  The navigation satellite constellations in medium-Earth orbit exist in a
background of third-body secular resonances stemming from the perturbing
gravitational effects of the Moon and the Sun. The resulting chaotic motions,
emanating from the overlapping of neighboring resonant harmonics, induce
especially strong perturbations on the orbital eccentricity, which can be
transported to large values, thereby increasing the collision risk to the
constellations and possibly leading to a proliferation of space debris. We show
here that this transport is of a diffusive nature and we present representative
diffusion maps that are useful in obtaining a global comprehension of the
dynamical structure of the navigation satellite orbits.
