a Ground Based Augmentation System (GBAS) to a Global Navigation Satellite System (GNSS) such as the Federal Aviation Administration (FAA) Local Area Augmentation System (LAAS) could possibly enable a commercial airliner to land automatically in Category III (CAT III) zero visibility condition.
The GPS constellation alone is not reliable enough to land a commercial airliner automatically without the assistance of ground based reference stations, even a single fault has to be detected and make known within split seconds so the entire Ground Based Augmentation System (GBAS) system can be shut down automatically with immediate effect if necessary.
The last thing you want is a wide-body commercial airliner with hundreds of living souls on-board slamming to the side of a runway at approximately 300 km/hr in zero visibility condition simply because the timing of the atomic clock on a GPS satellite drifted away uncorrected due to time dilation. It is all about redundancy and designing the system as fail-safe as much as possible.