1. R.L. Easton, R. Brescia, Continuously visible satellite constellations, Naval Research Laboratory Report 6896, 1969.
2. Some circular orbit patterns providing continuous Whole Earth coverage;Walker;Journal of the British Interplanetary Society,1971
3. J.G. Walker, Continuous Whole-Earth coverage by circular orbit satellite patterns, Royal Aircraft Establishment Technical Report No. 77044, 1977.
4. Satellite constellations;Walker;Journal of the British Interplanetary Society,1984
5. Three and four satellite continuous coverage constellations;Draim;Journal of Guidance, Control, and Dynamics,1985