Highly elliptical orbit
A highly elliptical orbit (HEO) is an elliptic orbit with high eccentricity.
Examples of inclined HEO orbits include:
- Molniya orbits, named after the Molniya Soviet communication satellites which used them.
- Tundra orbits
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. Bodies moving through the long apogee dwell appear to move slowly, and remain at high altitude over high-latitude ground sites for long periods of time. This makes these elliptical orbits useful for communications satellites. Geostationary orbits cannot serve high latitudes due to their altitude from ground sites being too low.
Sirius Satellite Radio used inclined HEO orbits, specifically the Tundra 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.