 2/3 approach and landing.There are stricter Safety requirements on GBAS systems relative to SBAS systems since GBAS is intended mainly for the landing phase where real-time accuracy and signal integrity control is critical, especially when weather deteriorates to the extent that there is no visibility (CAT-I/II/III conditions) for which SBAS is not intended or suitable.The US Nationwide Differential GPS System (NDGPS) is an augmentation system for users on U.S. land and waterways.