Sun-synchronous orbit

A near-polar orbit whose plane precesses at roughly one degree per day, matching Earth’s motion around the Sun and keeping passes over a location at nearly the same local solar time.

Orbital precession: The gradual rotation of an orbit’s orientation over time, caused by gravitational forces or other perturbations. The orbit depends on its orbital plane rotating eastward at a carefully selected rate.
Landsat program: A long-running series of Earth-observation satellites operated by the United States. Its satellites use sun-synchronous orbits to collect comparable land images over time.
Geostationary orbit: A circular equatorial orbit in which a satellite remains fixed above one longitude on Earth. It provides continuous regional viewing rather than sun-synchronous passes across changing ground tracks.
Local solar time: Time measured relative to the Sun’s position in the sky at a particular longitude. The orbit is designed to revisit locations at nearly the same local solar time.
Earth's oblateness: Earth’s slight equatorial bulge, which makes its gravitational field depart from that of a perfect sphere. The bulge produces the gravitational torque that drives the required precession.
Sentinel-2: A European Earth-observation mission that provides multispectral optical imagery of land and coastal areas. Its sun-synchronous orbit supports repeat imaging under a consistent lighting schedule.
Polar orbit: An orbit that passes over or near both poles on each revolution around a planet. Sun-synchronous orbits are near-polar, but polar geometry alone does not fix local solar time.
Ground track: The path traced across a planet’s surface directly beneath an orbiting object. Earth’s rotation shifts each pass, while the orbit’s precession governs how the track’s lighting time evolves.
J2 perturbation: An orbital perturbation caused by the leading nonspherical term in a planet’s gravitational field. Its effect on the ascending node sets the precession rate used in orbit design.
Earth observation: The collection of information about Earth’s surface and atmosphere using remote sensing and other measurement methods. Sun-synchronous geometry is widely used for systematic, repeatable satellite observations.

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