A Sun-synchronous orbit (sometimes called a heliosynchronous orbit) is a geocentric orbit which combines altitude and inclination in such a way that an object on that orbit ascends or descends over any given Earth latitude at the same local mean solar time. The surface illumination angle will be nearly the same every time. 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.
The uniformity of Sun angle is achieved by tuning the inclination to the altitude of the orbit (details in section "Technical details") such that the extra mass near the equator causes the orbital plane of the spacecraft to precess with the desired rate: the plane of the orbit is not fixed in space relative to the distant stars, but rotates slowly about the Earth's axis. Typical sun-synchronous orbits are about 600–800 km in altitude, with periods in the 96–100 minute range, and inclinations of around 98° (i.e. slightly retrograde compared to the direction of Earth's rotation: 0° represents an equatorial orbit and 90° represents a polar orbit).
Special cases of the sun-synchronous orbit are the noon/midnight orbit, where the local mean solar time of passage for equatorial longitudes is around noon or midnight, and the dawn/dusk orbit, where the local mean solar time of passage for equatorial longitudes is around sunrise or sunset, so that the satellite rides the terminator between day and night. Riding the terminator is useful for active radar satellites as the satellites' solar panels can always see the Sun, without being shadowed by the Earth. It is also useful for some satellites with passive instruments which need to limit the Sun's influence on the measurements, as it is possible to always point the instruments towards the night side of the Earth. The dawn/dusk orbit has been used for solar observing scientific satellites such as Yohkoh, TRACE, Hinode and Proba-2, affording them a nearly continuous view of the Sun.
Sun-synchronous orbits are possible around other oblate planets, such as Mars. But Venus, for example, is too spherical to have a satellite in sun-synchronous orbit. See the article Venus where a flattening coefficient of zero for this planet is cited.[improper synthesis?]
Technical details 
Equation (24) of the article Orbital perturbation analysis (spacecraft) gives the precession rate (per orbit) of an orbit around an oblate planet as
- is the coefficient for the second zonal term () related to the oblateness of the earth (see Geopotential model),
- is the gravitational constant of the Earth ()
- is the semi-latus rectum of the orbit,
- is the inclination of the orbit to the equator.
Such an orbit is sun-synchronous if and only if this equals , where P is the period and ρ is the mean motion of the Earth in its orbit around the Sun (.)
As the orbital period of a spacecraft is (where a is the semi-major axis of the orbit) and as for a circular or almost circular orbit it follows that
As an example, for a=7200 km (the spacecraft about 800 km over the Earth surface) one gets with this formula a sun-synchronous inclination of 98.696 deg.
Note that according to this approximation cos i equals −1 when the semi-major axis equals 12 352 km, which means that only smaller orbits can be sun-synchronous. The period can be in the range from 88 minutes (a=6554 km, i=96°) to 3.8 hours (a=12 352 km, but this orbit would be equatorial with i=180°). Thus, if one wants a satellite to fly over some given spot on Earth every day at the same hour, it can do between 7 and 16 orbits per day (but higher latitude locations require low a and so short periods). (A period longer than 3.8 hours may be possible by using an eccentric orbit with p<12 352 km but a>12 352 km.)
When one says that a sun-synchronous orbit goes over a spot on the earth at the same local time each time, this refers to mean solar time, not to apparent solar time. The sun will not be in exactly the same position in the sky during the course of the year. (See Equation of time and Analemma.)
The Sun-synchronous orbit is mostly selected for Earth observation satellites that should be operated at a relatively constant altitude suitable for its Earth observation instruments, this altitude typically being between 600 km and 1000 km over the Earth surface. Because of the deviations of the gravitational field of the Earth from that of a homogeneous sphere that are quite significant at such relatively low altitudes a strictly circular orbit is not possible for these satellites. Very often a frozen orbit is therefore selected that is slightly higher over the Southern hemisphere than over the Northern hemisphere. ERS-1, ERS-2 and Envisat of European Space Agency as well as the MetOp spacecraft of the European Organisation for the Exploitation of Meteorological Satellites are all operated in Sun-synchronous, "frozen" orbits.
See also 
- Orbital perturbation analysis (spacecraft)
- Geosynchronous orbit
- Geostationary orbit
- List of orbits
- Polar orbit
- World Geodetic System
- Shcherbakova, N. N.; Beletskij, V. V.; Sazonov, V. V. "Stabilization of heliosynchronous orbits of an Earth's artificial satellite by solar pressure.". Koheskie Issledovaniia, Tom 37 (4): 417 – 427.
- Rosengren, M. (November 1992). "ERS-1 - An Earth Observer that exactly follows its Chosen Path". ESA Bulletin (72).
- Sandwell, David T., The Gravity Field of the Earth - Part 1 (2002) (p. 8)
- Sun-Synchronous Orbit dictionary entry, from U.S. Centennial of Flight Commission
- NASA Q&A
- Boain, Ronald J. (2004-02). "The A-B-Cs of Sun Synchronous Orbit Design". Space Flight Mechanics Conference.