Microwave remote sensing operates in the microwave portion of the electromagnetic spectrum, generally using wavelengths greater than 3 cm and up to 1 m.
Microwaves are sensitive to different physical parameters than other regions of the electromagnetic spectrum. Microwaves interactions with objects are governed by geometric (structure, size, shape) and dielectric (water content) properties, whereas other regions of the electromagnetic spectrum reacts e.g. to object temperature or “color” (amount of reflection or absorption of the Sun light by a particular object).
As a general rule, microwaves interact with object at least as big as the wavelength. Smaller objects will therefore be transparent for the signal. Due to the large wavelengths, atmospheric particles are almost transparent to the signal and microwave remote sensing can penetrate clouds. Under very dry conditions, microwaves can even penetrate up to a few meters the top soil layers, therefore providing information that is not visible in other regions of the electromagnetic spectrum. Depending on the considered wavelength, microwave can also penetrate vegetation layers to different amounts.
In microwave remote sensing, three characteristics of the electromagnetic wave play an important role: its amplitude, its phase and its polarization. Depending on the application, either one characteristic or a combination of them is used to retrieve information.
There are two main types of microwave sensors: active RADAR systems and passive radiometers. RADAR is an acronym for RAdio Detection And Raging. An active radar system sends out pulses and records the echoes scattered back by the objects (scatterers) to the sensor. The systems use the two-way travel time of the radar pulse to determine the distance (range) to the illuminated object. Its backscatter intensity is determined by the radar system and object properties and depends on the quantity of energy coming back to the sensor. Active radar systems transmit a signal and record the amount of energy that is scattered back and depends of both dielectric and geometric properties. Passive radiometers record microwave energy, which is emitted by the Earth’s surface.
Depending on the type of system, microwave remote sensing can be used in multiple applications. Active sensors are principally used for diverse land cover mapping applications based on the particular backscattering mechanisms and characteristics of the objects on the Earth’s surface. Using multiple acquisitions, they are also favored for topographic, deformation and velocity mapping. Passive sensors are preferred for the determination of hydrologic variables such as soil moisture, precipitation, ice water content and sea-surface temperature.
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.