CANBUS spec to find GPS information from A3

HI, we have an M600 drone, and would like to get the GPS location information out of the A3 controller.

We have our own dedicated hardware mounted under the drone and want to use the canbus to read the positon of the drone as it moves around  so it can be recorded with the data.

I've looked around the DJI site but cannot find the canbus spec that would give this location information. The comms connection will be to a PIC32 with canbus capabilty, so it should be easy to implement if I know the commands needed.

Where can I find this information?