Sun Synchronous Orbit

A sun-synchronous orbit is an earth orbit which combines altitude and inclination in such a way that an object on that orbit passes over any given latitude of the Earth's surface at the same local solar time. The surface illumination angle will be nearly the same every time. Note that this does NOT mean that the satellite passes over the same spot at the same local solar time. For example, a satellite in sun-synchronous orbit might cross the equator twelve times a day each time at approximately 15:00 local time (for the sub-satellite point). This is achieved by having the orbital plane of the orbit precess (rotate) approximately one degree each day, eastward, to keep pace with the Earth's revolution around the sun. This trick is accomplished without use of power by balance of the satellite's own acceleration against gravity (which is greater near the equator, because earth is an oblate spheroid with greater mass near the equator).

Another way of visualizing a sun-synchronous orbit is that the satellite always maintains its basic aspect with respect to the Circle of Illumination. Of course, that Circle is in a different position relative to the Plane of Ecliptic at different times of year. Thus, the satellite has to "adjust" its position relative to the Plane of the Ecliptic, as shown in the diagram here, of a so-called dawn-to-dusk orbit, so called because the satellite's orbit will be approximately along the terminator. Clearly consistent lighting at approximately the same time of day is a strong point for the visible sensors of satellites in sun synchronous orbits.