Hi Will,
thanks for chiming in!
Where I'm headed with this is, please do keep nav in the forefront of mind - working nav though, not just occasionally maybe it's usable in a demo nav. I would love to use a phone with control over the TCB that would, when needed, get me from point a to point b with a minimum of fuss. 5 stars for " I'm thinking of an Open-Street-Map client and a messenger as particular examples". Also, wouldn't it be nice if the nav function were completely under the user's control, meaning that when the user turned it off, their location in realtime would not be tracked? I'm sure there are valid uses for point in time requests for a device to be located, but continual tracking, not so much.
your privacy interests are perfectly aligned with mine.
Regarding positional tracking, my idea is to have a GPS switch in the "Phone" part of Sculpt's UI. So the user is always in control to switch this modem feature on/off. If GPS is enabled, the modem driver feeds the current GPS coordinates as polled from the modem into a GPS report.
Now when integrating the OSM component, one can deliberately decide how to connect the GPS ROM requested by the OSM component. Normally, one would connect it to the GPS report that reflects the position provided by the modem. Alternatively, one could provide a hand-crafted GPS ROM (pretending that you are somewhere else), or none (by directing the GPS ROM request to the black hole).
When using an offline map, the OSM app would neither need network access nor write access to any file system. Therefore, we don't need to trust the app. That's the best protection against positional tracking we can achieve.
Note, however, that the network provider or a malicious/compromised modem firmware might still be able to track you. This could only be mitigated by taking the GPS position from a distinct GPS device that is independent from the modem, and leave the modem switched off.
From your posting I get that you have probably higher expectations from an OSM client application than me. I'd be happy enough with a plain map that helps me finding my way to the nearby Hostel when arriving by train at a foreign place. The crafting/porting of a more competent OSM app would be a perfect topic for contributing because as an actual user of such features, you have the incentives to get it right.
Since such an application would merely consume a GPS ROM, it can be developed without any physical GPS device or phone. The GPS position could come from a component that reports fake GPS positions. Compass sensory data could be handled similarly.
Cheers Norman