4. Earth Sensors

Earth Horizon Sensors (EHS) are the main means of directly determining the orientation of a spacecraft with respect to the Earth. Most of these sensors are frequently used in a servo loop for autonomous attitude control systems, based on real-time measurement of geocentric direction error signals, with angular accuracy requirements of 0.03° to 0.1°. EHSs are generally infrared equipment whose operation is based on the luminance contrast between space (black body at " 4 K) and the earth's horizon (black body at " 233 K at a wavelength of 15 µ m). However, some designs have prioritized signal amplitude over the spatial and temporal stability of the horizon, choosing to work in the ultraviolet or visible range.

To meet accuracy and stability requirements, EHSs need to operate in the 14-16 µ m infrared spectral band, which corresponds to the absorption band of the CO...