In order to use the Global Positioning System (GPS) as a major navigation system for civil aviation, International Civil Aviation Organization (ICAO) and aviation organizations around the world are working on the development of the Ground Based Augmentation System (GBAS) which is a solution to enhance the GPS service for civil aviation users. A GBAS ground facility prototype, Integrity Monitor Testbed (IMT), is presented in this paper to conduct a preliminary study on its performance in Taipei Flight Information Region (FIR). The IMT could enhance the accuracy and integrity of GPS through a series of comprehensive monitoring algorithms to supply the integrity messages to GPS users. The assessment of the Signal Quality Monitoring (SQM), the Data Quality Monitoring (DQM), and the Measurement Quality Monitoring (MQM) algorithms of IMT will be presented in this paper. The initial results of theses quality monitoring algorithms could provide the valuable reference for future implementation and operation of the GBAS in Taipei FIR.