Analysis of sun/moon interference on horizon crossing indicator

An extensive flight data analysis was performed to identify and study the weak inteference pheonomenon (WIP) associated with satellite earth-IR tracking erors induced by appearance of the sun or moon on the earth limb. Four years of flight data led to the classification of 14 types of interference. WIPs occurred before and after an eclipse and were analyzed in terms of separation error in order to perform computer simulations. The simulations accounted for the FOV, the earth radiance model, the threshold detector logic and the simulator on board the spacecraft. Attitude sensing errors of up to 1 deg were produced by low sensor sensitivity in the spectral region of the WIPs. The results indicate that careful consideration be given before designating the earth sensor as the primary attitude sensor.