A polar orbit (inclination ± 90°) passes over both poles; because the Earth turns underneath, the satellite eventually passes over every point. A special variant is the sun-synchronous orbit (SSO): at ± 600–800 km with an inclination of ± 98°, the orbital plane rotates exactly in step with the Sun. The satellite therefore always passes at the same local time (for example 10:30).
- For optical imagery: always the same lighting and shadows, so images from different days can be compared well (change detection).
- Dawn-dusk SSO: an orbit along the day-night line, where the satellite stays in sunlight almost all the time — useful for radar satellites (SAR), which use a lot of power.
For the analyst
Because the pass time is predictable, an opponent also knows when they are being watched. Moving activity to times without a pass, or camouflaging just before one, is a classic countermeasure (see 10.1).