Abstract: The Ground based Augmentation System (GBAS) is being developed to enable precision approach and landing operations using the Global Positioning System (GPS). Each GBAS installation provides services through a GBAS Ground Facility (GGF) which is located at the airport it serves. By monitoring the GPS signals, measurements, and navigation messages, the GGF is able to exclude unhealthy satellites and broadcast real-time range correction messages for healthy satellites to users via a VHF data link. Airborne users apply these corrections to remove errors that are common between the GGF and the aircraft. The GGF is also responsible for warning the aircraft of any potential integrity threats that cannot easily be resolved by excluding unhealthy satellites. One source of potential errors is the satellite broadcast ephemeris message, which users decode and use to compute GPS satellite positions. In GBAS, potential GPS ephemeris faults are categorized into two types, A and B, based upon whether or not the fault is associated with a satellite maneuver. Both type A and type B events can cause differential ranging errors. However, the ramifications of these failure classes differ in both likelihood of occurrence and means of detection. The likelihood of type B failures is relatively higher than type A because orbit ephemeris uploads and broadcast ephemeris changeovers are frequent (nominally once per day and once every 2 h, respectively, for each satellite), whereas spacecraft maneuvers are rare (no more than once or twice per year). This work focuses on aviation navigation threats caused by Type B faults for Category I (Cat-I) aircraft Precision Approach (PA). To detect and mitigate these threats, we investigate GGF monitors based on comparing projected satellite position estimates based on Zero-Order and First-Order Hold (ZOH, FOH) estimators, using prior days of data. Objectives: − To study the performance of ZOH and FOH estimators and − To define a statistical test for Type B anomalies detection, assuming a missed detection probability (MD) of 10-3 and Fault Free Alarm (FFA) probability of 1.9x10-4, according with CAT-I PA requirements of RTCA/MOPS. − To determine the covariance matrix of fault-free GPS broadcast orbit errors distribution. − To determine the Minimum detectable Error (MDE) and Ephemeris decorrelation parameter (PValue). − To select a candidate for a GBAS Type B orbit errors monitor for Cat-I PA operations. − To embed the prototype in a full GBAS GF system and to analyze its performance in an actual GBAS scenario with real data.