Radar satellite images and the "double-bounce" effect
During the last few weeks I've been playing around with radar images from satellite sensors, in order to figure out what are the best datasets that we can use for land cover mapping in the UK. The use of radar images is promising and we might be able to increase significantly the certainty of the next generation of UK Land Cover Maps.
That been said, analyzing and interpreting radar images is challenging and completely different from optical images. Currently I'm mainly working with Sentinel-1 satellite, which carries a Synthetic Aperture Radar (SAR) sensor. A SAR sensor is what is called an active sensor. In opposition to a passive sensor, which detects and processes the light reflected from the Sun (optical sensors such as Landsat), a SAR sensor emits a microwave beam and detects its reflection after interacting with the Earth surface.
The intensity of the beam that returns to the sensor will be a proportion of the emitted signal, backscattered to the sensor from a certain point in the Earth surface. This intensity is called backscatter coefficient. The coefficient will depend on the type of surface and also in things like its shape or roughness. Without entering into the "polarization" of the signal, we could simplify things saying that the roughest the surface is, the highest the signal intensity will be. In other words, we will see brighter pixels for rougher surfaces.
As an example, a calmed body of water will not backscatter much of the microwave beam (Fig. 1a). However, a more complex surface such as an heterogeneously vegetated land will backscatter some of the signal (Fig. 1b). Another curious case of reflection occurs when the beam encounters vertical structures. In this case, it can happen that the beam reflects in a flat horizontal surface and then reflects again in the flat vertical surface, returning to the sensor with a high intensity (Fig. 1c). This is called the double-bounce or the corner-reflection effect. The double-bounce effect commonly occurs with ships in the sea with calmed waters, or in tall buildings surrounded by flat concrete areas.
A few days ago I was exploring some Sentinel-1 images from the north-west of England (where I live), and I realized that this effect was very noticeable with some highly organized structures in the middle of the sea. These were the offshore wind farms of Morecambe bay. The Waley Wind Farm is one of the largest of the world and it is very spectacular to see it from space. It's also a great example of how radar satellite images can show us a different view of the Earth surface, difficult to sea with optical images. See how different it looks the Waley Wind farm from radar and optical images in the next image.
I'm enjoying exploring the capabilities of radar images. I will possibly write more entries with some curiosities about radar sensors in the future. I would like also to study how radar images can be used for automatic detection of bird colonies, but that will have to wait for now!