A polar orbit is one in which the orbit of the satellite crosses the north and south poles of the Earth. The part of the orbit during which the satellite travels from south to north is called the ascending pass, and the descending pass when the satellite travels from north to south.

In a polar orbit the Earth rotates beneath the orbiting satellite, giving global coverage over a period of time. ERS-2 and ENVISAT are both orbiting at an altitude of approximately 800 km, giving an orbital period of approximately 100 minutes. With this period a satellite will complete slightly more than 14 orbits in a single day.

However, as the Earth moves around the Sun in the course of the year, the angle of the Sun to the orbital plane of the satellite will change continuously. For the purposes of spectroscopic measurements involving sunlight, however, it would clearly be an advantage to have an orbit whose plane was always at the same angle to the Sun. This can be achieved by having an orbit that is slightly tilted to the north-south axis, and is which known as a near-polar orbit or an oblique orbit. The near-polar orbit is at an inclination of about 98.7° to the equator.

Fig 1.2.1.1.1: The ENVISAT orbit

Image: AT2-ELS