Testing is a project mainly written in ..., it's free.
PIXHAWK IMU / AUTOPILOT ONBOARD SOFTWARE
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See http://pixhawk.ethz.ch/software/imu/ for details.
The main function is in the file main.c. It contains startup code for all airframes. The mainloop for the individual airframes is in:
The mainloops for fixed-wing and quadrotors are in:
main/mainloop-quadrotor.c main/mainloop-fixed-wing.c