I have a Pixhawk system with all the standard 3DR peripherals (telemetry radio, GPS). I need to replace the 3DR GPS with another GPS unit so that I can conduct some tests with higher accuracy. I have a Trimble RTK unit and others at work that I want to try. I have no problem setting these up for the desired output so that is not a concern. I really just need to understand the Pixhawk/APM:Rover side better.

I've tried to find information about the configuration of the GPS port, but it is very sparse. Everything seems to try to push the 3DR modules (which I already have so I'm not cutting into their profits). The most information I have found has been on the Pixhawk webpage (https://pixhawk.org/modules/pixhawk) which let me know that the port is 3.3 V. But, there is no other information. I know it is possible to use other GPS systems. I can see discussion on the developer board about support for NMEA strings and custom RTK GPS modules.

Specifically, I'd like to know:

1. What protocol is used for the GPS TX/RX lines?

2. If it is UART, what baud rate is expected? If baud is configurable, where is it configured?

3. Do I need to change an APM:Rover parameter? I have found GPStype in the list, but NMEA is the only one I readily know how to setup. Are the others special for certain parts or are any generic protocols that I just didn't recognize?

4. What NMEA strings can it interpret and how fast can it handle them?

Thanks for any help anyone can provide.