I was working for the #5 US airline at the time Honeywell was in the testing phase for dGPS equipment.During that time we installed dual dGPS systems into an MD80 type a/c for flight testing and STC approval.The system that we installed in this a/c was capable of CAT III auto-land.During this test flight period, the only ground stations that were in operation were in EWR (our hub) and in MSP (Honeywell's home). Basically what happened was, the approach was surveyed, and when the a/c came in for landing, a VHF data-link would send the exact location of the touch-down zone to the GNSSU. All this was initiated by the a/p control panel selection of APP. The rest is just electronic magic out of anyones control. Hope that helps !