Passive Systems

Passive systems rely on the electromagnetic radiation emitted by the sun as a source of energy. These systems therefore measure the amount of energy reflected (visible wavelengths) or re-emitted (thermal infrared) from the Earth’s surface. There is currently a large number of satellites that use passive systems for data collection (see Table). Most of these satellites offer low to medium-resolution multispectral (colour) and higher resolution panchromatic (black and white) imagery. Although not very useful for regional exploration, several satellite systems now offer high-resolution colour imagery.

As opposed to the black and white snapshot of panchromatic sensors, multispectral systems acquire imagery in several bands that cover different portions of the electromagnetic spectrum. These spectral bands can then be combined to form colour images; either combinations more familiar to the eye or false-colour composites. In addition to being useful to discriminate the different materials found on the Earth’s surface, multispectral satellites enable to recognize exposed structure and the subtle surface expressions of obscured/buried structures related to vegetation and geomorphic patterns.

Example 1– Ouachita Mountains, Oklahoma
Example 2 – Structurally controlled geomorphic features