Satellites read Earth without ever touching the ground
Remote sensing gathers data about objects without touching them, especially Earth and other planets, using airborne or orbital sensors. Passive instruments catch reflected sunlight; active ones beam radar or laser and listen for returns from forests, ice, and cities. Landsat satellites have been imaging the planet in several wavelengths at once since the early 1970s.
Unlike in situ observation, remote sensing acquires information at a distance. Today that usually means satellite or aircraft sensors classifying Earth's surface, atmosphere, and oceans from electromagnetic signals. Passive methods measure emitted or reflected radiation—most often sunlight—while active methods illuminate targets with radar, lidar, or similar energy.
The payoff is access to dangerous or remote places: Amazon deforestation, polar glaciers, coastal depths, and military reconnaissance all rely on stand-off viewing. Orbital platforms sample many spectral bands; combined with aerial and ground data they help track El Niño and other long- and short-term climate swings. Multispectral analysis hinges on targets that reflect or emit differently from their surroundings.
Specialized tools fill niches: conventional radar for air traffic and large weather patterns; Doppler for speed enforcement and storm winds; lidar for ranging, chemical detection, and precise canopy or terrain heights.
Multispectral platforms such as Landsat have imaged Earth in several wavelengths since the early 1970s, yielding land-cover maps used to prospect for minerals and watch land use; current Landsat collection covers seven bands, while the Hyperion sensor on Earth Observing-1 resolves 220. Data quality is judged on four kinds of resolution: spatial, spectral, radiometric, and temporal. Pixels typically represent squares from 1 to 1,000 metres across, and sensors usually distinguish 8 to 14 bits of intensity, from 256 grey levels up to 16,384 shades per band. Gravity readings from above, first used to hunt submarines, expose small shifts in Earth's mass, as in the GRACE mission, and gamma-ray surveys search for uranium; in 1972 more than $2 million went into that kind of mineral prospecting.
Source: Remote sensing
