What are Highly Elliptical Orbits (HEO)?

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Aug 10, 2022

A Highly Elliptical Orbit (HEO) is a satellite orbit around the Earth that follows the path of an ellipse. This orbit path is highly elliptical or is said to have a high eccentricity (a measure of the deviation of an ellipse from a circle). An ellipse has 2 focal points and for an HEO, one of these focal points is the geo center of Earth and the other is the Apogee.

A major feature of an HEO is its apogee and perigee points. HEOs have a low perigee (the point of orbit closest to the Earth) with an altitude of under 1,000 Km and a high apogee (the point farthest from the earth) with an altitude of over 35,756 Km. These points refer to one of the key features of an elliptical orbit which is that a satellite in this type of orbit moves much faster when it is close to the earth than when it is farther away. This is because when the satellite is in perigee (the point of orbit closest to the Earth), the gravitational pull from the earth is high as compared to when the satellite is in apogee (the point farthest from the earth). Hence, the satellite will spend most of its time near the apogee where it moves very slowly and can be in view over its operational area for most of the time, and fall out of view quickly when the satellite comes closer to the Earth and passes over the blind side of the Earth. Near the apogee, the satellite will have the maximum coverage of the Earth’s surface it is facing and near the perigee when it is closest, it will have minimum coverage. To solve this problem, multiple satellites are placed on similar elliptical orbits and timed in such a way that one of them is always over the desired coverage area at all times thus achieving permanent coverage.

Another feature of HEOs is their plane of orbit. Some satellites may orbit around the equator, whereas others may have different orbits. The angle of inclination of a satellite orbit is the angle between a line perpendicular to the plane of the orbit and a line passing through the poles. This means that an orbit directly above the equator will have an inclination of 0° (or 180°), and one passing over the poles will have an angle of 90°. Those orbits above the equator are generally called equatorial obits, whilst those above the poles are called polar orbits.

Geostationary orbits cannot serve high latitudes because their elevation above the horizon from the ground sites is too low. HEOs have the advantage over a GEO by not being limited to equatorial orbits and thus do not have the resulting lack of high latitude and polar coverage. HEOs can be used to provide coverage over any point on the globe. As a result of this ability to provide high latitude and polar coverage, countries such as Russia which need coverage over polar and near-polar areas make significant use of highly elliptical orbits. With two satellites in any orbit, HEOs are able to provide continuous coverage. Their only main disadvantage is that the satellite's position from a point on the Earth does not remain the same and hence requires multiple satellites perfectly timed in the same path for continuous coverage.

Some examples of inclined HEO orbits include Molniya orbits, named after the Molniya Soviet communication satellites which used them, and Tundra orbits. Japan’s QZSS GNSS constellation satellites also use an HEO as their orbit. North American Radio company Sirius Satellite Radio used inclined HEO orbits, specifically the Tundra orbits, to keep two satellites positioned above North America while another satellite quickly swept through the southern part of its 24-hour orbit. The longitude above which the satellites dwell at apogee in the small loop remains relatively constant as Earth rotates. Their three separate orbits are spaced equally around the Earth but share a common ground track.

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