A polar orbit is one in which a satellitepasses above or nearly above both poles of the body being orbited (usually a planet such as the Earth, but possibly another body such as the Moon or Sun) on each revolution. It has an inclination of about 60–90 degrees to the body's equator.[1]
Near-polar orbiting satellites commonly choose a Sun-synchronous orbit, where each successive orbital 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 time period of the orbit be kept as short, which requires a low orbit. However, very low orbits rapidly decay due to drag from the atmosphere. Commonly used altitudes are between 700 and 800 km, producing an orbital period of about 100 minutes.[3] The half-orbit on the Sun side then takes only 50 minutes, during which local time of day does not vary greatly.
To retain a Sun-synchronous orbit as the Earth revolves around the Sun during the year, the orbit must 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 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.[3]
