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, for each revolution of the planet around the Sun, its orbital plane (specifically the longitude of the ascending node) precesses through one complete revolution around the planet.
Applications A Sun-synchronous orbit is useful for imaging, reconnaissance, and weather satellites, because every time that the satellite is overhead, the surface illumination angle on the planet underneath it is nearly the same. This consistent lighting is a useful characteristic for satellites that image the Earth's surface in visible or infrared wavelengths, such as weather and spy satellites, and for other remote-sensing satellites, such as 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.
Special cases of the Sun-synchronous orbit are the noon/midnight orbit, where the local mean solar time of passage for equatorial latitudes is around noon or midnight, and the dawn/dusk orbit, where the local mean solar time of passage for equatorial latitudes 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 that 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 TRACE, Hinode and PROBA-2, affording them a nearly continuous view of the Sun.
Orbital precession A Sun-synchronous orbit is achieved by having the osculating orbital plane precess (rotate) approximately one degree eastward each day with respect to the celestial sphere to keep pace with the Earth's movement around the Sun. This precession is achieved by tuning the inclination to the altitude of the orbit (see Technical details) such that Earth's equatorial bulge, which perturbs inclined orbits, 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 around Earth are about 600–800 km (370–500 mi) in altitude, with periods in the 96–100-minute range, and inclinations of around 98°. This is slightly retrograde compared to the direction of Earth's rotation: 0° represents an equatorial orbit, and 90° represents a polar orbit. Sun-synchronous orbits are possible around other oblate planets, such as Mars. A satellite orbiting a planet such as Venus that is almost spherical will need an additional perturbation to maintain a Sun-synchronous orbit.
Technical details The angular precession per orbit for an Earth orbiting satellite is approximately given by
Δ Ω = − 3 π J 2 R E 2 p 2 cos i , {\displaystyle \Delta \Omega =-3\pi {\frac {J_{2}R_{\text{E}}^{2}}{p^{2}}}\cos i,} where
J2 = 1.08263×10−3 is the coefficient for the second zonal term related to the oblateness of the Earth; RE ≈ 6378 km is the mean radius of the Earth; p is the semi-latus rectum of the orbit (in km); and i is the inclination of the orbit to the equator. An orbit will be Sun-synchronous when the precession rate ρ = dΩ/dt equals the mean motion of the Earth about the Sun nE, which is 360° per sidereal year (1.99096871×10−7 rad/s), so we must set nE = ΔΩE/TE = ρ = ΔΩ/T , where TE is the Earth orbital period, while T is the period of the spacecraft around the Earth. As the orbital period of a spacecraft is
T = 2 π a 3 μ , {\displaystyle T=2\pi {\sqrt {\frac {a^{3}}{\mu }}},}
where a is the semi-major axis of the orbit, and μ is the standard gravitational parameter of the planet (398600.440 km3/s2 for Earth); as p ≈ a for a circular or almost circular orbit, it follows that
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