Earth Sensor (Horizon Scanner)

An Earth sensor detects Earth's thermal infrared horizon — the boundary between the warm atmosphere (250 K) and cold space (3 K) — to determine the satellite's orientation relative to the local vertical (nadir direction). Scanning types mechanically sweep across the horizon; static types use arrays of IR detectors. Accuracy is typically 0.05–0.5°. Earth sensors are particularly important for GEO communications satellites that must keep their antennas pointed at specific ground regions, and for Earth observation satellites that need accurate nadir pointing. They complement star trackers and sun sensors in the ADCS sensor suite.