I'm putting this here since I'm unclear where this question would belong.
I want to build a ground station around a Raspberry Pi that can display basic telemetry.
Initially at least, I want to read only a minimal set of attributes, essentially what comes in the VFR_HUD, ATTITUDE and GLOBAL_POSITION_INT messages.
I understand that the MAVLINK data looks similar to what I find in a .tlog file. I think I understand MAVLINK message structure. I can parse these files enough to understand the basic structure of the protocol. I have a generator to construct packet types from the XML specifications.
What I'm missing is an understanding on the higher-level (session-level) protocol. Is there a message the ground station needs to send to APM to initiate a session? What does it need to tell APM? I'm entirely unclear on where I can find documentation that describes all this. I appreciate your pointers.