AuraUAV

http://diydrones.com/profiles/blogs/a-new-use-for-an-apm2-x Includes an advanced 15-state EKF developed by the University of Minnesota UAV lab and has seen action on all their UAV's, on a Lockheed martin project, and on Airborne Technologies (www.atiak.com) projects. Primarily tested on fixed wing aircraft. I figure there is no way I could catch up to the multirotor crowd once I got past that first 90% and was into the nuance and tuning.

https://github.com/AuraUAS

sensors apm2
https://github.com/AuraUAS/apm2-sensors Replacement firmware for an inexpensive APM2 board. This firmware converts the APM2 into a standalone sensors module and adds a full suite of inertial and position sensors to any host computer or application. This firmware maintains APM2 support for reading RC receiver values in, and driving output servos so for some applications it could serve as a robot controlle

links
Gimbal, Gimbal lock