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In the global navigation satellite system (GNSS) carrier phase data processing, cycle slips are limiting factors and affect the quality of the estimators in general. When differencing phase observations, a problem in phase ambiguity parameterization may arise, namely linear relations between some of the parameters. These linear relations must be considered as additional constraints in the system of observation equations. Neglecting these constraints, results in poorer estimators. This becomes significant when ambiguity resolution is in demand. As a clue to detect the problem in GNSS processing, we focused on the equivalence of using undifferenced and differenced observation equations. With differenced observables this equivalence is preserved only if we add certain constraints, which formulate the linear relations between some of the ambiguity parameters, to the differenced observation equations. To show the necessity of the additional constraints, an example is made using real data of a permanent station from the network of the international GNSS service (IGS). The achieved results are notable to the GNSS software developers.

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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).

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