太阳同步轨道
A Sun-synchronous orbit (SSO), also called a heliosynchronous orbit, is a nearly polar orbit around a planet, in which the satellite passes over any given point of the planet's surface at the same local mean solar time. More technically, it is an orbit arranged so that it precesses through one complete revolution each year, so it always maintains the same relationship with the Sun. A Sun-synchronous orbit is useful for imaging, reconnaissance satellite, and weather satellites, because every time that the satellite is overhead, the surface illumination angle on the planet underneath it will be nearly the same.