Earth Albedo Effects on the Orbital Variations of Echo I and PAGEOS I

Abstract

A study is performed to determine the effects on satellite orbits of the force of solar radiation diffusely reflected from the Earth. In particular, the orbits of Echo I and PAGEOS I are studied to determine whether unexplained variations in their orbital elements are due to Earth albedo. Ground-based photometric measurements have confirmed the essential stability of the average radii of curvature of these two balloon satellites during the time interval studied. Expressions are developed which give the effects of albedo on the orbital elements. It is found that this perturbing effect generally acts to decrease the effect of direct solar radiation on the eccentricity and the argument of perigee, but to increase the effect of direct solar radiation on the semi-major axis. Unexplained orbital variations are observed for Echo I from January 1963 to December 1964 and for PAGEOS I from April 1967 to October 1968. Corrections are made for earth oblateness, lunar and solar gravitational fields, atmospheric drag, and direct solar radiation. Using mean albedo measurements from TIROS VII radiometer data, these unexplained variations are identified as earth albedo radiation force perturbations. These albedo perturbations are significant and must be accounted for in order to infer accurate density variations at various global positions at over 1000 km from the Echo I orbital decay, and at over 2000 km from the PAGEOS I orbital decay.