Earth Sensor (Horizon Scanner)

An Earth sensor — properly an Earth horizon sensor — is an infrared attitude sensor that locates the nadir direction by sensing where Earth's thermally glowing disc meets the cold of space. Working in the carbon-dioxide emission band around 14–16 μm, where the planet radiates smoothly day and night, it detects the limb crossing in several directions; balancing those crossings yields the direction to Earth's centre, typically to better than 0.1°. For the Earth-pointing majority of satellites — communications platforms holding beams on territory, imagers holding nadir — this was for decades *the* pitch and roll reference: scanning designs swept a small mirror in a cone through the horizon, while static designs stare at the whole limb with thermopile arrays, no moving parts. The sensor's limits trace its physics: it cannot sense yaw (rotation about the nadir axis looks identical), accuracy suffers from atmospheric radiance variations — cold cloud tops, seasonal gradients — and it works only where a warm disc fills a predictable angle. Modern fleets increasingly derive the same knowledge from star trackers plus orbit position, but horizon sensors persist where simplicity, autonomy and independence from catalogues earn their keep.