A ground based augmentation system (GBAS) is a system proposed by the International Civil Aviation Organization (ICAO) to meet the stringent navigation requirements of civil aviation. We have already developed a GBAS prototype, so this time, we installed the experimental reference receivers near Taichung International Airport (ICAO code: RCMQ) in Taiwan. Then, we conducted flight tests at the airport and evaluated the navigation performance of the GBAS prototype. The experiment was conducted to validate the GBAS performance under GBAS approach service type (GAST) C specifications. Before the flight tests, we conducted a field test at Kaohsiung International Airport (ICAO code: RCKH). We installed reference stations and a static ground user near the runway at RCKH. The field test data was used to validate the performance of the GBAS prototype under the required navigation performance (RNP) of a Category (CAT) I precision approach and landing. The result showed no hazardously misleading information (HMI) in the finite data we received. In this paper, we evaluate the GBAS prototype with the flight test data. There were three flight tests in the experiments. The first flight test was carried out on October 24, 2019. Each flight test took approximately three hours. We focus our analyses on the approach phase since the GBAS is aimed toward supporting precision approaches and landing. The results of this validation showed that an HMI event occurred, and the performance of the GBAS prototype appeared to fail to meet the CAT I requirement based on the flight test data. We speculated that the HMI was due to either multipath or noise. A detailed analysis of the resulting navigation capabilities is carried out in this paper.