Polar orbit
A polar orbit is one in which a satellite passes above or nearly above both poles of the body being orbited (usually such a planet as the Earth, but possibly another such body as the Sun) on each revolution. It therefore has an inclination of (or very close to) 90 degrees to the equator. A satellite in a polar orbit will pass over the equator at a different longitude on each of its orbits.
Polar orbits are often used for earth-mapping, earth observation, capturing the earth as time passes from one point, reconnaissance, as well as for some weather satellites.[1] The Iridium satellite constellation also uses a polar orbit to provide telecommunications services. The disadvantage to this orbit is that no one spot on the Earth's surface can be continuously sensed from a satellite in a polar orbit.
Near-polar orbiting satellites commonly choose a Sun-synchronous orbit, wherein each successive orbital pass occurs at the same local time of day. A Sun-synchronous orbit can be particularly important for such applications as remotely sensing atmospheric temperature, which require periodic measurements. However better shorter orbits keep time, atmospheric drag rapidly decays very low orbits of a few hundred kilometers. A commonly used altitude is approximately 1000 km, which produces an orbital period of about 100 minutes.[2] The half-orbit on the Sun side then takes only 50 minutes, when local time of day little varies.
To retain the Sun-synchronous orbit as the Earth revolves around the Sun during the year, the orbit of the satellite must precess at the same rate. A satellite that passes directly over the pole would not whereas, because of the Earth's equatorial bulge, one that is slightly inclined experiences a torque that causes precession: an angle of about 8 degrees from the pole produces the desired precession in a 100-minute orbit.[2]
A satellite can hover over one polar area much the time, albeit from far away, using a polar highly elliptical orbit with its apogee above that area.
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