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Often they rely heavily on GPS for time and frequency synchronization (think 5G TDD, where all operators need to be aligned in time or they jam eatch other‘s uplink). The GPS antenna is also often quite visible from the outside of a base station.

Still, what‘s also possible is to use PTP as backup (e.g., over eCPRI) and use the fiber / microwave backhaul to some master clock with, say, a ribidium time source as a backup, so I think it‘s not entirely unreasonable to assume that cell phone signals will remain available in the presence of a GPS outage.