Heli-OS advances, helicopter hovers using quaternions

Heli-OS, obviously for Helicopter Operating System, is a project I started a year ago with a friend of mine. The challenge is to render a radio-controlled helicopter entirely autonomous (no human input allowed for flight). Progress has been made recently as I grabbed latitude and longitude from a GPS in a C# application (not much of a feat, but I made a PCB to render the GPS USB), made a bridge between MatLAB/SimuLink and X-Plane (through UDP), and finally, made a successful hover of the helicopter (in simulation) using PID controllers on each axis (pitch, roll, yaw) and on altitude. Attitude is measured and controlled with quaternions, which allows me to define the error in the control loop as another quaternion, representing the rotation between current attitude and desired attitude.