Satellites such as DMSP, POES, NPOESS, and TIROS circle the globe in so-called polar orbits, passing over the North and South Poles as Earth rotates beneath them. Each satellite crosses any point on Earth up to two times a day and has an orbital period of about 100 minutes, thus providing nearly complete global coverage every six hours. In contrast with geostationary satellites, which remain far enough away to match Earth's daily rotational period, polar-orbiting satellites travel at a low altitude, generally around 850 kilometers. They can therefore provide images with relatively high resolution.
In addition to spinning on its axis, Earth also circles the sun, at a rate of about 365 days per revolution. There are 360 degrees in a circle, so Earth is moving, or precessing, nearly 1 degree per day. If a satellite were in a truly polar orbit, circling the globe at 90 degrees relative to the equator, it would move with the planet, experiencing different conditions throughout the year. However, if the orbit is tilted or inclined slightly away from a true north-south orbit, the asymmetric gravitational pull of the planet will cause a slow precession in the orbital plane. At an inclination of 98.7 degrees, the orbital plane will precess nearly 1 degree per day, the same rate as Earth's yearly rotation around the sun. Thus, the orbital plane will seem fixed with respect to the sun, and the satellite will cross the equator at the same solar time every day. This so-called sun-synchronous orbit is particularly useful for meteorological satellites.
For example, sun-synchronous spacecraft can provide imagery of a certain point under the same lighting conditions every day. Of course, the clouds change with each orbit, but their broad patterns and positions remain mostly unchanged in the short orbital periods involved. Daily mosaics can be made from the swaths, which are a good general summary of global weather patterns for that period.
A further refinement of the sun-synchronous orbit is the dusk-to-dawn orbit. In this case, the satellite never casts a shadow on Earth, but travels with its orbital plane always facing the sun, riding above the line that separates day from night. Thus, the satellite can constantly view clouds at dawn or dusk, where cross light from the sun best highlights their shapes and sizes as well as the ocean texture beneath. In dual-satellite constellations, one satellite might maintain a dusk-to-dawn orbit, while a second might be oriented closer to the noon-midnight line, passing through the point of brightest day and the darkest night.