Repeat orbit characteristics and maneuver strategy for a synthetic aperture radar satellite

ri newly proposed synthetic aperture radar Earth-imaging mission, LightSAR, requires the orbit to be maintained within a 125-m-radius tube centered on a reference orbit over the lifetime of the satellite. A reference orbit was defined that repeats every eight days and has a ground-track closure of about 8 cm and a radial closure of about 16 m at the end of the repeat interval. Major perturbing forces were investigated to determine the length of time before the perturbed orbit violated the tube constraint. The forces examined include Earth gravity (including tides), luni-solar gravity, atmospheric drag, and radiation pressure. Of these perturbations, atmospheric drag and luni-solar gravity were the dominant perturbations leading to violation of the tube constraint. A maneuver strategy was developed to meet the stringent orbit maintenance requirement of staying within 125 m of the reference orbit. A series of maneuvers directed in the tangential and normal directions and applied at fixed time intervals was able to maintain the satellite position within the tube for eight days. The additional application of a maneuver in the radial direction allows the maneuver strategy developed to be effective for up to four repeat cycles, nearly 32 days. In all, 31 maneuvers with a total Delta V of 910.659 mm/s were required in the assumed dynamic environment to maintain the spacecraft within 125 m of the reference orbit during this interval.