InSAR technology is capable of measuring ground deformations with extreme precision using data from satellites, orbiting hundreds of kilometers high up in the sky. It might seem unimaginable to detect movements of a few millimeters from space (I was amazed when I discovered it) but that’s how it works.
If you are a civil engineer, geo-technician, geologist, or if you are involved in any phase of the life cycle of a large infrastructure (planning, design, construction or operation) you should know InSAR technology or at least the most basic capabilities of it. Those who are responsible for the management or control of groundwater exploitation; managers or operators of linear infrastructures, dams, bridges, tunnels, mines, hydrocarbon extraction sites, experts in earthquakes, plate tectonics, volcanoes… should also know and integrate this incredible technology in their daily work.
To make use of this information from the satellite data, it is really not necessary to have a deep knowledge of the complex technology that makes it possible. But it is useful to have an understanding about the basic aspects which to comprehend the results obtained, the quality and accuracy of the data or the direction in which InSAR measures movements and deformations…
What is insar
Synthetic Aperture Radar Interferometry (InSAR) uses information from Synthetic Aperture Radar (SAR) sensors. These active sensors emit electromagnetic pulses in the microwave spectrum, operate both day and night, and are capable of penetrating clouds.
Many SAR applications make use of the amplitude of the signal returning to the sensor and ignore the phase information of the wave. In contrast, SAR interferometry-based techniques use the phase information of the backscattered radiation. If you don’t quite remember what the phase of a wave is, the internet offers a lot of resources for a quick refresh.
By calculating the phase differences of two SAR images of the same area acquired at different times we generate maps called interferograms. From these interferograms, we can then infer the displacement or deformation of the ground surface or an infrastructure between the two dates covered by the interferogram. Displacements measured using a single interferogram may contain errors due to the state of the atmosphere or a step in the processing chain called phase unwrapping. To minimize errors and separate the deformation signal from the other signals (such as atmospheric signal and noise) in the phase, a long time series of interferograms are analyzed.
The most advanced InSAR techniques, such as PSI (Persistent Scatterer Interferometry) or SBAS (Small Baseline Subset), uses multiple SAR images acquired at the same location, and generate many interferograms to develop a processing flow that allows the separation of the deformation signal of interest from other components.
In which direction does InSAR measure deformations?
The SAR sensor emits the pulses in a side-looking geometry, in contrast to the overhead aerial perspective of the optical sensors on earth observation satellites. Measurements made with InSAR technology are one-dimensional, i.e. they measure the movements on the line connecting the satellite to each observed point (what we call line-of-sight (LOS) movements).
The SAR instrument onboard the satellites illuminates the same area in its ascending pass (when the satellite travels from south to north) as in its descending pass (when it travels from north to south). The figure below shows how the same area can be seen from these two different perspectives (ascending and descending geometry). To obtain the displacements in the vertical axis and planimetry it is necessary to decompose the Los movements, taking into account the geometry of the system.
If the planimetric movements are negligible compared to the vertical motions, the calculation of vertical velocity can be performed from a single LOS geometry using the cosine of the angle of incidence of the satellite in that area.
The following figure shows the line-of-sight measurement of the deformations in the ascending geometry, as well as the color code used by Detektia´s tools and web platform.
If the horizontal movements are not negligible, we can decompose the LOS movements from the two acquisition geometries (ascending and descending) into vertical and horizontal (East-West) displacements, as shown in the following figure.
In the other posts we talk about the different satellites and constellations of satellites that carry SAR sensors, the different bands or wavelengths and the differences between them. We will also be reviewing some missions that are no longer operational. Remember that the first satellites with SAR sensors were launched in the 90s, but from their archive images we can reconstruct the terrain or infrastructures deformations that occurred 10, 20 or even more years ago, as in this case of the M30 Tunneling Works.