I read on the website of European Space Agency that the altitude of Galileo satellites, which is 29600 Km from the center of the Earth, is chosen to avoid gravitational resonances so that station keeping manoeuvres are not needed during the lifetime of a satellite.

Note: I read before that GPS satellites' manoeuvres are controlled from the ground. GPS satellites are at an altitude of 26561 Km.

Note 2: Eccentrity of Galieo and GPS satellites orbits can be considered as zero.

1. What is gravitational resonance?
2. How it is calculated?