Passive vs. Active Sensing: Common Active and Passive Sensing Systems

So far, 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 reemitted, as it is for thermal infrared wavelengths.

Passive remote sensing system measure energy that is naturally available (Left), while active remote sensing systems provide their own energy source for illumination (Right)

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 large amount of energy to adequately illuminate targets. Some examples of active sensors are a laser fluorosensor and synthetic aperture radar (SAR).

Common Active Remove Sensing Systems

The most widely used active remote sensing systems include:

RADAR, based on the transmission of long-wavelength microwaves (e. g. , 3 – 25 cm) through the atmosphere and then recording the amount of energy back-scattered from the terrain;

LIDAR, which is based on the transmission of relatively short-wavelength laser light (e. g. , 0.90 mm) and then recording the amount of light back-scattered from the terrain; and

SONAR, which is based on the transmission of sound waves through a water column and then recording the amount of energy back-scattered from the bottom or from objects within the water column.

Common Passive Remove Sensing Systems

The most widely used passive remote sensing systems include:

Multispectral Sensor: Multispectral sensors measure reflected energy within several specific sections (also called bands) of the electromagnetic spectrum. Multispectral sensors usually have between 3 and 10 different band measurements in each pixel of the images they produce. Examples of bands in these sensors typically include visible green, visible red, near infrared, etc. Landsat, Quickbird, and Spot satellites are well-known satellite sensors that use multispectral sensors.

Hyperspectral Sensor: Hyperspectral sensors measure energy in narrower and more numerous bands than multispectral sensors. Hyperspectral images can contain as many as 200(or more) contiguous spectral bands. The numerous narrow bands of hyperspectral sensors provide a continuous spectral measurement across the entire electromagnetic spectrum and therefore are more sensitive to subtle variations in reflected energy. Images produced from hyperspectral sensors contain much more data than images from multispectral sensors and have a greater potential to detect differences among land and water features. For example, multispectral imagery can be used to map forested areas, while hyperspectral imagery can be used to map tree species within the forest.