Here's a block diagram of the wireless, Bluetooth Low Energy based solar powered GPS & compass dongle:

The broken white box left of the nRF51822 should've been a Bluetooth 'B' symbol. The nRF51822 controls the power switch (A P-channel MOSFET) for the GPS power (to turn the GPS off when no one is listening). The GPS serial output voltage level will be too high for the nRF chip, so it need sto be shifted down with a voltage divider. However, the GPS should be able to read the output from the nRF chip just fine.

It would be nice to have the nRF51822 be able to read the battery voltage, but I am not sure if I'll add this. A simple voltage divider will bleed off too much current I'm afraid and drain the battery. Will need more calculations.

The compass / accellerometer board is a GY-511 board. I have not yet been able to find a schematic for it, but it's based on an ST LSM303DLHC chip.

I believe I can charge the LiPo battery safely with an adjustable buck converter like this.

I've ordered the PCBs for the nRF51822 breakout and the Raspberry Pi On/Off board. I'll update GitHub with the latest files soon.

I have all the components to start building and testing this. Until I get my own nRF 51822 breakout board, I've ordered a BLE400 to start experimenting.

I also just received the prize Pi Zero, now I'm just waiting for a micro HDMI adapter to start testing it. I'm also waiting for an SWD to JTAG adapter for the BLE400 board before I can program the nRF chip.

It seems like there is no GPS / compass service definition in BLE GATT, so I'll have to create my own. Does anyone know of one?

Since the electronics should hopefully not draw more than 13 mA while running, and the solar cell should be able to provide 150 mA in full sunlight, I should get 10 hours of runtime out of 1 hour of sunlight. A sufficiently large battery, and I should be able to manage a week without sunshine.

So much to do and so little time!