S1TBX TOPSAR Interferometry with Sentinel-1 Tutorial.pdf

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TOPS Interferometry Tutorial Interferometry Tutorial The goal of this tutorial is to provide novice and experienced remote sensing users with step-by- step instructions on interferometric processing with Sentinel-1 Interferometric Wideswath products. What is Interferometry? Interferometric synthetic aperture radar (InSAR) exploits the phase difference between two complex radar SAR observations taken from slightly different sensor positions and extracts information about the earth’s surface. A SAR signal contains amplitude and phase information. The amplitude is the strength of the radar response and the phase is the fraction of one complete sine wave cycle (a single SAR wavelength). The phase of the SAR image is determined primarily by the distance between the satellite antenna and the ground targets. By combining the phase of these two images after coregistration, an interferogram can be generated whose phase is highly correlated to the terrain topography. For an introduction to interferometric concepts, please see ESA’s InSAR Principles: Guidelines for SAR Interferometry Processing and Interpretation (ESA TM-19). Sentinel-1 Interferometric Wide Swath Products The Interferometric Wide (IW) swath mode is the main acquisition mode over land for Sentinel-1. It acquires data with a 250 km swath at 5 m by 20 m spatial resolution (single look). IW mode captures three sub-swaths using Terrain Observation with Progressive Scans SAR (TOPSAR). With the TOPSAR technique, in addition to steering the beam in range as in ScanSAR, the beam is also electronically steered from backward to forward in the azimuth direction for each burst, avoiding scalloping and resulting in homogeneous image quality throughout the swath. TOPSAR mode replaces the conventional ScanSAR mode, achieving the same coverage and resolution as ScanSAR, but with a nearly uniform SNR (Signal-to-Noise Ratio) and DTAR (Distributed Target Ambiguity Ratio). 2

TOPS Interferometry Tutorial TOPSAR Acquisitions IW SLC products contain one image per sub-swath and one per polarisation channel, for a total of three (single polarisation) or six (dual polarisation) images in an IW product. Each sub-swath image consists of a series of bursts, where each burst has been processed as a separate SLC image. The individually focused complex burst images are included, in azimuth- time order, into a single sub-swath image with black-fill demarcation in between, similar to ENVISAT ASAR Wide ScanSAR SLC products. Fogo Volcano 2014 Eruption In this tutorial, we will process two Sentinel-1 IW SLC products over the 2014 eruption of the Fogo volcano in Cape Verde. The eruption began on November 23, 2014 and continued to be active until February 8, 2015. 3

TOPS Interferometry Tutorial Pico de Fogo in Cape Verde off the coast of Africa Opening a Pair of SLC Products In order to perform interferometric processing, the input products should be two or more Single Look Complex (SLC) products over the same area acquired at different times. Open Product button in the top toolbar and browse Step 1 - Open the products: Use the for the location of the Sentinel-1 Interferometric Wide (IW) swath products. Select the manifest.safe file from each Sentinel-1 product folder and press Open Product. Press and hold the Ctrl button on the keyboard to select multiple products at a time. If the product is zipped, you may also select the zip file. 4

TOPS Interferometry Tutorial Opening a Product Step 2 - View the product: In the Products View you will see the opened products. Within the product bands, you will find two bands containing the real (i) and imaginary (q) parts of the complex data. The i and q bands are the bands that are actually in the product. The virtual Intensity band is there to assist you in working with complex data. 5

TOPS Interferometry Tutorial In Sentinel-1 IW SLC products, you will find 3 subswaths IW1, IW2, IW3. Each subswath is for an adjacent acquisition by the TOPS mode. Fogo Island can be seen in IW3. Products View Footprint of Subswaths on World Map Step 3 - View a band: To view the data, double-click on the Intensity_IW3_VV band. Zoom in using the mouse wheel and pan by clicking and dragging the left mouse button. 6

TOPS Interferometry Tutorial Intensity IW3 Band Within a subswath, TOPS data is acquired in bursts. Each burst is separated by demarcation zones. Any ‘data’ within the demarcation zones can be considered invalid and should be zero- filled but may contain garbage values. Coregistering the Data For interferometric processing, two or more images must be coregistered into a stack. One image is selected as the master and the other images are the slaves. The pixels in slave images will be moved to align with the master image to sub-pixel accuracy. Coregistration ensures that each ground target contributes to the same (range, azimuth) pixel in both the master and the slave image. For TOPS InSAR, S-1 TOPS Coregistration should be used. Step 4 - Coregister the images into a stack: Select S-1 TOPS Coregistration in the Radar menu. Select S-1 TOPS Coregistration 7

TOPS Interferometry Tutorial TOPS Coregistration consists of a graph that reads two products, select a single subswath with TOPSAR-Split, applies a precise orbit correction with Apply-Orbit-File and performs a DEM assisted Back-Geocoding coregistration. S-1 TOPS Coregistration In the first Read operator, select the first product [1]. This will be your master image. In Read(2) select the other product. This will be your slave image. In the TOPSAR-Split tab, select the IW3 subswath for each of the products. 8

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