Sentinel-1 ToolboxTOPS Interferometry TutorialIssued May 2015Luis VeciCopyright 2015 Array Systems Computing Inc.

TOPS Interferometry TutorialInterferometry TutorialThe goal of this tutorial is to provide novice and experienced remote sensing users with step-bystep instructions on interferometric processing with Sentinel-1 Interferometric Wideswathproducts.What is Interferometry?Interferometric synthetic aperture radar (InSAR) exploits the phase difference between twocomplex radar SAR observations taken from slightly different sensor positions and extractsinformation about the earth’s surface.A SAR signal contains amplitude and phase information. The amplitude is the strength of theradar response and the phase is the fraction of one complete sine wave cycle (a single SARwavelength). The phase of the SAR image is determined primarily by the distance between thesatellite antenna and the ground targets.By combining the phase of these two images after coregistration, an interferogram can begenerated whose phase is highly correlated to the terrain topography.For an introduction to interferometric concepts, please see ESA’s InSAR Principles: Guidelinesfor SAR Interferometry Processing and Interpretation (ESA TM-19).Sentinel-1 Interferometric Wide Swath ProductsThe 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 modecaptures 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 beamis 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 andresolution as ScanSAR, but with a nearly uniform SNR (Signal-to-Noise Ratio) and DTAR(Distributed Target Ambiguity Ratio).2

TOPS Interferometry TutorialTOPSAR AcquisitionsIW SLC products contain one image per sub-swath and one per polarisation channel, for a total ofthree (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 aseparate SLC image. The individually focused complex burst images are included, in azimuthtime order, into a single sub-swath image with black-fill demarcation in between, similar toENVISAT ASAR Wide ScanSAR SLC products.Fogo Volcano 2014 EruptionIn this tutorial, we will process two Sentinel-1 IW SLC products over the 2014 eruption of theFogo volcano in Cape Verde. The eruption began on November 23, 2014 and continued to beactive until February 8, 2015.3

TOPS Interferometry TutorialPico de Fogo in Cape Verde off the coast of AfricaOpening a Pair of SLC ProductsIn order to perform interferometric processing, the input products should be two or more SingleLook Complex (SLC) products over the same area acquired at different times.Step 1 - Open the products: Use theOpen Product button in the top toolbar and browsefor the location of the Sentinel-1 Interferometric Wide (IW) swath products.Select the 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 TutorialOpening a ProductStep 2 - View the product: In the Products View you will see the opened products. Within theproduct bands, you will find two bands containing the real (i) and imaginary (q) parts of thecomplex data. The i and q bands are the bands that are actually in the product. The virtualIntensity band is there to assist you in working with complex data.5

TOPS Interferometry TutorialProducts ViewIn Sentinel-1 IW SLC products, you will find 3 subswaths IW1, IW2, IW3. Each subswath is for anadjacent acquisition by the TOPS mode. Fogo Island can be seen in IW3.Footprint of Subswaths on World MapStep 3 - View a band: To view the data, double-click on the Intensity IW3 VV band. Zoom inusing the mouse wheel and pan by clicking and dragging the left mouse button.6

TOPS Interferometry TutorialIntensity IW3 BandWithin a subswath, TOPS data is acquired in bursts. Each burst is separated by demarcationzones. Any ‘data’ within the demarcation zones can be considered invalid and should be zerofilled but may contain garbage values.Coregistering the DataFor interferometric processing, two or more images must be coregistered into a stack. One imageis selected as the master and the other images are the slaves. The pixels in slave images will bemoved to align with the master image to sub-pixel accuracy.Coregistration ensures that each ground target contributes to the same (range, azimuth) pixel inboth 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 SARProcessing menu.Select S-1 TOPS Coregistration7

TOPS Interferometry TutorialTOPS Coregistration consists of a graph that reads two products, select a single subswath withTOPSAR-Split, applies a precise orbit correction with Apply-Orbit-File and performs a DEMassisted Back-Geocoding coregistration.S-1 TOPS CoregistrationIn 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

TOPS Interferometry TutorialSelect a Subswath and PolarisationIn the Apply-Orbit-File tab, select Sentinel Precise Orbits. Orbit auxiliary data containinformation about the position of the satellite during the acquisition of SAR data.The Precise Orbit Determination (POD) service for SENTINEL-1 provides Restituted orbit filesand Precise Orbit Ephemerides (POE) orbit files. POE files cover approximately 28 hours andcontain orbit state vectors at fixed time steps of 10 seconds intervals. Files are generated one fileper day and are delivered within 20 days after data acquisition.If Precise orbits are not yet available for your product, you may select the Restituted orbits whichmay not be as accurate as the Precise orbits but will be better than the predicted orbits availablewithin the product.Orbit files for Sentinel-1 are automatically downloaded by the Toolbox.9

TOPS Interferometry TutorialSelect Sentinel Precise OrbitsIn the Back-Geocoding tab, select the Digital Elevation Model (DEM) to use and the interpolationmethods.Areas outside the DEM or in the sea may be optionally masked out.Select to output the Deramp and Demod phase if you require Enhanced Spectral Diversity toimprove the coregistration.Back-Geocoding Parameters10

TOPS Interferometry TutorialIn the Write tab, specify the output folder and the target product name.Specify the output name, format and folderPress Process to begin processing the coregistration.The resulting coregistered stack product will appear in the Products View.Coregistered Stack ProductInterferogram Formation and Coherence EstimationThe interferogram is formed by cross multiplying the master image with the complex conjugate ofthe slave. The amplitude of both images is multiplied while the phase represents the phasedifference between the two images.The interferometric phase of each SAR image pixel would depend only on the difference in thetravel paths from each of the two SARs to the considered resolution cell.The interferometric phase variation Δϕ is then proportional to ΔR divided by the transmittedwavelength λ.11

TOPS Interferometry TutorialStep 5 - Form the Interferogram: Select the stack and select Interferogram Formation fromthe InSAR Products menu.Select Interferogram FormationThe phase difference can have contributions from five different sources: Δϕflat is called flat Earth phase which is the phase contribution due to the earthcurvature. Δϕelevation is the topographic contribution to the interferometric phase. Δϕdisplacement is the surface deformation contribution to the interferometric phase. Δϕatmosphere is the atmospheric contribution to the interferometric phase. It isintroduced due to the atmospheric humidity, temperature and pressure change betweenthe two acquisitions. Δϕnoise is the phase noise introduced by temporal change of the scatterers, differentlook angle, and volume scattering.12

TOPS Interferometry TutorialContributors to SAR Interferometric PhaseThrough the interferometric processing, we shall try to eliminate other sources of error to be leftwith only the contributor of interest which is typically the elevation or the displacement.In the interferogram formation step we shall remove the flat-Earth phase. The flat-Earth phase isthe phase present in the interferometric signal due to the curvature of the reference surface. Theflat-Earth phase is estimated using the orbital and metadata information and subtracted from thecomplex interferogram.13

TOPS Interferometry TutorialInterferogram DialogThe interferogram product produced will contain a band for the interferometric phase.Interferometric Phase BandIf you open the interferometric phase band, you will still see the demarcation zone between twobursts. This will be removed once TOPS Deburst is applied.14

TOPS Interferometry TutorialInterferometric PhaseInterferometric fringes represent a full 2π cycle. Fringes appear on an interferogram as cycles ofarbitrary colours, with each cycle representing half the sensor’s wavelength. Relative groundmovement between two points can be calculated by counting the fringes and multiplying by half ofthe wavelength. The closer the fringes are together, the greater the strain on the ground.Flat terrain should produce a series of regularly spaced, parallel fringes. Any deviation from aparallel fringe pattern can be interpreted as topographic variation.With the same operator, you may also generate the coherence estimation in the same processingstep as the interferogram.The coherence between master and slave images can show you if the images have strongsimilarities and are therefore good candidates for generating a DEM. Loss of coherence canproduce poor interferometric results.Loss of coherence could be caused by temporal (time between acquisitions), geometric (orbiterrors), volumetric (vegetation) or processing.Coherence ContributionThe coherence band shows how similar each pixel is between the slave and master images in ascale from 0 to 1. Areas of high coherence will appear bright. Areas with poor coherence will bedark. In the image, vegetation is shown as having poor coherence and buildings have very highcoherence.15

TOPS Interferometry TutorialTOPS Deburst and TOPS MergeTo seamlessly join all burst data into a single image, apply the TOPS Deburst operator from theSentinel-1 TOPS menu.Select TOPS DeburstOpen the resulting deburst interferometric phase band and deburst coherence band.Interferometric Phase16

TOPS Interferometry TutorialCoherenceTo process multiple swaths, repeat steps 4 & 5 selecting the other subswaths in the TOPS Splitoperator. To merge the results of the processing from all subswaths, use the TOPS Mergeoperator.Note: In the September 2015 release of the Toolbox, it should be possible to process allsubswaths all at once.Topographic Phase RemovalThe Interferogram can then be flattened by removing the topographic phase. The operator willsimulate an interferogram based on a reference DEM and subtract it from the processedinterferogram.Step 6 - Remove Topographic Phase: Select the Interferogram product and go to the InSARProducts menu. Select Topographic Phase Removal.Select Topographic Phase Removal17

TOPS Interferometry TutorialTopographic Phase Removal DialogThe resulting product will have an interferogram with topographic phase removed and a band forthe topographic phase.Topographic Phase RemovedPhase FilteringInterferometric phase can be corrupted by noise from: Temporal decorrelation Geometric decorrelation Volume scattering18

TOPS Interferometry Tutorial Processing errorWhere there is loss of coherence, the interference pattern is lost.To be able to properly unwrap the phase, the signal-to-noise ratio needs to be increased byfiltering the phase.Step 7 - Phase Filtering: Select the Interferogram product and go to the InSAR Tools menu.Select Goldstein Phase Filtering.Select Phase Filtering19

TOPS Interferometry TutorialPhase Filtering DialogFiltered Phase Band20

TOPS Interferometry TutorialFor more tutorials visit the Sentinel Toolboxes website comments to the SNAP Forum

TOPS Interferometry Tutorial 8 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].