This paper introduces GBAS and its Approach Service Types, the technology conceptual framework for CAT III GBAS based on single-frequency GPS (GAST D) was described in detail, the integrity monitoring scheme and broadcasted messages of ground equipment as wells as additional algorithms and monitors of airborne equipment were studied. For the impact of ionospheric anomalies on GBAS operation, ionospheric gradient anomalous event was simulated based on anomalous model, GAST D GBAS ground and airborne simulation verification platform were designed and developed, and the performance of GAST D GBAS under ionospheric abnormal condition was verified and analyzed.