Research Highlights

"DEM Error Correction in InSAR Time-Series"

by Ph.D. Student: Heresh Fattahi


We present a mathematical formulation for the phase due to the errors in Digital Elevation Models (DEMs) in InSAR time-series obtained by the Small Baseline (SB) or Small BAseline Subset (SBAS) method. We show that the effect of the DEM error in the estimated displacement is proportional to the perpendicular baseline history of the set of SAR acquisitions. This effect at a given epoch is proportional to the perpendicular baseline between that epoch and the reference acquisition. Therefore the DEM error can significantly affect the time-series results even if small-baseline interferograms are used. We propose a new method for DEM error correction of InSAR time-series, which operates in the time domain after inversion of the network of interferograms for the displacement time-series. This is in contrast to the method of Beradino et al. (2002) in which the DEM error is estimated in the interferogram domain. We show the effectiveness of this method using simulated InSAR data. We apply the new method to Fernandina volcano in the Galapagos Islands and show that the proposed DEM error correction improves the estimated displacement significantly.

DEM 1

Figure 1. Application of the new DEM error correction algorithm on the simulated data considering different temporal variations of ground displacement: (a) Baseline-time plot of network of interferograms, (b) baseline history. (c-f) Simulated displacement history (dashed lines), estimated displacement history without DEM error correction (filled circles), DEM error corrected using the new algorithm (triangles) and using the original SBAS method (plus signs) for a pixel with (c) zero, (d) linear, (e) exponential, and (f) complex time-variable displacement. The baseline history is taken from real Envisat SAR data acquired over Fernandina volcano (see section V). Note that the proposed and original SBAS methods achieve identical results for cases presented in c, d and e.


DEM

Figure 2. (a) Estimated displacement history for a pixel in the center of Fernandina caldera before (circles) and after (triangles) DEM error correction. (b) Uncorrected displacement history (filled circles, dashed line) and baseline history (open circles, solid line) for the 2003-2005.5 time period. This period is characterized by rapid baseline variation (see Fig. 2(b)). The histories are proportional because of a DEM error. Please note that the total offset due to the 2009 eruption may have been underestimated in C-band ASAR data because of high phase gradient problem, since recent results from L-band ALOS PALSAR data shows more than one meter offset for this eruption [21].



Reference:

Fattahi, H., and F. Amelung (2012), DEM Error Correction in InSAR Time-Series, IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING.