The radar operates in the microwave portion of the electromagnetic (EM) spectrum with a wavelength from 1 millimeter to 1 meter. Imaging radars are independent of weather conditions and can operate day or night. EM-waves are polarized. Normally only the horizontal (H) or vertical (V) linear polarizations are used. The radar system is characterized by combination of polarization of transmitted and received pulse: HH, HV, VH or VV. When making a contact with a scatterer, the polarization of the EM-wave can change, depending on the geometrical and dielectrical properties of the scatterer.The data can be acquired from both the ascending (northwards) and descending (southwards) satellite passes. Water clouds can interfere with the radars operating below 2 cm in wavelength. The effects of rain can be generally ignored at wavelengths above 4 cm. For longer wavelengths (above 20 cm), an effect called Faraday rotation caused by the ionosphere, i.e., free charges (electrons) and the Earth’s magnetic field, can lead to a rotation of the polarization plane. In the presence of Faraday rotation, the data, usually fully polarimetric, should be corrected. The radar systems operate in different bands that uses different wavelengths. The most common frequences/wavelengths (frequency = Speed of Light / wavelength) for environmental applications are X (5,75-10,90 GHz), C-(4,20-5,75 GHz), S-(1,550-4,20 GHz), L-(0,390-1,550 GHz) and P-(0,255-0,390 GHz) band. The selection of SAR system for acquiring data depends on their application. Longer wavelengths are mainly devoted to communication and navigation purposes. Radars penetrate atmosphere and clouds. For example for forestry, longer wavelengths starting from C- or S-band are preferred.