The Pixhawk PX4 was developed by the ETH Zurich in the first place and was later published as an open source project. The main goal while developing the Pixhawk PX4 was to realize the flight of a UAV using image processing. The current PX consist of 2 units wich are combined on a PCD. The PX4-FMU monitors the flight while the IO-PX4 controls the external components that come into use.
Due to the popularity of the Pixhawk series there is a hughe amount of additional components such as GPS, IMu or Machine Vision Boards, that can be used with the PX4. The PX4FLOW as an example can be used together with the Pixhawk controller and monitors the underground aswell as the distance measurement.
More Pixhawk Flight & UAV Controller aswell as fitting accessorie can be found HERE.