Remote sensing

Remote sensing is the practice of gathering data about an object or event without touching it, the opposite of on-site observation. While it can apply to other planets, it is most commonly used to study Earth. The technique serves a wide array of fields—from geophysics, geography, and land surveying to most Earth sciences like hydrology, ecology, and meteorology—as well as military, intelligence, commercial, economic, planning, and humanitarian efforts.

Today, remote sensing typically refers to sensors on satellites or aircraft that detect and classify objects on Earth’s surface, atmosphere, and oceans using propagated signals, such as electromagnetic radiation. These methods fall into two categories. Passive remote sensing detects natural radiation, like reflected sunlight, using devices such as film cameras, infrared sensors, charge-coupled devices, and radiometers. Active remote sensing, by contrast, emits its own signal (e.g., radar or lidar) toward a target and measures the reflection or backscatter; the time delay between emission and return reveals the object’s location, speed, and direction.

This technology allows data collection from dangerous or hard-to-reach places. Applications include tracking deforestation in the Amazon, monitoring glacial changes in polar regions, and measuring coastal and ocean depths. During the Cold War, it was used for stand-off surveillance of hostile borders. Remote sensing also replaces slow, costly ground surveys and avoids disturbing the areas or objects under study.

Orbital platforms gather data across the electromagnetic spectrum, which, combined with aerial or ground-based analysis, helps researchers track phenomena like El Niño and other long- and short-term trends. Additional uses span natural resource management, agriculture (land use and conservation), greenhouse gas monitoring, oil spill detection, and national security—including overhead and border surveillance.

The basis of multispectral analysis is that objects or areas reflect or emit radiation differently from their surroundings. Conventional radar supports air traffic control, early warning, and large-scale weather data. Doppler radar helps law enforcement monitor speed and enhances meteorological collection, such as wind speed, direction, and precipitation intensity. Other active systems study plasma in the ionosphere. Interferometric synthetic aperture radar creates precise digital elevation models of large terrain (e.g., RADARSAT, TerraSAR-X, Magellan). Laser and radar altimeters on satellites measure gravity-induced water bulges to map seafloor features within about a mile, and they gauge ocean wave height and wavelength to determine wind speed, direction, and surface currents. Ultrasound and radar tide gauges measure sea level, tides, and wave direction near coasts.

Lidar is used for weapon ranging, laser-guided projectiles, and detecting chemical concentrations in the atmosphere. Airborne lidar measures ground object heights more accurately than radar, often producing Digital Surface Models or Digital Elevation Models. Vegetation monitoring is a key lidar application.

The most common instruments are radiometers and photometers, which collect reflected and emitted radiation across many frequencies—primarily visible and infrared, followed by microwave, gamma-ray, and occasionally ultraviolet. They can detect chemical emission spectra, providing atmospheric concentration data. Radiometers work at night by sensing artificial light, a signature of human activity; this enables remote sensing of population, GDP, and infrastructure damage from war or disasters. Radiometers and radar on satellites also monitor volcanic eruptions. Research at the U.S. Army Research Laboratory has shown that spectropolarimetric imaging helps track targets, as manmade objects (e.g., military trucks) have polarimetric signatures absent in natural ones.