A
Sun-synchronous orbit (
SSO, also called a
heliosynchronous orbit) is a
geocentric orbit that combines
altitude and
inclination in such a way that the satellite passes over any given point of the planet's surface at the same local solar time. Such an orbit can place a satellite in constant sunlight and is useful for imaging, spy, and weather satellites. More technically, it is an orbit arranged in such a way that it
precesses once a year. The surface
illumination angle will be nearly the same every time that the satellite is overhead. This consistent lighting is a useful characteristic for
satellites that image the Earth's surface in visible or infrared wavelengths (e.g. weather and spy satellites) and for other remote sensing satellites (e.g. those carrying ocean and atmospheric remote sensing instruments that require sunlight). For example, a satellite in sun-synchronous orbit might ascend across the equator twelve times a day each time at approximately 15:00 mean local time. This is achieved by having the
osculating orbital plane
precess (rotate) approximately one degree each day with respect to the
celestial sphere, eastward, to keep pace with the Earth's movement around the
Sun.