I now have the NRF51822 board talking to the compass board - reading the magnetometer and accellerometer values, and then calculating the tilt-compensated heading in a fixed point algorithm, and I think sending it over the Bluetooth Location and Navigation service.

I think I will spin off yet another sub-project for the navigation dongle, and make a special GitHub repo for it, and upload the code there.

I'm still pondering how to do the solar charging. Adafruit has a really nice board (the USB / DC / Solar Lithium Ion/Polymer charger - v2), but at $17,50 it's a bit expensive, so I'm considering my options.

Perhaps one way would be to have the process or monitor the solar cell voltage, and through a transistor add load to decrease the output current if the solar cell voltage falls under 6 volts. I think I'd still want a 4.2V LDO to ensure that I don't feed too high a voltage to the battery.

On the other hand, if I need to start soldering a circuit, perhaps I might as well build one for the Microchip MCP73871 IC that Adafruit uses; the chip itself is only $1,79.

I'm waiting for my PCBs, which have been ordered. I have ordered all my components, so I'm ready to start assembling them when they arrive.

Talking to the GPS is another next step.