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A series of Earth-observation missions using repeated optical and multispectral imaging from near-polar Sun-synchronous orbits.

A geostationary orbit provides continuous viewing of approximately the same terrestrial region, favouring fast cloud evolution, storm tracking and the diurnal cycle. The large range makes fine spatial resolution demanding, high latitudes have poor viewing geometry, and a single platform has no global coverage. A polar orbit gives much smaller range and broad latitude coverage as Earth rotates beneath it, favouring detailed global mapping; any one location is sampled intermittently. A Sun-synchronous orbit can hold approximately fixed local solar time for comparing observations, but that sampling can miss much of the diurnal evolution. The science therefore sets a trade between spatial coverage, spatial resolution and temporal sampling.

The Moon's very slow, tidally locked rotation makes a lunar stationary orbit fundamentally problematic. Let its orbital angular velocity around Earth be $n$, approximately also its spin rate. For lunar mass $m$, terrestrial mass $M$, and separation $a$, the two-body estimates giveThus $r_{\mathrm{sync}}\simeq 3^{1/3}{r}_H$ lies outside the lunar Hill sphere. Earth's tidal perturbations prevent the desired simple stable, lunar-centred stationary orbit. This is stronger than merely saying that the required orbit is high; specially maintained three-body orbits are a different construction.

Many optical land-mapping missions acquire daylight images in the midmorning, roughly 10–11 local solar time, using a Sun-synchronous orbit. This supplies substantial solar illumination while often avoiding the stronger afternoon convective cloud development. It also gives useful relief shadows without the very long shadows and weak illumination of sunrise. Repeated local-time viewing reduces illumination changes when comparing scenes.^[ref]

The time is a mission design choice, not a law applying to every optical satellite; afternoon acquisitions also exist. Sun-synchrony approximately fixes local solar time, not the same civil clock time everywhere, and seasonal solar-angle changes remain.

A Sun-synchronous orbit maintains approximately constant orientation relative to the mean Sun, so corresponding passes over a location occur at nearly the same local solar time. It is usually a nearly polar, slightly retrograde low Earth orbit. Earth rotates beneath the successive tracks, allowing broad coverage; the satellite is not stationary over a location and does not necessarily revisit that location every day.

It is attained by selecting altitude, orbital eccentricity and orbital inclination so that nodal precession caused by Earth's equatorial bulge matches the mean apparent annual motion of the Sun, about $360^{\circ }$ per year eastward. To leading order,Here $a$ is the semi-major axis, $e$ the orbital eccentricity, $i$ the orbital inclination, $n=\sqrt{G{M}_E/a^3}$ the mean motion, and $J_2$ Earth's dimensionless quadrupole coefficient. A retrograde inclination has $\cos i<0$, giving the required positive precession; typical imaging altitudes yield inclinations around $97^{\circ }$–$99^{\circ }$. The launch establishes the required orbital plane, and subsequent orbit maintenance limits drift.

This geometry suits repeated optical and multispectral imaging of land, vegetation and oceans, because comparable local time reduces illumination and shadow differences. Landsat and Sentinel-2 are examples. Radar missions such as Sentinel-1 also use this orbit for regular global sampling even though their illumination is supplied by the radar. Constant local time helps comparison but does not remove seasonal changes in solar elevation.^[ref]

A European radar mission using Sun-synchronous orbits and supporting interferometric synthetic-aperture radar.

A European optical multispectral imaging mission using Sun-synchronous orbits for repeated observations of land and coastal areas.