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A Tundra orbit (Russian: Тундра) is a highly elliptical geosynchronous 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. With an orbital period equal to the sidereal day, the orbit will not be synchronized with the apparent motion of the sun around earth, but with that of the fixed stars, and will not hover at the zenith of the same surface locations at the same time every day. To that end, its orbital period should equal the length of the solar day instead. These orbits are conceptually similar to Molniya orbits, which have the same inclination but half the period (about 12 hours). The only current known users of Tundra orbits are Sirius Satellite Radio and the EKS satellite. The EKS satellite is part of a Russian early warning constellation for missile launches,〔http://www.nasaspaceflight.com/2015/11/soyuz-2-1b-eks-1-russian-early-warning-system/〕 while Sirius operates a constellation of three satellites used for satellite radio.〔http://www.americaspace.com/?p=43791〕 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° 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. The Tundra and Molniya orbits use a 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. ==References== 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「tundra orbit」の詳細全文を読む スポンサード リンク
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