||This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. (April 2009)|
A Tundra orbit (Russian: "Тундра") is a type of highly elliptical geosynchronous orbit with a high inclination (usually near 63.4°) and an orbital period of one 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. Synchronization with the solar day ensures that the satellite, viewed from a point on the ground, describes a predictable trajectory in the sky at the same time day after day, and moves at a moderate angular speed over the area of interest. The ground track of a satellite in a tundra orbit is a closed "figure-eight".
These are conceptually similar to Molniya orbits which have the same inclination but half the period (about 12 hours). The only current user of tundra orbits is Sirius Satellite Radio, which operates a constellation of three satellites. The RAAN and mean anomaly of each satellite is offset by 120 degrees so that when one satellite moves out of position, another has passed perigee and is ready to take over.
Tundra and Molniya orbits are used to provide high latitude users with higher elevation angles than a geostationary orbit. Neither the Tundra nor Molniya orbits are 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 degrees places apogee at the northernmost point of the orbit. (An argument of perigee of 90 degrees would likewise serve the high southern latitudes. An argument of perigee of 0 or 180 degrees 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.)
The Tundra and Molniya orbits use a sin−1 √ ≈ 63.4° inclination to null the secular perturbation of the argument of perigee caused by the Earth's equatorial bulge. With any inclination other than 63.4 degrees or its supplement, 116.6 degrees, the argument of perigee would change steadily over time so that apogee would occur either before or after the highest latitude is reached.