Descending Sun-Synchronous Orbits with Aerodynamic Inclination Correction

Earth observation spacecraft use sun-synchronous orbits because they enable observations of ground targets with similar illumination conditions over different passes. To achieve these orbits, spacecraft shall be orbiting the Earth at a particular inclination, which is a function of the orbiting altitude. In the low-Earth-orbit range, spacecraft experience aerodynamic drag, which makes the spacecraft orbit decay while the orbit inclination remains unchanged, hence loosing the sun-synchronous aspect of the orbit if no corrective measures are taken. A novel method is proposed whereby the sun-synchronous inclination is maintained, using aerodynamic lift, while the spacecraft decays due to aerodynamic drag. To achieve it, a lift-to-drag ratio in the range of 1.0–1.6 is required. This lift-to-drag ratio is not feasible with currently characterized surface properties but it may be achievable in the future. To apply this method in the present, propulsion that partially compensates the drag would be required to lower the lift-to-drag ratio requirement to a feasible level. This method could lower the propulsion requirements on low-altitude sun-synchronous spacecraft by letting them decay, but at the same time maintaining the sun-synchronous aspect of their orbits.