High Precision Attitude Determination by Sensing the Earth and Lunar Horizon in the Infrared

Abstract

The attitude of a space vehicle must be determined and controlled for a variety of purposes some of which are the firing of course correcting rockets or retrorockets for re-entry, automatic docking, and the pointing of on-board cameras, antennas, and other instruments. In deep space the stars must be used for this purpose, but when orbiting a planet the thermal interface between the edge of the planetary disc and space provides an excellent attitude reference. Infrared sensors have been used for detecting the position of the earth horizon for many years and have generally exhibited accuracies of about 0.5°. Advanced space missions are demanding accuracies of the order of 0.02° to 0.1° and in this paper the theory and hardware techniques necessary to obtain such accuracies with the earth and lunar horizons will be discussed.Fundamental to obtaining high accuracy is the identification of the infrared horizon. The true or hard horizon of the earth can only rarely be seen from space, being obscured by clouds, haze, and atmospheric absorption. Therefore, the thermal horizon appears as a radiance gradient fading into the space background, which can subtend as much as 2° at low orbiting altitudes. This gradient has a different appearance and stability in various spectral regions. From experimental and theoretical studies of these profiles, stable characteristics have been found which permit identification of a point on the profile, which is always a fixed altitude above the true horizon. A signal processing technique to detect such a point has been called a “locator.” It has been shown that by spectral filtering to the 14.5 - 15.5 micron band, and use of a suitable locator, an accuracy of 0.02” can be achieved from low orbiting altitudes .This problem is non-existent on the moon because of the absence of an atmosphere, however, another and equally disturbing phenomenon occurs, namely, the large variation in horizon radiance over the lunar disc. In practical sensors, a horizon “locator” is required to overcome this effect as well.A high precision sensor has been developed incorporating these “locator” signal processing techniques. This is essentially a tracking telescope which locks onto the “locator” identification point and continuously displays the angle between this point and a fiducial reference. This sensor has been tested in a series of suborbital flights of the X-15 and the results will be presented.