In practice, low Earth orbit (LEO) and medium Earth orbit (MEO) satellite networks consist of multiple orbits, each populated with many satellites. A widely used spatial architecture for satellites is the Walker constellation, where the longitudes of orbits are equally spaced and the satellites are periodically distributed along the orbits. In this paper, we develop a stochastic geometry model for Walker constellations by constructing orbits that are invariant with respect to (w.r.t.) Earth's spin and satellites that are invariant w.r.t. orbit-wise rotation. This model enables an analysis based on dynamical systems, which allows one to address structural properties such as periodicity and ergodicity. It also enables a stochastic geometry analysis where, for a typical user at a given latitude, we derive the performance of downlink communications as a function of key constellation parameters, including orbit inclination and altitude, the number of orbits, the number of satellites, and the user latitude.

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