Highly elliptical orbit

Highly Elliptical Orbits (HEOs) are orbits with high eccentricity, where one side of the orbit is significantly closer to the Earth than the other. HEOs have a low perigee (the point of orbit closest to the Earth) altitude, under 1,000 km, and a high apogee (the point farthest from the earth) altitude of over 35,756 km. Examples of inclined HEO orbits include Molniya orbits and Tundra orbits.

The name "Highly Elliptical" refers to the elliptical shape of the orbit caused by its eccentricity.

Diagram showing an HEO

With high orbital eccentricity comes significant changes in the satellite’s speed. HEO satellites move faster when closer to the earth, compared to when they are farther away. As a result, applications can take advantage of long dwell times above a fixed point in the sky to provide better coverage as the satellite approaches and returns from its apogee (point furthest from the Earth). However, near the perigee, the satellite passes over any point of interest very fast limiting its use.

HEOs are used for satellites to provide coverage over fixed points on Earth irrespective of latitude. HEO satellites can provide high latitude and polar coverage, unlike equatorial orbits such as geostationary orbits. However, as their position above Earth does not remain the same during their orbit, two HEO satellites are required to provide continuous coverage.

HEOs are used for communications, satellite radio, remote sensing, astrophysics and astronomy missions, and other applications. Countries such as Russia, which need coverage over polar and near-polar areas, make significant use of HEOs.

• Satellites have a medium round-trip delay.
• Satellite to Earth path loss is smaller on one side of the orbit and larger on the other side.
• The satellite's coverage area is small on one side of orbit and larger on the other side.
• HEO satellites have less exposure to atmospheric drag than LEO and MEO satellites.
• It offers services to high-latitude regions of the Earth, unlike Geostationary orbits.
• It offers long dwell times above fixed points on the Earth.

• It requires two or more satellites for continuous coverage.
• IIt only provides coverage above the selected area.
• Satellites are more complex compared to satellites for other types of orbit.
• High propagation delay occurs on one side of the Earth.
• The cost of manufacturing and launching HEO satellites is higher.

Examples of highly elliptical orbits include the following:

• Molniya orbits—used in the Molniya Soviet civil and military communication system

• Tundra orbits—a highly elliptical geosynchronous orbit with high inclination typically used for communications satellites in high latitude regions that other orbits cannot serve.
• The GX10A and GX10B Inmarsat satellites.

The Molniya series of satellites developed by the Soviet Union was launched in a highly elliptical orbit to enable them to reach the far northern regions of the country. The first satellite in this series (Molniya 1) was launched on April 23, 1965. By 1967 six Molniya satellites provided coverage throughout the Soviet Union. During the 50th anniversary of the Soviet Union on October 1, 1967, the annual parade in Red Square was broadcast nationwide via the Molniya satellite network.