The key element in remote sensing technology is the sensor. A sensor detects and quantitatively records the electromagnetic radiation (EMR) from an object remotely; hence the term ‘sensing’. For an object to be sensed, it must radiate energy either by reflecting the radiation impinging on it from an illumination source or by ‘shining’ by itself. If a sensor provides its own illumination source it is an active sensor; otherwise, if it depends on an independent illumination source, such as the sun, or the radiation from the object itself, such as the earth thermal emission, it is then a passive sensor. Synthetic aperture radar (SAR) is a typical active sensor system, as it sends microwave radiation pulses to illuminate the target area and receives the returned signals to produce an image. In contrast, the most commonly used panchromatic and multi-spectral optical sensors are typical passive sensors. A camera is a passive sensor in general, but it can become an active sensor when it is used with a flashlight in the dark. In this case, the camera provides its own light source to illuminate the object and meanwhile takes a picture.
