What is HEO?
A Highly Elliptical Orbit (HEO) is characterised by a very elongated ellipse: a low perigee (typically 600–1,000 km) and a very high apogee (up to 40,000 km). The most famous HEO is the Molniya orbit, used extensively by Russia, with a 63.4° inclination and a 12-hour period.
The Molniya orbit
Named after the Soviet communications satellite series launched from 1965, the Molniya orbit has a perigee of ~600 km and an apogee of ~39,700 km. At the specific inclination of 63.4°, the argument of perigee does not drift due to Earth's oblateness — a phenomenon known as the critical inclination — which keeps the apogee permanently over the northern hemisphere. A satellite spends roughly 8 of its 12-hour period near apogee, appearing quasi-stationary above latitudes of 60–70°N.
Use cases
HEO is the solution of choice for communications and broadcasting over Arctic and sub-Arctic regions where GEO satellites offer insufficient elevation angles. Russia's Molniya and Meridian military communications satellites, the Arktika-M weather monitoring system, and early Soviet TV broadcasting all used this orbit family. In the West, the Sirius satellite radio network (now SiriusXM) used a Tundra orbit — a similar HEO variant — before transitioning to GEO.
Trade-offs
Continuous coverage requires a minimum of three satellites in coordinated Molniya orbits to maintain at least one satellite always near apogee. Ground antennas must track moving satellites, adding mechanical complexity compared to fixed GEO dishes.