1. ELY, T. A. “Stable Constellations of Elliptical Inclined Lunar Orbits,” The Journal of Astronautical Sciences, Vol. 53, No. 3, July–September 2005, pp. 301–316.
2. GIACALIA, G. E.O. “Third Body Perturbations on Satellites,” Proceedings of the Artificial Satellite Theory Workshop, U.S. Naval Observatory, November 8–9, 1993.
3. WALKER, J.G. “Continuous Whole Earth Coverage by Circular Orbit Satellites,” Proceedings of the International Conference on Satellite Systems for Mobile Communications and Surveillance, March 1973 (Institution of Electrical Engineers, London), pp. 35–38.