Tundra orbit

A Tundra orbit (Russian: Тундра) is a highly elliptical geosynchronous orbit (note: not geostationary orbit) with a high inclination (usually near 63.4°) and an orbital period of one sidereal day (about 4 minutes less than a solar day). A satellite placed in this orbit spends most of its time over a chosen area of the Earth, a phenomenon known as apogee dwell. The ground track of a satellite in a tundra orbit is a closed figure eight.

These orbits are conceptually similar to Molniya orbits, which have the same inclination but half the period (about 12 hours). The only current known user of Tundra orbits is the EKS satellite, part of a Russian early warning constellation for missile launches. Until 2016, Sirius Satellite Radio, now part of Sirius XM Holdings operated a constellation of three satellites in Tundra orbits for satellite radio. The RAAN and mean anomaly of each satellite was offset by 120° so that when one satellite moved out of position, another had passed perigee and was ready to take over. The three satellites were launched in 2000 and moved into circular disposal orbits in 2016; Sirius XM now broadcasts only from geostationary satellites.

Tundra and Molniya orbits are used to provide high latitude users with higher elevation angles than a geostationary orbit. Neither the Tundra nor Molniya orbit is geostationary because that is possible only over the equator, so both orbits are elliptical to reduce the time that the satellite is away from its service area. An argument of perigee of 270° places apogee at the northernmost point of the orbit. An argument of perigee of 90° would likewise serve the high southern latitudes. An argument of perigee of 0° or 180° would cause the satellite to dwell over the equator, but there would be little point to this as this could be better done with a conventional geostationary orbit.

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