Refer to figure.
GBAS - GROUND BASED AUGMENTATION SYSTEM
GBAS is a system that attempts to reduce the natural errors within the GNSS system by providing very localised augmentation to the satellite signals, via a VHF data broadcast (VDB) that a suitably equipped aircraft can receive and use to fix any position errors. It is also called "differential GPS", and can correct for errors induced by satellite clocks, ephemeris and ionospheric propagation to make the augmented GNSS signal very accurate. Errors in the receiver, multipath signals, and some small atmospheric propagation errors can still occur, but the intention is to get the accuracy below 1m for aircraft around aerodromes and on final approach. The GBAS can also, like its satellite based cousin SBAS, give integrity warnings about faulty satellites, as it should be able to detect this at the nearby antennas.

GBAS works by placing a few GNSS receivers around an aerodrome, each with an exact known location. The receivers then compare their uncorrected GNSS position with their actual position to work out the GNSS error in that location. That is the calculate into pseudorange corrections for each individual satellite, and broadcast to aircraft within 30 nm via the VHF data broadcast.

To be clear, the satellites do not know this is happening, and do not care either; the GBAS just helps the aircraft augment its final position calculation slightly. because of this very localised correction, GBAS can make position accuracy extremely accurate, often much below 1 m accuracy.