So far, throughout this chapter, we have made various references to the sun as a source of energy or radiation. The sun provides a very convenient source of energy for remote sensing. The sun's energy is either reflected, as it is for visible wavelengths, or absorbed and then re-emitted, as it is for thermal infrared wavelengths. Remote sensing systems which measure energy that is naturally available are called passive sensors. Passive sensors can only be used to detect energy when the naturally occurring energy is available. For all reflected energy, this can only take place during the time when the sun is illuminating the Earth. There is no reflected energy available from the sun at night. Energy that is naturally emitted (such as thermal infrared) can be detected day or night, as long as the amount of energy is large enough to be recorded.
Active sensors, on the other hand, provide their own energy source for illumination. The sensor emits radiation which is directed toward the target to be investigated. The radiation reflected from that target is detected and measured by the sensor. Advantages for active sensors include the ability to obtain measurements anytime, regardless of the time of day or season. Active sensors can be used for examining wavelengths that are not sufficiently provided by the sun, such as microwaves, or to better control the way a target is illuminated. However, active systems require the generation of a fairly large amount of energy to adequately illuminate targets. Some examples of active sensors are a laser fluorosensor and a synthetic aperture radar (SAR).
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...a camera provides an excellent example of both passive and active sensors. During a bright sunny day, enough sunlight is illuminating the targets and then reflecting toward the camera lens, that the camera simply records the radiation provided (passive mode). On a cloudy day or inside a room, there is often not enough sunlight for the camera to record the targets adequately. Instead, it uses its own energy source - a flash - to illuminate the targets and record the radiation reflected from them (active mode).
... radar used by police to measure the speed of traveling vehicles is a use of active remote sensing. The radar device is pointed at a vehicle, pulses of radiation are emitted, and the reflection of that radiation from the vehicle is detected and timed. The speed of the vehicle is determined by calculating time delays between the repeated emissions and reception of the pulses. This can be calculated very accurately because the speed of the radiation is moving much, much faster than most vehicles...unless you're driving at the speed of light!
Is there a passive equivalent to the radar sensor?
The answer is ...
Whiz quiz - Answer
Indeed. The passive microwave radiometer, for instance, does not carry an illumination source, relying instead on detecting naturally emitted microwave energy. Such an instrument can be used for detecting, identifying and measuring marine oil slicks, for instance.