copter.ardupilot.com/wiki/ac2_followme/
There’s already a follow mode built into the code, so it already has the GPS updating thing down.
The next step for you is figuring out how to get your flight controller to talk to your Nexus. See if you can rig up an Arduino-based Bluetooth shield to your flight controller, for your Nexus - Flight Controller communications. It’s easy, fairly cheap, it won’t have super range, but for 3rd person it’s all you need. Fast too.
Also, just as an aside, since I’m attempting something similar, (reasonably affordable) GPS is reliable within 2-10 meters. That’s way too loose for 3rd person unless you just want a loose chase cam.
And, consider your GPS unit’s refresh rate. 1 Hz = 1 cycle/second = 1 GPS reading/second. This is also quite slow for 3rd person as it will reposition every second, and not necessarily be very smooth. 5+ Hz GPS units have massively dropped in price these days, get one. (You will however, be limited by your Nexus’ GPS unit, which is probably only 1 Hz, since why the hell would you need more than that in a tablet?)
Take into consideration the processing requirements that would be necessary for faster waypoint update calculations. I’m not entirely sure the APM is powerful enough to handle the calculations involved in high speed GPS update calculations.
AND, if you’re going much further than this, read up about NMEA messaging. It’s how GPS is communicated between most devices. Super useful. Nearly everything uses it.
I will say this though, this is a project I’m interested in and is similar to my own. I’ll also say that my programming isn’t fantastic, but I have gained a bit of a grasp on what’s required of the project. Perhaps we could trade what we figure out as we do. Lord knows there’s going to be a ton of tuning involved.