Highly elliptical orbit
A highly elliptical orbit (HEO) is an elliptic orbit with a low-altitude (often under 1,000 kilometres (540 nmi)) perigee and a high-altitude (often over 35,786 kilometres (19,323 nmi)) apogee. The 'highly elliptical' term refers to the shape of the ellipse, and to the eccentricity of the orbit, not to the high apogee altitude.
Such extremely elongated orbits have the advantage of long dwell times at a point in the sky during the approach to, and descent from, apogee. Visibility near apogee can exceed twelve hours of dwell at apogee with a much shorter and faster-moving perigee phase. Bodies moving through the long apogee dwell can appear still in the sky to the ground when the orbit is at the right inclination, where the angular velocity of the orbit in the equatorial plane closely matches the rotation of the surface beneath. This makes these elliptical orbits useful for communications satellites.
Sirius Satellite Radio uses HEO orbits to keep two satellites positioned above North America while another satellite quickly sweeps 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 the earth rotates. The three separate orbits are spaced equally around the Earth, but share a common ground track.
Examples of HEO orbits offering visibility over Earth's polar regions, which most geosynchronous satellites lack, are:
- Molniya orbits, named after the Molniya Soviet communication satellites which used them.
- Tundra orbits