Passive Sensors
Passive Sensors are the most common sensors onboard Earth Observing Satellites such as Landsat or SPOT and meteorological satellites such as GOES. These sensors are commonly known as radiometers and spectrometers. Passive sensors measuring the reflectivity (r) and transmittance (t) of the surface and atmosphere. The amount of absorption (a) can be derived by knowing how much energy the surface reflects and transmits. The values of a, r, and t are ratios of the total incident solar energy so that
a + r + t = 1 and the conservation of energy is observed.
Energy falling upon the surface is either absorbed, reflected, or transmitted through the surface. Studying the reflectance, transmittance, and absorbtion of a surface reveals information about the nature of the surface and the atmosphere.
Radiometers and spectrometers are collecting data in several different wavelengths of the Electromagnetic (EM) Spectrum. This type of data (typically referred to as multispectral data) is beneficial because different types of features on Earth will have different responses to different wavelengths. For instance, vegetation may reflect only a small amount of energy (5%-10%) in a visible band (.45 microns), but will reflect a large amount of energy (45%-50%) in an infrared band (1.0 microns).
