21 September, 2021

Understanding the difference between SAR and InSAR

Synthetic Aperture Radar (SAR) and Interferometric Synthetic Aperture Radar (InSAR) are two powerful remote sensing technologies used for studying the Earth's surface. While both of these technologies utilise radar signals to generate images, they differ in their approach and the type of data they provide.

What is SAR?

SAR is a type of radar that uses microwaves to create high-resolution images of the Earth's surface. It is particularly useful for studying areas that are difficult to access or that experience extreme weather conditions, such as the polar regions. SAR images are generated by transmitting a microwave signal toward the Earth's surface and then recording the reflections of this signal that bounce back. By analysing the time it takes for the signal to travel to the Earth's surface and back, SAR can generate 2D and 3D images with a high level of detail.

Comparison of SAR and optical imagery of the Volcano Fagradalsfjall ©ICEYE and Maxar Technologies.
Comparison of SAR and optical imagery of the Volcano Fagradalsfjall ©ICEYE and Maxar Technologies.

The value of SAR lies in its ability to detect objects that may not be as visible in optical imagery, with the potential for achieving high resolutions through advanced processing techniques. Man-made structures such as roads, boats, and planes can be easily identified, as well as environmental disturbances like oil spills. The unique features of natural formations, such as the fissures and cones of the Fagradalsfjall volcano, can also be captured with remarkable detail using SAR.

For instance, a comparison of a SAR image and an optical image over Palm Jumeirah in Dubai, UAE, highlights the clear advantage of SAR in identifying road systems, buildings, and boats with great accuracy.

©Capella Space and Maxar Technologies.
Comparison of radar and optical imagery of Palm Jumeirah ©ICEYE and Maxar Technologies.

What is InSAR?

In contrast, InSAR is a technique that uses two or more SAR images of the same area to create a high-precision map of the Earth's surface. By comparing the phase difference between the radar signals in these images, InSAR can detect even tiny changes in the height of the Earth's surface. This makes InSAR particularly useful for studying geological processes such as earthquakes, volcanic activity, and land subsidence.

One of the main advantages of InSAR is its ability to detect small changes in the Earth's surface over time (down to 30mm with a +/- 4mm accuracy). To employ the technique, at least two high-precision SAR images are needed, captured at different times. If there has been any surface movement, the resulting phase shift is measured and recorded through an interferogram.

To better understand the concept of an interferogram, let's refer to the image below, which relates to the aforementioned volcano.

Interferogram of Fagradalsfjall Volcano ©ICEYE and Maxar Technologies.
Interferogram of Fagradalsfjall Volcano ©ICEYE and Maxar Technologies.

Although the image may seem quite vibrant, it is actually a standard representation of InSAR data. The different colours on the image correspond to different depth values, which in turn indicate the extent of surface displacement resulting from the phase shifts.

Another advantage of InSAR is its ability to generate highly precise topographic maps. By combining multiple SAR images, InSAR can generate maps with vertical resolutions on the order of centimetres, making it a valuable tool for studying terrain features such as mountains and valleys.

Are you interested in learning more about InSAR?

As geospatial specialists, Geoimage is well-equipped to discuss InSAR techniques and explore how they can benefit your project in greater depth. For further information, please refer to our Surface Movement Monitoring page. We hope this article has provided you with a better understanding of SAR and InSAR imagery, and how they offer valuable means of gathering data insights.

If you have any questions or would like to discuss your project requirements, please do not hesitate to contact us by sending a message.

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