According to
the FAA, "certain aircraft equipped with the Rockwell Collins GPS 4000-100 and select ADS-B out GPS receivers are indicating 'ADS-B fail, unavailable, TCAS fail or transponder fail' messages." Aircraft without working transponders were directed to coordinate with the FAA before departure, a prior
directive had instructed those planes not to fly above 28,000 feet.
...
Various online reports suggested impacted planes were mostly CRJ jets made by Canada’s Bombardier, but also some Boeing 737s, 717s and possibly a 767 as well. "We are working to determine the cause of the problem," the FAA said in a statement, "which may have resulted from a software update to the aircraft navigation systems."
There was no immediate response or comment available from Collins Aerospace.
"GPS isn’t down," Hackaday reported, as anyone with a smartphone can see easily enough. "However, it is degraded.