Hardware sensors that allow aircraft state measurement.
In Paparazzi project we use sensors that differ on size, mounting and working principles. Sensors rely on heat, light, ultrasound, magnetic fields, gravitational field, pressure and so on.
Almost any sensor can be adopted for use with any board. Of course there is no need to have all types of sensors onboard. Typically it is enough to have two types of sensors: orientation (IMU or IR) and altitude. Everything you install beyond that can further increase orientation quality and give your aircraft improved abilities.