Remote Sensing

Remote sensing measurement devices are not in direct contact with the objects they sense. Remote sensing involves observing objects indirectly either actively or passively. Active remote sensors emit electromagnetic waves that travel to an object and are reflected back toward the sensor. Examples of active remote sensors include a police radar gun that emits electromagnetic waves to determine the speed of objects; X-rays that use electromagnetic waves to produce images of the human body; and sonar that uses reflected sound waves. One active remote sensor used by meteorologists and hydrologists measures the water level in a stream or lake by transmitting and receiving acoustic waves. The sonar bridge gauge is generally mounted under a bridge and measures water that flows beneath it.

Passive remote sensors observe electromagnetic waves emitted by objects. One example of this is the camera. For us, sight is a passive remote sensor because light waves are scattered toward our eyes.