2nd URSI-Regional Conference on Radio Science (URSI-RCRS-2015) 16-19 November, 2015 Effect of coherence on the accuracy of DEM generated using interferometry SAR- A case study carried out using TerraSAR-X Imagery Manoj Joseph1, Chetna Soni2, AK Bera3, Suparn Pathak3, EVS Sitakumari1, AVV Prasad1 1National Remote Sensing Centre, ISRO, Hyderabad; 2Banasthali Vidyapith, Tonk; 3Regional Remote Sensing Centre-West, NRSC,ISRO, Jodhpur National Remote Sensing Centre, ISRO

Introduction A Digital Elevation Model (DEM) is the digital cartographic representation of the elevation of the terrain at regularly spaced intervals in X and Y directions, using Z- values related to a common vertical datum. Digital Elevation Models (DEMs) are used in many applications such as in topographic mapping, environmental modeling, rainfall-runoff studies, landslide hazard zonation, seismic source modeling, etc. SAR interferometry is an established technique for generating high quality DEMs from spaceborne and airborne data. Interferometric SAR utilizes the phase difference information extracted from a pair of complex valued SAR images acquired from different orbit positions. Accuracy of this technique depends strongly on time interval between the observations used and the spatial baseline. Interferometry can be possible from various satellites like RADARSAT, ERS, TerraSAR-X, ALOS-PALSAR etc. Coherence calculated from a pair of interferometric images, plays a significant role in constructing an accurate Digital Elevation Models (DEM). The quality of coherence depends on various factors like baseline, wavelength and time span. Weather conditions especially rainfall during SAR acquisitions also cause significant change in the moisture conditions on the earth surface and hence affects coherence.

SAR interferometry SAR 1 SAR 2 Phase of a Pixel in ... ... SAR-Image #1: ... SAR-Image #2: ... Interferogram: (if !) Ref:- http://saredu.dlr.de

Objective Generation of Digital Elevation Model (DEM) using SAR interferometry. Validation of results using Field measured GPS observations. Analyze the effect of coherence on the accuracy of DEM generated using interferometry SAR.

Study Area Jodhpur City & Surrounding Area, Rajasthan The study area comprises various land features like built-up, agriculture land, scrub land, waterbodies and barren rocky area. Relatively plane with elevation ranging from 125m to 300m TerraSAR-X image HH polarisation DoP: 10-09-2014

Data Software TerraSAR-X interferometry pair- Strip map mode DoP: 30th August 2014 & 10th September 2014 Incident angle : 21.50 HH-polarisation Single look complex format Normal Baseline(m) 53.506 Critical Baseline (m) 3970.622 2 PI Ambiguity height (m) 73.008 Software ENVI-Sarscape

Methodology Two coherent SAR images are required to produce an interferogram. –Master & Slave The images are first co-registered for finding the offset and difference in geometry between two amplitude images. One SAR image is then re-sampled to match the geometry of the other, meaning each pixel represents the same ground area in both images. The interferogram is generated by multiplication of the first image (master) to the complex conjugate of the second image (slave), The interferometric phase due to the reference ellipsoid is removed; the process is referred to as flattening. The interferogram is filtered using an adaptive power spectrum filter to amplify the phase signal.

Field observations Elevation information has been collected using GAGAN enable GPS receiver

Process complex SAR image 1: 30th August 2014 complex SAR image 2: 10th September 2014 phase of a complex SAR image pixel:

Interferogram Flattened Interferogram Interferogram interferogram:

Coherence

Phase unwrapping The phase angle that results when two signals are interfered is restricted to the range of 0 t0 2π. For generation of digital elevation models, the consecutive fringes present in the interferogram have to be unwrapped, which involves interpolating over the 0-2π phase jumps to produce a continuous deformation field. Minimum cost flow algorithm has been used

Results Hill shade view generated from DEM DEM

Coherence variation due to rainfall Due to the rainfall occurred during the second data acquisition date (10th September2014), coherence of the interferometry pair has been poor in many places. Most of the agriculture fallow lands have higher soil moisture content. This reduces the coherence of the study area. But built up and hilly area have better coherence.

Coheremce Vs DEM accuracy Accuracy of the derived DEM has been analyzed by comparing it with the field measured GPS values Sample points were selected at locations which have different coherence values. It is observed that that as the coherence increases elevation accuracy also improves. Areas having coherence greater than 0.5 have higher DEM accuracy

Conclusion DEM of Jodhpur city & surrounding area has been generated using TerraSAR-X data using Interferometry technique. Effect of coherence on the accuracy of DEM has been analyzed. DEM is of good matching with GPS measured elevation in high coherent areas

References U. G. Sefercik and I. Dana, “Crucial points of interferometric processing for DEM generation using high resolution SAR data,” ISPRS archives, Vol XXXVIII-4/W19, 2011, pp. 289-296 Mark A. Richards, “. A Beginner’s Guide to Interferometric SAR Concepts and Signal Processing,” IEEE A&E systems magazine, vol. 22, no.9, 2007, pp. 5-29 “InSAR Principles: Guidelines for SAR Interferometry Processing and Interpretation”, ESA, TM-19,February, 2007 Vivek Kumar Singh, P. K. Champati ray and A. T. Jeyaseelan, “Digital Elevation Model (DEM) Generation Using Insar: Garhwal Himalaya, Uttarakhand,” International Journal of Earth Sciences and Engineering, Vol. 03, No. 01,2010, pp. 20-30 Umut gunes Sefercik, Naci Yastikli, Iulia Dana, “DEM Extraction in Urban areas using High- resolution TerraSAR-X imagery,” Journal of Indian Scociety of Remote Sensing, Vol. 42, No. 2,2014, pp. 279-290 W.Y. Lau, D. Meng, H.C. Chang, L. Ge, X. Jia, I. Lee, “Seasonal Effect on InSAR Derived DEMs” , Int. Symp. on GPS/GNSS, Hong Kong, 8-10 December 2005, paper 10A-02

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