Molniya Orbit

The Molniya satellite orbit is a highly elliptical Earth orbit designed to provide long-duration visibility at high latitudes, particularly over the Northern Hemisphere. It typically has an orbital period of one-half of a sidereal day (about 12 hours), a high eccentricity (approximately 0.7–0.75), and an inclination of about 63.4°. In a representative Molniya orbit, the apogee is located over the Northern Hemisphere at an altitude of roughly 40,000–46,000 km, while the perigee lies over the Southern Hemisphere at an altitude of several thousand kilometers. The first Soviet Molniya satellite was launched on 13 April 1965. Molniya’s inclination is carefully chosen for two main reasons. First, the orbit’s eccentricity is such that the satellite is quasi-geosynchronous and the orbit period is one-half of a sidereal day. Secondly, the inclination is chosen so that the satellite is pseudo-geostationary at a latitude of approximately 60°.

One effect of the Earth’s oblateness is to cause a rotation of the line of apsides. However, at an inclination of 63.4° to the plane of the equator, the rotation is zero so that the apogee remains fixed over a particular spot on the Earth’s surface. These orbits are sometimes called the 63° slot. During each 12-hour orbit, a Molniya satellite spends the majority of its time—typically about 8 to 11 hours—near apogee, providing high elevations and favourable link geometry to users in high-latitude regions.

Continuous regional coverage can be achieved by a constellation of three or four satellites spaced appropriately in their orbital planes. In operational systems, payloads are often activated only when the satellite is above a specified altitude, typically on the order of 15,000–20,000 km, to optimise power usage and service quality.

Molniya orbits have been used for television broadcasting, telecommunications, military communications, relaying, weather monitoring, and early warning systems. Molniya satellites were also used for the first US-USSR hotline (Moscow to Washington). The primary link was initially land-line telegraph (then fax) and in September 1971, the terrestrial circuits were supplemented with two new satellite communication links using two Intelsat satellites and two Soviet Molniya II satellites ¹.

From 1971, US signals intelligence and reconnaissance satellites are reported to make use of Molniya orbits. ² From 1976, the US Satellite Data System (SDS) is reported to make use a mix of GEO and Molniya orbits, the latter having an apogee of approximately 39,000 km and an apogee of approximately 300 km, to relay imagery from low-flying reconnaissance satellites ³.

HEO orbits are also used for the Tundra orbit (apogee approximately 39,700 km, perigee approximately 25,000 km, eccentricity 0.25), and for mobile satellite systems to avoid shadowing at high and low latitudes.