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Remote Sensing

How does a SAR sensor see?

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

Have you ever been flying on an airplane, excited that you got a window seat, imagining all of the amazing landscapes you will see on your journey, only to be thwarted by pesky clouds obscuring your view of the ground? 

This problem is all too common to optical Earth-observing satellites such as Sentinel-2 and Landsat-9, which are unable to observe the ground when there is cloud cover. This issue leads to the loss of crucial information that can affect decision making processes in natural resource management, agriculture and food security, and more. High cloud cover percentages combined with long revisit times of optical satellites means that regions in the tropics may only have very scarce satellite imagery.

Fortunately, there is a rapidly growing technique that allows us to “see through” the clouds: synthetic aperture radar (SAR). SAR is called 'synthetic aperture radar' because it combines radar acquisitions to simulate a radar with a very large antenna, allowing us to get high-resolution radar images from space! For more information about this technique (known as the synthetic aperture principle), readers are directed to Section 2.1.3 of the SAR Handbook.

SAR sensors are active systems that transmit microwave radiation to the Earth at an oblique angle and measure the portion of the signal that is “scattered” back towards the sensor. This is different from a passive system (e.g. Sentinel-2) that just measures radiation. In other words, passive sensors must rely on existing sources of energy (typically sunlight), whereas active sensors like SAR create their own.

The amplitude of a SAR signal is quantified by a variable called the radar cross section  (RCS, sometimes referred to colloquially as backscatter), which is the ratio of received energy to transmitted energy. The RCS amplitude is impacted by a multitude of factors associated with the sensor itself and the surface that it is imaging, unlike the amplitude of an optical signal which is influenced only by the radiometric properties of the surface. 

Because SAR is an active system with many different factors influencing the amplitude of the signal, the way we interpret SAR imagery is very different from how we interpret optical imagery. But before we get to SAR imagery, let’s first review how we understand optical imagery.

How we understand optical imagery 

Passive optical sensors like Sentinel-2 measure electromagnetic radiation that has been emitted by the sun and reflected by the Earth. The amount of energy received by the sensor – a measurement known as radiance – depends on how strongly energy is reflected by the Earth’s surface. The radiance is measured at different bands, which are ranges of wavelengths in the electromagnetic spectrum. In the visible spectrum of electromagnetic radiation (between roughly 400 and 700 nanometers), different bands measured by optical sensors correspond to different colors. 

But how can we understand the signals measured by an optical sensor? To create an image, computer displays use three different channels, one for red, one for green, and one for blue. This system was created to simulate the red, green, and blue “cones” in the human eye. By assigning the radiance of the red, blue, and green bands to their corresponding channels, we are able to form a “true-color composite” image. This image roughly simulates what we would see with our eyes if we were standing on the satellite. In fact, we can think of our own eyes as passive, optical sensors–the signals received by the red, blue, and green cones in our eyes give us ample information to understand the world around us. Optical satellites, unlike our eyes, can see in many different bands beyond just the visible spectrum, giving us far more information with which to interpret the surface beyond just color.

How we understand SAR imagery 

Like optical imagery, SAR data measures the intensity of the energy received by the sensor – known as amplitude. But unlike optical, this intensity is not at all dependent on the surface’s radiometric properties, but rather its physical properties such as surface and moisture. SAR data is commonly represented by a grayscale image conveying the RCS amplitude for each pixel. High-amplitude RCS is displayed as white, whereas low-amplitude RCS is displayed as black. As discussed previously, parameters of both the sensor as well as the imaged surface influence the RCS amplitude.

To understand how scientists interpret high and low amplitudes, or to learn more about the parameters impacting RCS, readers are directed to this article on Parameters influencing SAR signal amplitude.

For more information regarding data access and availability, or a more comprehensive overview  of SAR, check out Chapter 2 of SERVIR’s SAR handbook.

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Remote Sensing
Last edited: 08-05-2024

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