Molniya Orbit

Molniya orbit is a type of highly elliptical orbit with an inclination of 63.4 degrees, an argument of perigee of −90 degrees and an orbital period of one half of a sidereal day. Molniya orbits are named after a series of Soviet/Russian Molniya (Russian: "Lightning") communications satellites which have been using this type of orbit since the mid 1960s.

A satellite in a highly eccentric orbit spends most of its time in the neighborhood of apogee which for a Molniya orbit is over the northern hemisphere, the sub-satellite point at apogee having a latitude of 63.4 degrees North. As the apogee altitude is as high as 40,000 km it will therefore, for a considerable period around apogee, have an excellent visibility from the Northern hemisphere, from Russia but also from northern Europe, Greenland and Canada.

To get a continuous high elevation coverage of the northern hemisphere at least three Molniya spacecraft are needed.

The reason why the inclination should have the value 63.4° is that then the argument of perigee is not perturbed by the J2 term of the gravitational field of the Earth but stays −90°.