If you want to use GPS with Revo, there are a few things you should know:
1. Contrary to what most people think at the beginning, GPS sensor cannot be used alone.
This is because GPS alone has a very bad huge error of 2-4m. If you intend to use GPS for navigation, it ONLY gives good position when data is combined through sensor fusion algorithm, with all (no exception) other sensors.
So again adding GPS to aircraft, won't make it fly better if it wasn't already.
2. Each sensor properly calibrated and giving good data is absolutely essential in the first place before attempting to use GPS for navigation
Before even attempting to use GPS for navigation, it is absolutely essential to have calibrated and good data from all sensors with no exception.
Each sensor measures different type of information that is minimum what you need. For example GPS does not know heading, that is only obtained from magnetometer...
3. The sensor fusion algorithm (INS31) which is basically Extended Kalman Filter, is designed to deal with noise. Although it does it to some extend, if one of your sensor feeds completly wrong data to the filter, it won't be able to handle this.
That's why all your sensors must be working properlly.
Now possibles problems you might encounter
- Internal magnetometer is too noisy, you must use external magnetometer mounted far from electronics/motors etc.. That's why it is convinient to have Magnetometer in the GPS unit, mounted on a 14-16cm not rotating mast. It is essential to calibrate Magnetometer outdoors, far from walls, and metallic objects (best done over OPLink radio).
- Baro might be noisy (some clones have problems), once baro is giving good data, you should tune Altitude PIDs such that your aircraft can maintain altitude in Complementary Filter (Basic) mode.
- You should tune your PIDs such that aircraft is responsive but not oscillating.
- You should eliminate vibrations.
Once your aircraft flys great in Complementary Filter, you can start thinking about adding GPS.
Hope that helps.
Also please read post by Cliff - The ABC Steps of Autonomous Flight on Multicopters
https://forum.librepilot.org/index.php?topic=15.msg36#msg36It will pay off