The Librem 5 looks quite appealing to me, especially after the unfortunate production errors which prevented me from getting a GTA04A5, but there’s one little bit of information I’d like to know before I sign up for a pre-order.
That would be the GPS isolation. I’m not sure what form the GPS functionality would take in your design - whether it would use functionality built in to the cellular baseband chip if present (I know that a lot of Qualcomm modems have built-in GPS capability, not sure about other manufacturers) or whether it would be a separate chip. Additionally, if it’s the latter option, whether the GPS would be connected to the baseband through its own dedicated lines or would need to pass any requests through the main CPU. I believe that most baseband chips have a dedicated UART line of some sort to talk to a GPS module, partly for things like AGPS but also for… less benign purposes.
For reference, my ideal situation would be a separate GPS chip which connects only to the power management chip and to the main CPU, so that the cellular baseband can’t get GPS information (yes, this will make AGPS harder to implement). Since the baseband runs software which isn’t under our control, it only makes sense to treat it as the enemy - especially given things like 3GPP TS 23.271, part of the GSM standards which says how the network can ask a device to report its position. That has valid uses (eg. when you dial the fire brigade and say “help, my cat is on fire”, you want them to know where you are), but also plenty of scope for abuse - especially when all you need is SS7 access to send such queries.
So: how would the GPS functionality and connectivity be implemented in the Librem 5?