Editing Polar orbit (section)

==Usage==
Polar orbits are used for [[Earth observation satellite|Earth-mapping]], [[reconnaissance satellite]]s, as well as for some [[weather satellite]]s.<ref>Science Focus 2nd Edition 2, pg. 297</ref>
The [[Iridium satellite constellation]] uses a polar orbit to provide telecommunications services.

{{anchor|nearPolarOrbit}}Near-polar orbiting satellites commonly choose a [[sun-synchronous orbit]], where each successive orbital [[pass (spaceflight)|pass]] occurs at the same local time of day. For some applications, such as [[remote sensing]],  it is important that ''changes'' over time are not aliased by changes in local time. Keeping the same local time on a given pass requires that the [[frequency|time period]] of the orbit be kept as short, which requires a low orbit. However, very low orbits rapidly [[orbital decay|decay]] due to [[drag (physics)|drag]] from the atmosphere. Commonly used [[altitude]]s are between 700 and 800&nbsp;km, producing an [[orbital period]] of about 100 minutes.<ref name="phy6">{{cite web |url=http://www.phy6.org/Education/wlopolar.html |title=Polar Orbiting Satellites |first=David P. |last=Stern |date=2001-11-25 |access-date=2009-01-21}}</ref> The half-orbit on the Sun side then takes only 50 minutes, during which local time of day does not vary greatly.

{{anchor|precessingSV}}To retain a Sun-synchronous orbit as the [[Earth's orbit|Earth revolves]] around the Sun during the year, the orbit must [[Nodal precession|precess]] about the Earth at the same rate (which is not possible if the satellite passes directly over the pole).
Because of Earth's [[equatorial bulge]], an orbit [[orbital inclination|inclined]] at a slight angle is subject to a [[torque]], which causes [[precession]]. An angle of about 8° from the pole produces the desired precession in a 100-minute orbit.<ref name="phy6" />