This tracks with what I has seem elsewhere. I don't have the details handy now, but the communication system used for long range handoffs for flights above 18,000 ft seems to be what is impacted. Keeping planes below that deck would keep them bouncing more frequently between shorter range towers but at least still tracked and in communication.

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My view on ADSBexchange is centered on CMH, zoomed out to Washington D.C. to the east, and filtered by 18K up I've got 142 aircraft.