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Synthetic Aperture Radar (SAR) Basics.pptx
- 1. Synthetic Aperture Radar(SAR) Basics
- 2. • Synthetic ApertureRadar (SAR) is an emerging technology in remote sensing. • In fact, Sentinel-1 is equipped with this active type of sensor. Likewise, Radarsat and TerraSAR use synthetic aperture radar. • The main advantage behind this technology is how it can synthetically produce higher resolution images in any weather condition and even at night.
- 3. SAR’s Bat-Like Capabilities •It’s said that synthetic aperture radar is similar to how bats use echolocation to navigate in a cave. • When bats fly in a cave, it uses sound to navigate. Generally, they create sound waves from 50 to 120 dB. When this sound bounces off a wall and returns to the bat, it understands distance based on the echo. • In general, the same principles apply to SAR. The satellite sends microwave pulses to Earth. The pulse returns back to the satellite and the sensor makes an image from the returned echoes.
- 4. • Generally, syntheticaperture radar is side- looking. This means they don’t look completely down at Nadir, but at an angle. • As mentioned earlier, microwave radar can see at night and through clouds and smoke. At any time of the day or in any type of weather condition, SAR works. • Actually, longer wavelengths can penetrate clouds better and even the ground. For example, L-band (~24 cm) radar has longer wavelengths than C-band (~6 cm) and X-band (~3cm).
- 5. Types of RadarScattering • Side-looking radar interacts with different types of terrain. • The 3 main types of scattering mechanisms are: • Specular • Diffuse • Double-bounce
- 6. Smooth Surface • SmoothSurface reflection comes from flat terrains like roads or water. For this type of scattering, very little energy of the transmitted pulse returns to the sensor (similar to a mirror). • In this example, pixels will appear black typically with values less than -20dB.
- 7. Rough Surface • Roughsurface scattering such as plowed farm fields and vegetation. Scattering goes in all directions diffusely. • For example, typical pixel values will be greater than -20dB and in grey.
- 8. Double-bounce • Often, Doublebounce occurs off structures and man-made objects. The reflected pulse hits one surface after the other and back to the sensor. • For example, typical pixel values will appear white with values greater than -10dB.
- 9. A Little Deeperin the Technology • Polarization refers to the orientation of the radar wave from the SAR antenna. • Both electric and magnetic lines of force are at right angles to each other, but it’s the electric field that determines the direction of polarization of the wave. • Synthetic Aperture Radar uses an antenna that can transmit in either the horizontal (H) or vertical (V) polarization.
- 10. • When theelectromagnetic wave scatters from a target, the polarisation state of an electromagnetic wave can change. • When the sensor receives the returning wave, it measures the degree of change in polarisation from the target. For example, it can either be H or V polarization or both simultaneously.
- 11. • For single-polarization,these are typical transmit and receive pairs. • HH – for horizontal transmit and horizontal receive • VV – for vertical transmit and vertical receive • HV – for horizontal transmit and vertical receive • VH – for vertical transmit and horizontal receive
- 12. • Finally, it’ssynthetic aperture because it can create higher resolution images. • Because it receives backscatter along the length of the synthetic aperture radar, it can synthetically generate a higher resolution image for a point target on Earth. • The entire length of the synthetic aperture radar has the backscatter information for the point target. When all the backscatter information is merged, it’s like a “synthetic aperture”.
- 14. SAR Image Interpretation •Now that you know the basics of synthetic aperture radar, let’s look at a SAR image with these types of scattering. In this Radarsat-2 example, the image clearly shows all three types of backscatter.
- 16. Specular Reflection • Inthis scene, there is a river that flows in the east-west direction. As shown in the schematic above, very little energy reflects back to the radar sensor. In this case, the pixel is dark with a low dB. • This can also be seen in the southeast portion with the road/airport paved surface. Again, this is a specular reflection off a smooth surface. •
- 17. Double-Bounce Scattering • Onthe other hand, the bright white in the center of the image can be interpreted as an urban feature. The radar is receiving double-bounce backscatter, meaning the transmitted pulses are returning to the sensor. • It’s unclear at this scale what this object is but it’s due to double-bounce returns. Because of its values greater than -10dB, pixels will appear as a bright white. •
- 18. Diffuse Scattering • Finally,the majority of the radar image is rough surface scattering. You have a bit of specular and double-bounce scattering. • This may be from annual cropland, vegetation, grasses, or other features. It is diffuse scattering because there’s not a high or low amount of backscatter in the image.
- 19. Synthetic Aperture RadarUse Cases • In search and rescue missions, weather conditions are often poor. In the case of forest fires, smoke could completely block visibility. Because microwave SAR is not affected by these types of conditions, rescuers use it to find man-made objects on the ground. Specifically, it looks for double-bounce scattering where the crash site occurred. Or even where flooding occurs, if there is a specular reflection (dark pixels).
- 20. • Scientists usesynthetic aperture radar to estimate surface elevation with the Space Shuttle Radar Topography Mission inSAR. This satellite used interferometry (InSAR) generating one of the most accurate elevation models of the whole globe.
- 21. • SAR isabout understanding surface characteristics based on backscatter. That’s why we use SAR during oil spills and to understand ocean waves. During an oil spill, oil floats on water suppressing waves.