Molniya orbit

A Molniya orbit (Russian: Молния; IPA: [ˈmolnʲɪjə], "Lightning") 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 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 the Russian Federation 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 that 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 at −90°.

Figure 2: Illumination zones (at least 10° elevation) from a Molniya orbit. At apogee, the green illumination zone applies, three hours before or after apogee, the red zone applies, four hours before or after apogee, the blue zone applies. The plane of the figure is the longitudinal plane of apogee rotating with the Earth. In the eight-hour period centered at the apogee passage, the longitudinal plane is almost fixed, the longitude of the satellite varies by only ±2.7°. The views of the Earth from these three points are displayed in figures 3–8

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