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  • 3020 IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 48, NO. 7, JULY 2010

    Topographic Correction for ALOS PALSAR Interferometry

    Sergey Samsonov

    Abstract—L-band synthetic aperture radar (SAR) interferome- try is very successful for mapping ground deformation in densely vegetated regions. However, due to its larger wavelength, the capacity to detect slow deformation over a short period of time is limited. Stacking and small baseline subset (SBAS) techniques are routinely used to produce time series of deformation and average deformation rates by reducing the contribution of topographic and atmospheric noise. For large sets of images that are presently avail- able from C-band European Remote Sensing Satellites (ERS-1/2) and Environmental Satellite (ENVISAT), the standard stacking and SBAS algorithms are accurate. However, the same algorithms are often inaccurate when used for processing of interferograms from L-band Advanced Land Observing Satellite Phased Array type L-band SAR (ALOS PALSAR). This happens because only a limited number of interferograms is acquired and also because of large spatial baselines often correlated with the time of acquisition. In this paper two techniques are suggested that can be used for removing the residual topographic component from stacking and SBAS results, thereby increasing their accuracy.

    Index Terms—Advanced Land Observing Satellite (ALOS) Phased-Array-type L-band synthetic aperture radar (SAR) (PALSAR), interferometry, SAR interferometry (InSAR), small baseline subset (SBAS) topographic correction, stacking.

    I. INTRODUCTION

    D IFFERENTIAL synthetic aperture radar (SAR) inter-ferometry (DInSAR) is a powerful tool for measuring ground deformation on a large spatial scale with high resolution and high accuracy [14], [15]. Presently, DInSAR is routinely used for mapping seismic and volcanic deformation [11], [12] as well as deformation caused by various anthropogenic sources such as mining and extraction of groundwater, oil, and gas [7], [8]. There are a few factors limiting the accuracy of interferometry, such as temporal and spatial decorrelation, and atmospheric and topographic noise. The decorrelation effect is perhaps the largest limitation, which has been partially overcome by developing a permanent scatterer (PS) technique [4], [6]. By using the PS approach, it is possible to calculate linear deformation rates and even to reconstruct nonlinear time series of each PS [5]. This technique works best when a dense network of PSs is found, which is not often the case for many

    Manuscript received May 12, 2009; revised July 21, 2009, November 11, 2009, and January 15, 2010. Date of publication April 8, 2010; date of current version June 23, 2010. This work was supported by the Foundation for Research, Science and Technology, New Zealand.

    The author is with GNS Science, Lower Hutt 5010, New Zealand, and also with Department of Earth Sciences, the University of Western Ontario, London, ON N6A5B7, Canada (e-mail: ssamson@uwo.ca).

    Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org.

    Digital Object Identifier 10.1109/TGRS.2010.2043739

    regions. In areas of moderate to good coherence, it is possible to apply stacking technique [18] that produces mean deformation rates, and also small baseline subset (SBAS) technique [1] that produces nonlinear time series as well as linear (average) de- formation rates. The SBAS technique works only on pixels that are coherent above a chosen threshold on all interferograms.

    When a large set of SAR images for a single region is available, it is almost always possible to pick the best co- herent interferometric pairs that have small spatial baseline, thereby reducing sensitivity to residual topography. This is of- ten the case for the C-band European Remote Sensing Satellites (ERS-1/2) and the Environmental Satellite (ENVISAT), which have been operational for many years and have produced large amounts of data since the early 1990s. Unfortunately, in some regions such as those covered by dense vegetation or snow, C-band interferometry is unsuccessful. The only option then is to use L-band data from the only presently operational sensor, Phased-Array-type L-band SAR (PALSAR), onboard the Advanced Land Observing Satellite (ALOS) [16].

    In our previous work [17], we showed that ALOS PALSAR interferometry produces excellent results in the New Zealand environment where the standard C-band approach is mostly unsuccessful. Interferograms are collected over a time span of two–three years with spatial baselines of up to 2–3 km. Due to the relatively small number of images available and because of good coherence, it is often impossible and unnecessary to apply PS techniques. On the other hand, stacking and SBAS techniques are very successful for calculating the time series of deformation and for producing linear (average) deformation rates. Both these techniques remove random noise, therefore increasing the sensitivity of the measurements. However, in [17], it was shown that the residual topographic noise cannot be considered random in case of ALOS PALSAR, and it is believed to be the single largest source of error limiting the interpretation of results (Fig. 1). In this paper, sensitivity of the stacking and SBAS results to the residual topography is explained, and methodology for removing this signal by cal- culating a topographic correction is suggested.

    II. METHODOLOGY

    A. Stacking

    A stack of K interferograms is calculated for each pixel by the following equation:

    Vobs = ∑

    φkobs∑ tk

    (1)

    0196-2892/$26.00 © 2010 IEEE

  • SAMSONOV: TOPOGRAPHIC CORRECTION FOR ALOS PALSAR INTERFEROMETRY 3021

    Fig. 1. Differential interferogram (path 325, HH) January 13–February 28, 2007, Bperp = 826 m. Full color cycle (purple to pink) corresponds to about 12 cm of line-of-sight displacements. Topographic component was removed with 90-m SRTM DEM. Residual topographic noise is clearly observed.

    where Vobs is the average deformation rate measured in radians/year and φkobs is the observed phase of the kth interfer- ogram (measured in radians) calculated over a time period tk

    (measured in years). It is generally assumed that φkobs consists of deformation φkdef , topographic φ

    k topo, and atmospheric φ

    k atm

    components, i.e.,

    φkobs = φ k def + φ

    k topo + φ

    k atm. (2)

    Here, it is assumed that the atmospheric component also includes all other small errors that are uncorrelated in time (orbital, thermal noise, and so on). By substituting (2) into (1) one gets

    Vobs = ∑

    φkdef + ∑

    φktopo + ∑

    φkatm∑ tk

    . (3)

    Since the atmospheric component is uncorrelated in time, it is assumed that the quantity

    ∑ φkatm/

    ∑ tk is small and therefore

    can be omitted, and φktopo can be expressed as dependent on the perpendicular baseline length

    Vobs = ∑

    φkdef∑ tk

    + ∑

    φktopo∑ tk

    = ∑

    φkdef∑ tk

    − H R sin(θ)

    ∑ Bk⊥∑ tk

    (4)

    where R is the line-of-sight distance, θ is the look angle, H is the residual topographic height, and Bkperp is the perpendicular baseline of the kth interferogram.

    Fig. 2. Temporal variation of perpendicular baseline relative to first acquisi- tion for descending path 628 and ascending path 325. Nonrandom dependence is clearly observed. A similar pattern is observed for many ascending and descending paths of the ALOS satellite over New Zealand.

    TABLE I ALOS PALSAR DIFFERENTIAL INTERFEROGRAMS FROM DESCENDING

    PATH 628 FRAME 4400 (HH POLARIZATION). LAST LINE SHOWS CUMULATIVE PERPENDICULAR BASELINE B⊥ AND TIMESPAN t

    From this equation, it is clear that the total topographic con- tribution depends on

    ∑ Bkperp and is negligible only when this

    term is close to zero. However, for most ALOS PALSAR tracks, this value is presently large and therefore cannot be omitted. At the same time, it is desirable to remove the topographic component in order to improve the accuracy of the calculated stack.

    In [13], it was proposed to remove this term by calculating a topographic correction based on dependence of the topographic phase φktopo on perpendicular baseline B

    k perp. This would be

    a method of choice if variation of Bkperp is random in time. However, in Fig. 2, it is clearly observed that the ALOS PALSAR perpendicular baselines are correlated with the time of acquisition. For example, for a subsets of eight images from path 325 and four images from path 628 acquired between February 2007 and June 2008, all possible interferograms have perpendicular baselines of the same sign, and therefore, the

  • 3022 IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 48, NO. 7, JULY 2010

    Fig. 3. Area of 360 × 360 pixels (approximately 10 × 10 km) of central TVZ in New Zealand from descending path 628. Two distinctive regions of subsidence are (top left) Tauhara and (center) Wairakei geothermal fields. Residual topographic noise is clearly observed on (a) original stack and removed from (b) corrected stack. (c) Residual topographic correction calculated with proposed technique. (a) Original stack. (b) Corrected stack. (c) Topographic correction.

    absolute value of ∑

    Bkperp is large. In this case of correlated baselines, it is clear that the topographic component φktopo of the observed signal φkobs cannot be separated from the deformation component φkdef , and therefore, standard approach cannot be applied.

    In order to estimate and remove the to

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