Although an infinite number of polar orbits are possible, for earth observation satellites one particular type, the sun-synchronous near-polar orbit, is in widespread use. This orbit is often loosely referred to as just "polar orbit" and is used for the current polar weather satellites, including EUMETSAT's planned Metop system.

At its simplest, a polar orbit is inclined at (or nearly at) 90° to the equatorial plane, so that a satellite is able to pass over both poles of the Earth. With the plane of the orbit lying north-south and almost fixed in space, the spinning Earth results in a polar-orbiting satellite sweeping over different "swaths" of ground with each orbit, eventually covering the entire globe.

The orbit is designed to ensure that the angle between the orbital plane and the sun remains constant, resulting in consistent lighting conditions. This is achieved by a careful selection of orbital parameters to produce a precession of the orbit equal to the apparent motion of the sun as seen from Earth orbit, i.e. about one degree eastward each day. The satellite's orbital plane must be inclined away from a true north-south polar orbit. With an inclination of 98.7° to the equatorial plane, the asymmetric gravitational pull of the Earth causes the orbit to precess by the required amount. (Note that the satellite's motion is actually retrograde - it moves to the west, not the east.) A key feature is that, in each half of this orbit, the satellite always crosses a particular line of latitude at the same local solar time. The angle of the sunlight (in the daytime half) will therefore be consistent, only varying slowly as the seasons change in the course of a year.

The altitude of a satellite in polar orbit is a compromise between different requirements:

• High ground resolution and a short orbital period for frequent coverage - these result from a low orbit
• A swath of observation that is wide enough such that successive orbital swaths overlap. This ensures complete ground coverage, and is favoured by a higher orbit

As a result, a typical polar satellite moves in a circular orbit with an altitude of about 850 km and a period of 100 minutes. The satellite scans a swath about 3000 km wide on the Earth's surface, which is also wide enough to cover the poles despite the north-south orbital inclination of 8.7°. With these parameters, the satellite makes just over 14 orbits in a day, and every point on the Earth is covered at least twice.

The two types of weather satellite, polar and geostationary, should be seen as complementary. Each type has advantages and disadvantages, and an ideal observing system combines both elements.