Ground Based Augmentation Systems (GBAS) using Global Positioning System (GPS) and Galileo is an opportunity navigation system which is used for aircraft precision approach, landing and taxiing with guaranteed accuracy, robustness, integrity, and availability for all weather operations. Galileo supposed to be in operation by year 2013. The Institute of Communications and Navigation of Deutsches Zentrum für Luft- und Raumfahrt (DLR), as one of the leaders in developing GBAS system, is participating in the project. The first aim of this thesis is to study the state of the art of Ground Based Augmentation System, Global Positioning System, Galileo, and how they are working by now. We will present the state of the art Global Positioning System (GPS) and give a brief introduction to GBAS, and Galileo constellation. We will give an overview on measurement and error sources. We will study the effect of several kinds of error sources from real data provided by DLR and how these error sources can affect the GPS measurement. We also study the effect of error sources related to the Ground Accuracy Designator (GAD), which plays an important role on GBAS accuracy. The main goal of this study is to select the GAD, in order to be able to broadcast to civil users. Another goal of the work is to implement B-Values and B- Values threshold algorithms for future systems. The analysis of results will be evaluated and we will conclude the result with respect to the required performances.