Understanding the difference between SAR and InSAR
You are most likely familiar with seeing images of Earth from above. Google Maps for example is one we often see. But in no way does it stop there. Images ‘from above’, come in different forms and can be collected within our atmospheric layers (imaging systems mounted on moving aircrafts), or via satellites in the exosphere (the outermost layer of our atmosphere).
To enable imagery to be collected from space a complex process involving advanced technology and imaging systems are mounted to satellites. Not only that – you have the Earth’s orbit and 4 other layers of Earth’s atmosphere to compete with!
Let’s break it down further.
What is SAR?
Unlike optical imagery, SAR (Synthetic Aperture Radar) is a form of radar that can penetrate through cloud cover, smoke, haze and also darkness to see features other imaging techniques cannot.
Radar is seriously impressive!
This recent image of a volcano (named Fagradalsfjall) erupting in Iceland, shows just how SAR data can be visualised. It has been compared to an optical image with cloud cover over the same volcano taken only a year earlier. (Points if you can pronounce it’s name in one go).
The volcano is a great example of a radar or SAR image captured this year. You can see the fissure line clearly through the smoke and ash of the volcano.
In a basic form, SAR captures data through sending microwaves (or radio waves) down towards the Earth from specialised imaging equipment. The microwaves interact with moisture and structures on the Earth, resulting in reflected waves back to the imaging equipment. A complete image is able to be formed as a result of the reflected microwaves. There is much more detail that goes into how SAR works, but for now we’ll keep it simple.
What is the value in SAR data?
The value in SAR comes from being able to detect things you may not as easily be able to see from optical imagery. Through clever processing, very high resolutions are possible. Spotting man made material such as roads, boats or planes show up quite well. Oil spills are much easier to detect and definitions of nature such as the above Volcano Fagradalsfjall with its fissures and sculpted cones come up brilliantly.
Take a look at another comparison of a SAR image to an optical image over Palm Jumeirah in Dubai, (UAE). Road systems, buildings and boats are very easy to identify with SAR.
What is InSAR?
Interferometric Synthetic Aperture Radar (known as InSAR) is a technique that uses SAR data. To put it simply, it is a type of mapping technique for ground movement through the use of SAR. Interferometry uses the SAR data in order to detect surface movements and ground displacement of down to 30mm with a +/- 4mm accuracy. The technique requires two or more precise SAR images taken at different times. If there has been any surface movement, then what is known as a phase shift occurs, where these are measured and recorded via an interferogram.
Lets take it back to the volcano and look at this image below to see what we mean by interferogram.
This image sure is colourful, but this is how InSAR data is displayed. Each of the colours represent a value of depth to reveal surface movement resulting from the phase shifts.
InSAR is a highly accurate land deformation detection technique and is great for many applications. Imagine monitoring urban developments. I mean… Leaning Tower of Pisa much? Construction, town planning, transportation, mining, oil and gas industries can all benefit from the brilliance of InSAR. Natural occurrences like detecting landslides or identifying tectonic deformations are core founding use cases of InSAR. The technology is advancing all the time with new imaging equipment.
Want to learn more on InSAR?
We recommend watching this short video for a great visual description of InSAR by Hannes Dekyvere.
Want even more information on InSAR?
Don’t forget, Geoimage are geospatial experts so we can talk with you about the InSAR technique and how we can help your project in further detail. Our page on Surface Movement Monitoring has more information as well. We hope you have a better understanding of SAR and InSAR imagery and how it can provide alternative ways of obtaining data insights.