Highly Elliptical Orbit

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Highly Elliptical Orbit (HEO) is an elliptic orbit characterized by a relatively low-altitude perigee and an extremely high-altitude apogee. These extremely elongated orbits can 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 matches the rotation of the surface beneath. (For the Earth, this inclination is 63.4 degrees.) This makes these elliptical orbits useful for communications satellites.

Examples of HEO orbits offering visibility over Earth's polar regions, which most geosynchronous satellites lack:

Molniya orbit, after the former Soviet communications satellite network which used such an orbit.
Tundra orbit, also developed for Soviet use.

Much of Russia is at high latitude, so geostationary orbit does not provide full coverage of the region. These Soviet HEO orbits include polar coverage.

HEO orbits are used by Sirius Satellite Radio.