Thermopile

A thermopile is a horizon sensor that works by detecting thermal infrared radiation, or radiated heat. Blue/black skies do not emit or reflect much heat, while the ground absorbs most of the Sun's visible-light radiation, and emits that energy as thermal infrared. This difference allows a high-flying sensor that is not pointed directly up or down to detect the horizon, where the "cold" sky changes to the "hot" ground. Several autopilots and FPV-assist systems have been based on this type of sensor, as it is practical for a fixed-wing plane without using GPS or expensive/drift-prone IMUs.