Passive and Active Remote Sensing Systems

Remote sensing is the science of gathering information about an object or area without direct physical contact, typically by recording the electromagnetic energy that the target reflects or emits. The term itself is usually credited to Evelyn Pruitt, a geographer at the U.S. Office of Naval Research, who coined it around 1960 to distinguish satellite- and aircraft-based observation from traditional aerial photography. All remote sensing exploits some portion of the electromagnetic spectrum: the visible range (roughly 0.4 to 0.7 micrometers), the near-infrared and shortwave infrared (0.7 to about 2.5 micrometers), the thermal infrared (approximately 8 to 14 micrometers, associated with emitted heat rather than reflected sunlight), and the microwave region (wavelengths from millimeters to meters, used by radar). The foundational distinction within the field is between passive and active systems, a division that determines what a given sensor can observe, when, and under what conditions.