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Remote Sensing

Parameters influencing SAR signal amplitude

Publication date: 08-05-2024, Read time: 5 min

Have you ever thought about how extraterrestrial life may perceive the world around it? What if instead of seeing colors, it could only sense the moisture and roughness of a surface? What if this sense of perception could penetrate the surface? 

While humans are limited to our sense of sight, we can take advantage of synthetic aperture radar – SAR – to determine these properties of the Earth’s surface. The amplitude of a radar cross section (RCS) signal depends on the parameters of both the sensor and the surface that it’s imaging. For a definition of RCS, or to understand how SAR is different from optical data, check out this article on an introduction to SAR.  In this article, we will review how a few of these parameters affect RCS, and how Earth observation scientists understand these parameters.

Because SAR is an active system, we can control the wavelength and polarization (the plane orthogonal to the signal’s travel direction in which the wave oscillates) of the signal in both transmission and receiving directions. Both of these sensor properties influence the magnitude of the RCS we observe. We will focus here on the effects of polarization on RCS; for more information regarding the impact of wavelength on RCS, readers are invited to read Chapter 2 of SERVIR’s SAR handbook

Most modern SAR sensors are linearly polarized, meaning the polarization is constant along the direction the electromagnetic wave is propagating in. They are defined arbitrarily as vertical or horizontal with respect to the surface of the Earth. Polarization codes are two letter codes, the first of which refers to the emitted signal’s polarization, and the second refers to the received signal’s polarization, with V meaning vertical and H meaning horizontal (e.g. the code VH means that the SAR sensor transmitted microwave radiation that was vertically polarized, and measured scattered radiation that was horizontally polarized). If a signal has the same polarization when it's transmitted and received, it’s referred to as single-polarized, whereas if it has a different polarization when it's transmitted and received, it’s referred to as cross-polarized. 

The two main drivers of the RCS controlled by the imaged surface are its roughness and its dielectric constant. The dielectric constant is a measure of a substance’s permittivity, which characterizes the tendency of a charge to distort in the presence of an electric field. While the details of the dielectric constant are beyond the scope of this article, we are interested in it because it is a good proxy for moisture. 

Both the surface roughness and dielectric constant govern how much of the incoming radiation is scattered at the surface. However, only the dielectric constant impacts whether or not and how deep the signal penetrates into the scattering surface. The response of both of these parameters depends on the wavelength of the radiation emitted by the SAR sensor. Here we will focus on how the roughness of the surface affects the amplitude of the RCS. 

Most bare or low-vegetation surfaces allow very little penetration for microwave radiation, meaning that surface roughness is the main driver of RCS response. In general, the rougher a surface is, the more the signal will be scattered towards our sensor, and the stronger the return will be. The smoother a surface is, the more the signal will be scattered away from our sensor (known as specular reflection).

Earth Observation Scientists approximate any surface on Earth to be one of three types of scattering surfaces: (1) rough surface scatterers, (2) double-bounce scatterers, and (3) volume scatterers. Rough scatterers include low-vegetation fields and bare soils, as well as roads and paved surfaces.  Double-bounce scatterers occur when a radar signal bounces off the ground, then bounces off a vertical structure such as a building or a tree trunk before returning to the sensor. Volume scatterers (e.g. vegetation canopies) are reflected multiple times before a final reflection scatters the signal back towards the sensor.

These different scattering surfaces reflect different polarizations of SAR data differently. Rough surfaces reflect VV-polarized signals stronger than HH-polarized signals, both of which are reflected stronger than cross-polarized signals. Double-bounce scatterers reflect HH-polarized signals stronger than VV-polarized signals, both of which are reflected stronger than cross-polarized signals. Volume scatterers are the main source of cross-polarized signals. Thus, if a certain surface shows a high RCS value in the cross-polarized channels, but a low RCS value in the single-polarized channels, it can be said that the surface is dominated by volume scattering and is likely vegetation. Differently polarized signals are commonly represented as different layers in the same SAR data. Thus, Earth observation scientists can assign different polarizations to different color channels (e.g. assigning VV polarization to the red channel, HH to the green channel, and VH or HV to the red channel). The resulting colors in the image convey the relative scattering strength across all polarizations, allowing us to draw conclusions about the surface.

For more information regarding data access and availability, different applications of SAR, readers are encouraged to read this introductory article on SAR  by NASA Earthdata. Readers wishing to learn more about the topics covered in this article are invited to dive into Chapter 2 of SERVIR’s SAR handbook.

Remote Sensing
Last edited: 08-05-2024

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