Authors– Ashish Joshi, Shefali Agrawal, R P Singh

Satellite remote sensing is well known to provide magnificent visual perception of different earth features, at the same time, it also provides a quantitative understanding of various earth processes. Concepts of interferometry, when used in space-based imaging of the Earth’s surface using Synthetic Aperture Radar (SAR), help in a detailed and accurate three-dimensional relief map of the surface. Interferometry is a technique to perform precise measurements using the analysis of interference patterns generated from the coherent superposition of two or more electromagnetic waves. It is used in various disciplines of Astronomy, Metrology, Geophysics, Remote Sensing, etc. Interferometric synthetic aperture radar (InSAR) technique is an imaging technique in which two or more SAR images taken over the same region are combined coherently to reveal 3-dimensional surface topography and its motion.

Radar interferometry was introduced more than five decades ago, but such concepts paved the way for operational missions like Sentinel-1, NISAR, etc., which aim to study the dynamics of the solid Earth.  In SAR data, each pixel of the image contains information on both the intensity and phase of the signal backscattered from the Earth’s surface. However, it is the phase information only (and not the intensity) that is used in the InSAR technique. The phase in the SAR image is directly proportional to the distance travelled by the electromagnetic waves or range to the target.Traditionally, a SAR interferometer consists of two SAR antennas separated by a fixed distance or baseline. Both antenna measures the backscattered signal simultaneously, which is coherently processed to generate interference fringes.  Present satellites like Sentinel-1, EOS-4, NISAR, etc., use the concept of multi-pass InSAR in which they synthesize interference fringes by ingenious use of the repeat radar observations of the same scene from satellite locations sufficiently close together (optimal baseline). The baseline is the distance between the SAR satellites in two SAR acquisitions. The optimum baseline is very crucial for the interferogram formation.

Figure 1: Interferogram generated from EOS-4 SAR data acquisition

The complex images from each pass are superimposed (or interfered) as though they were from a single SAR interferometer. Phase values are subtracted from each pixel corresponding to the same area of the ground in both images to produce the interferogram (Fig. 1). Figure 1 shows the interferogram generated through ISRO’s EOS-4 SAR Satellite. The interferogram has been generated of the Alwar, Rajasthan area from the EOS-4 data acquired on 13th April 2022 & 30th April 2022.  This phase difference is a measure of the difference in path length from the given pixel to each antenna of the SAR interferometer. Using the knowledge of the orbit parameters, the phase interferogram can be related directly to the altitude on to pixel basis to generate a digital terrain model of the terrain.  Differential use of Radar interferometry can be assessed in terms of detection of small surface changes due to volcanic activities, glacier flowing, landslides, surface subsidence, etc (Fig. 2).  Differential InSAR involves taking three (or more) images of the same feature.  Passes 1 and 2 are used to form an interferogram using the basic InSAR technique. Similarly, passes 2 and 3 produce a further interferometer of the same area. The two interferograms are then themselves differenced to reveal any changes that have occurred in Earth’s surface. The coherence between two SAR images is the major challenge in the repeat pass interferometry. Any changes in the acquired area (other than topographical changes) will affect the formation of the interferogram.

Figure 2: Interferogram generated from Sentinel-1A data over the Tibet earthquake-affected region.

The InSAR is the extension of the Young double slit experiment in space. In the Young double slit experiment, when the light passes through two slits, there will be the formation of bright and dark fringes due to constructive and destructive interference between the electromagnetic waves. In the same way, when spaceborne SAR acquired the same area through single or repeat pass, there will be formation of the fringes due to constructive and destructive interference between the electromagnetic waves. Figure 2 shows the interferogram generated through the Sentinel-1A SAR satellite of the Tibet earthquake 2025. The powerful earthquake struck magnitude of 7.1 in the Tibet region on 7th January 2025. The interferogram has been generated from the Sentinel-1A data acquired on 5th January 2025 & 17th January 2025. The fringes in the Interferogram represent the phase difference between the features. Each fringe in the interferogram corresponds to half of the SAR wavelength. Thus, the displacement can be calculated by multiplying the number of fringes by half of the SAR wavelength.

References:

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