Safe Switching of the GRACE Formation using an Eccentricity/Inclination Vector Separation;

Abstract

After more than two years in orbit, a longitude swap maneuver is required to exchange the leading and trailing spacecraft of the GRACE formation. While the two satellites are nominally separated by about 200km in along-track direction, a close encounter will take place during the swap sequence. Based on the successful use for co-location of geo-stationary satellites, the concept of eccentricity/inclination vector separation is suggested for safe proximity operations in this mission phase. Taking care of the natural evolution of the relative orbital elements of GRACE 1 and 2, optimum maneuver dates are identified. By proper timing of the maneuvers a safe limit for the minimum distance during the encounter can then be guaranteed even in case of arbitrary thruster performance errors. This allows the use of a fuel optimal maneuver sequence with single drift start and stop maneuvers.