I'm assuming you're using a UBlox M8 or F9P or something similar? Most of the built units have a small battery or supercap that allows them to retain almanac an

I'm assuming you're using a UBlox M8 or F9P or something similar? Most of the built units have a small battery or supercap that allows them to retain almanac and ephemeris data while powered off. If the ephemeris data is less than 4 hours old this will allow it to very quickly re-sync to the visible SVs and get a lock again. It's quite impressive, though, at how the hardware has improved. For a complete cold start one of the challenging issues is to actually find a single satellite to listen to. For GPS there's 32 different spreading codes and each transmitted signal is affected by Doppler shift depending on its position and velocity relative to where you are on the Earth. You basically have to iterate through (spreading code, Doppler shift) pairs and run a correlator to see if there's a signal present. Contemporary hardware has a ton of correlators that can run in parallel which can significantly improve the TTFF (time to first fix). As soon as you find one SV and start to get a bit of almanac data you can quickly eliminate (spreading, Doppler) options from the problem space by dropping SVs that you know are not going to be visible.