fraction of available data for TID activity assessment. We describe the data retrieval and preparation system and show first results (case studies and preliminary results).