An earth sensor for determining the orientation of a space vehicle with respect to the earth with a mean time to failure of 29 years, far higher reliability than former sensors, is being developed. The sensor uses a new approach of detecting the earth. This method requires no moving parts and only about ten per cent of the electronic parts required by existing earth sensors. The far infrared radiation from the earth is imaged on temperature sensitive elements which absorb infrared radiation and are heated by it. The sensor uses the difference in temperature between elements to determine the offset angle.