1. Cell sites have their own high precision clock, so they can go at least 10-20 minutes and probably longer without any difficulty.
2. The site really only needs to see a single satellite to get time synchronization if it has the almanac and ephemeris because it knows exactly where it is and how far away the satellite is. In fact, in the presence of noise, it can integrate over a long period to pull a jammed signal out from under the noise floor.
3. There are already a number of other terrestrial sources of time. TV stations have precise timing, and a defined offset to GPS time. They also have huge output power that is more difficult to jam.
4. The cell networks can synchronize with each other. If the network simply adjusts the time to be an average of neighboring cells and its own clock, they will stay in sync. It's not required for the mobile phone network to have correct absolute time, only correct time relative to other nearby network elements.
Does anyone know if a hobbyist can pull the time from TV signals like you can with an RPi/GPS-hat/etc? I'd love to have a bedside clock that pulls from TV signal for no other reason than I didn't even know that existed until now.
If you're in a country that still broadcasts Teletext[0] then it's available over analog or DVB-T. VCRs used to do this to set their clock automatically in Europe.
Also, if you already have a time, you can keep it using the 50/60 Hz of the electric grid (which is specifically regulated to keep cheap alarm clocks in sync) or the TV refresh rate (which was also kept in sync deliberately for clocks in the analog days, dunno about now)
Cell towers could do all those things you suggest. They don't though. They just use a GPS receiver. They have an internal TXCO (Temperature Compensated Crystal Oscillator) or similar, but they drift eventually. 3G and earlier tech was actually more resilient. It could use GPS for more accurate timing, but didn't require it.
1. Cell sites have their own high precision clock, so they can go at least 10-20 minutes and probably longer without any difficulty.
2. The site really only needs to see a single satellite to get time synchronization if it has the almanac and ephemeris because it knows exactly where it is and how far away the satellite is. In fact, in the presence of noise, it can integrate over a long period to pull a jammed signal out from under the noise floor.
3. There are already a number of other terrestrial sources of time. TV stations have precise timing, and a defined offset to GPS time. They also have huge output power that is more difficult to jam.
4. The cell networks can synchronize with each other. If the network simply adjusts the time to be an average of neighboring cells and its own clock, they will stay in sync. It's not required for the mobile phone network to have correct absolute time, only correct time relative to other nearby network elements.