Comment by entrope

To meet its integrity budget, a GBAS needs at least triple redundancy, not just a single station.

If one has a lot of patience, one can read the details of how it works at https://elibrary.icao.int/product/299828 (along with SBAS, the ICAO name for what WAAS does; the constellations they can augment; GRAS, a weird hybrid between GBAS and SBAS that apparently went nowhere; ILS; VOR; DME; and VHF marker beacons). Annex 10 Vol I probably doesn't say explicitly that triple redundancy is required, because if you had a single receiver with an MTBF of something like 10 billion hours then you could use just that one.

This particular 30-foot error should go away almost entirely for future receivers that implement DFMC SBAS or ARAIM.