A fully operational GLONASS constellation consists of 24 satellites, with 21 used for transmitting signals and three for on-orbit spares, deployed in three orbital planes. The three orbital planes’ ascending nodes are separated by 120° with each plane containing eight equally spaced satellites. The orbits are roughly circular, with an inclination of about 64.8°, and orbit the Earth at an altitude of 19,100 km, which yields an orbital period of approximately 11 hours, 15 minutes. The planes themselves have a latitude displacement of 15°, which results in the satellites crossing the equator one at a time, instead of three at once. The overall arrangement is such that, if the constellation is fully populated, a minimum of five satellites are in view from any given point at any given time.
Each satellite is identified by a “slot” number, which defines the corresponding orbital plane and the location within the plane; numbers 1–8 are in plane one, 9–16 are in plane two, and 17–24 are in plane three.
A characteristic of the GLONASS constellation is that any given satellite only passes over the exact same spot on the Earth every eighth sidereal day. However, as each orbit plane contains eight satellites, a satellite will pass the same place every sidereal day. For comparison, each GPS satellite passes over the same spot once every sidereal day.