Remote sensors measure and record the magnitude and frequency of reflected energy from an object where the ‘energy’ is generally either electromagnetic radiation (light) or acoustic (sound). Remote sensing devices mounted on aircraft and satellites normally use imaging sensors that measure reflected energy from objects under surveillance; the mostly commonly used sensors for underwater detection use acoustic systems although the results are often presented as images. Imaging sensors fall into two general categories: active sensors and passive sensors. Passive sensors monitor only the natural solar reflected light or electromagnetic energy from an object and form the majority of the airborne and satellite based sensors in use today. Active image sensors provide their own energy which is transmitted to the object and then reflected back to the sensor. Acoustic systems, RADAR and LiDAR (based on a laser) are all active sensors.