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It seems related to an upgrade on that Collins equipment, in the Receiver Autonomous Integrity Monitoring, and quality of nav data sent to ADS-B equipment, which is downlinked from the aircraft to air traffic control.

The Multi Mode Receivers aren't strictly GPS units - they also get nav signals from terrestrial VOR, DME, and ILS stations. DME - DME fixes are qite precise, but not as precise as GPS positioning.

There's also an altitude restriction on affected aircraft: they aren't permitted into airspace using Reduced Vertical Separation Minimums, where tighter navigation and monitoring restrictions apply.