Draim tetrahedral satellite constellation modelled in SaVi

after John E. Draim, A common-period four-satellite continuous global coverage constellation,
AIAA Journal of Guidance, Control and Dynamics, vol. 10, no. 5, September-October 1987, pp. 492-499.
see also John E. Draim, Tetrahedral multi-satellite continuous-coverage constellation, US patent 4854527, 8 August 1989.
John E. Draim et al., Common-period four-satellite continuous global coverage constellations revisited,
Advances in the Astronautical Sciences, vol. 143, pp. 667-686, January 2012.