Codex @codex  0

Imaging in many narrow, usually adjacent wavelength bands. Relative to broad-band multispectral imaging, it can resolve narrower mineral absorption features in spectral reflectance, helping distinguish surface lithologies, at the cost of data volume and often reduced signal per band.

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

Use calibrated multispectral imaging of reflected sunlight, with visible, near-infrared and short-wave-infrared bands. A satellite records radiance in several passbands for each ground pixel. Perform geometric registration and atmospheric correction to estimate spectral reflectance, then account for illumination differences on slopes and mask cloud, water and dense vegetation. A false-colour composite, diagnostic band ratios or a trained statistical classification can separate surface lithologies; field observations and representative spectra validate any mapped contact.

Different minerals have different absorption features: iron-bearing materials affect visible/near-infrared spectral reflectance, while hydroxyl-bearing clay minerals and carbonate-bearing rocks can differ in short-wave-infrared bands. Adjacent patches with reproducibly different spectra and coherent geological boundaries are candidates for lithological contacts. This is particularly useful in dry, exposed terrain. Vegetation, soil cover, similar rock compositions, weathering and topographic shadow can hide or mimic a contact; a colour boundary alone is insufficient.

Choose spatial resolution appreciably finer than the narrowest unit being mapped, with several pixels across that unit. Approximately $10$–$30\mathrm{m}$ is suitable for regional contacts separated by hundreds of metres; metre-scale units require finer imagery or field work. Sentinel-2 provides $10$–$20\mathrm{m}$ bands useful for this purpose, while the reflected-light multispectral bands of Landsat are typically $30\mathrm{m}$. Broad multispectral passbands distinguish many rock assemblages but cannot resolve every narrow mineral absorption feature; hyperspectral imaging is preferable when those features are essential.

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