Instruments on satellites do not sense the entire electromagnetic (EM) spectrum but they are sensitive to different ranges of frequencies.

Multispectral optical sensors can sense both visible and near-infrared wavelengths. However, optical sensors cannot see through clouds and some features are not visible if the optical image has a cloud cover over the study area.

Wavelengths belong to the microwave portion of the EM spectrum, can penetrate clouds, and therefore, the data collected by an instrument on satellite that makes use of microwaves—Synthetic Aperture Radar (SAR)—are useful, even when the sky is cloudy. The microwave region consists of the wavelengths, from about 0.75 cm to about 100 cm. The SAR instrument can collect data within different smaller wavelength ranges of the microwave region called bands—C, X, and L.

The optical sensor is passive. It does not provide energy but it depends on Sun's energy. The SAR, which is an active sensor, provides energy using microwaves.

The interpretation of optical images is easier than SAR images. Information that is generated using the data collected by optical sensors, resembles the information that a human receives, through his naked eyes.