Synthetic Aperture Radar

This post will explore some of the uses of  Synthetic Aperture Radar within remote sensing.

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Since it’s invention in the 1950s, Synthetic Aperture Radar has cemented itself as a valuable tool for remote sensing, providing data on surfaces through interactions with radio pulses emitted by the sensor at a resolution which would have been impossible without an unfeasibly enormous antenna. SAR imaging has several advantages over optical imagers, namely the ability to collect data in cloudy and night-time conditions and penetrate tree cover and porous surfaces in the right conditions. Rapid responses to disaster areas are made possible with this imaging technique, whether it’s mapping out flood zones during heavy rainfall, or identifying minute changes to the ground beneath our feet following an earthquake using interferometry.

SAR antenna can be carried by either aircraft or satellites, allowing to cover a large ground area. By analysing the radio waves which are returned to the antenna, we can gain information on the type of surface the waves scattered off. Flat surfaces bounce waves away from the sensor, rough surfaces scatter the waves in all directions, and tall surfaces like buildings or tree trunks reflect waves directly back at the sensor.

SAR_scattering
SAR scattering on surfaces with different roughness (Source: Humboldt Geospacial Online)

 

In the final SAR image (following a series of pre-processing and correction steps) these reflections appear as changing brightness over each area with a different reflection method, as highlighted below:

SAR plymouth
SAR image over Plymouth, UK, showing different scattering methods (Source: ESA)

One of the uses for SAR data mentioned above is mapping out the extent of flooding. As water appears much darker in SAR than objects on land, performing a comparison between an older, non-flooded image of a regions and a recent image to see where new areas of water have appeared becomes relatively simple. SAR’s ability to image through the rain clouds causing the flood, and even record this data at night if the situation requires it, mean the comparison can be performed whenever the SAR image becomes available.   

At ARGANS, we are currently researching a similar SAR water detection technique to identify waterline boundaries along coastlines. This will assist in determining the effect of erosion on vulnerable areas, and provide more information to use across our other avenues of research.

The increased accessibility of recently acquired SAR data through the European Space Agency’s Sentinel 1a and 1b satellites allows anyone with an internet connection to make use of it.  The multitude of analysis techniques developed by researchers for SAR gives the data great flexibility, allowing it to form the foundation of many remote sensing products produced around the world. Complementing optical remote sensing systems worldwide, SAR imaging systems continue to improve and provide us with ever more detailed surface roughness information in all conditions.