ERS SAR INTERFEROMETRY FOR TIDAL FLAT DEM

Joong-Sun Won and Sang-Wan Kim

Department of Earth System Sciences, Yonsei University 134 Shinchon-dong, Seodaemun-gu, Seoul, 120-749, Korea

e-mail: jswon@yonsei.ac.kr

Abstract— It is essential to construct a high precision digital elevation model (DEM) annually or seasonally in tidal flats to

monitor coastal erosion. To monitor active coastal changes using the tidal flat DEM, a vertical accuracy of higher than 20 cm is required. We apply space-borne radar interferometry (InSAR) to the Korean tidal flats to test the feasibility of InSAR in tidal flats. We also investigate favorable conditions for the data acquisition of interferometric pair. We first carried out a simulation of radar backscattering using parameters of the r.m.s. height, correlation length, and moisture content measured in the test sites. The simulation results led us to a conclusion that C-band VV-polarization would be the most effective combination for InSAR applications in tidal flats, while L-band HV-polarization might be useful for discriminating surface conditions. Under favorable conditions, we successfully constructed tidal flat DEMs using ERS-1/2 tandem pairs. In that case, the tidal flat DEM construction from ERS-1/2 tandem pairs was as effective as a waterline method. However, it was not always successful to obtain coherent interferometric pairs in tidal flats even though the bottom surface was fully exposed to the air. The results indicate that tidal conditions are not the one and only parameter accounting for interferometric coherence. One interesting result was that the coherence of the ERS interferometric pairs generally agreed with the reflectance of Landsat TM bands 4 and 5. The correlation coefficient R was about 0.7. The correlation was higher in middle and upper tidal flats, while it was lower in lower tidal flats. The results imply that the tidal flat parameters controlling the optical reflectance of the near and mid infrared are closely related to the parameters governing radar backscattering. Using sophisticated future single-pass or near single-pass space-borne SAR systems, a high precision tidal flat DEM will possibly be constructed if data acquisition plans are properly designed.

Keywords; Tidal flat, DEM, InSAR, ERS SAR, Landsat TM.

I.

II.

INTRODUCTION Tidal flat is an active zone between land mass and ocean. Tidal flats of about 6,000 km2 develop along the west coast of the

Korean peninsula. The average slope of the Korean tidal flats is about 0.1° and the elevation variation is about 4-7 m. The tidal flat digital elevation models (DEMs) in a time series can be used to estimate sediment budget annually or seasonally. To monitor such active coastal changes, we need a DEM having an accuracy of higher than 20 cm. It is very difficult to estimate morphologic change from field observation alone. Remote sensing, combined with in situ surveying, is an effective tool for monitoring tidal flats. Therefore, field measurements and remote sensing techniques have become accepted as complementary tools in geomorphology [1]. Several attempts to conduct high precision topographic mapping in tidal flats have recently been made using sophisticated remote sensing techniques, including Light Detection and Ranging (LIDAR), airborne radar interferometry (InSAR), and the waterline method. LIDAR [2] or airborne InSAR [3],[4] can be utilized to generate a precise intertidal digital elevation model (DEM). However, neither space-borne synthetic aperture radar (SAR) nor LIDAR are effective ways to obtain the appropriate data on tidal flats, mainly because of the low probability of finding favourable tidal conditions [5]. Therefore, the waterline method [6] is the only current useful approach in the practical application of satellite remote sensing to tidal flat environments. It was pointed out by that the expected DEM accuracy from the waterline method would be 14 cm at best [7].

In this study, we investigate the feasibility of spaceborne radar interferometry to tidal flat mapping mainly using ERS-1/2 SAR data. The results of radar backscattering model, coherence of interferograms, and favourable conditions for data acquisition are discussed.

RADAR BACKSCATTERING MODELING Optical remote sensing alone cannot fully satisfy demands for tidal flat studies. SAR could compensate for the problem. Radar

backscattering modeling was conducted: i) to determine optimal combination of frequencies and polarization; and ii) to estimate the expected maximum range of radar backscattering intensity within tidal flat.

Using ground truth data including moisture content, surface roughness, and grain size, we estimated radar backscattering numerically. We have sampled at 22 sites of typical sand and mud flats. Soil moisture content ranged from 12 % to 43 %. From the measured surface roughness, correlation length (l) and r.m.s. height (s) were estimated by Gaussian model. The ks and kl for C-band had ranges of 0.45-2.65 and 3.0-22.8, respectively. The ks and kl for L-band had ranges of 0.1-0.62 and 0.7-5.1, respectively. Two numerical models were used: SPM model [8] and an empirical model developed by [9]. The SPM model was used only for

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Proc. of FRINGE 2003 Workshop, Frascati, Italy,1 – 5 December 2003 (ESA SP-550, June 2004) 30_won

(a) (b)

Figure 1. The results of radar backscattering models for (a) C-band and (b) L-band with respect to different r.m.s. heights.

small ks and kl values, and the Oh's empirical model was applied to most cases. Both L- and C-band cases were estimated for HH-, VV-, and HV-polarization. The variation in terms of kl values was less than 3 dB in both C- and L-band cases. L-band HV-polarization was more sensitive to moisture content and resulted in maximum variation of about 8 dB, while C-band HH-polarization showed a minimum variation of about 3 dB. The r.m.s. height ks was turned out to be the most significant parameter among the three, and Figure 1 displays the results of C-band and L-band cases. The maximum variation might be observed by using L-band HV-polarization up to 17 dB, while only up to 5 dB differences is expected by C-band VV-polarization. The results imply that L-band HV-polarization is the most effective to investigate the surface conditions of tidal flats while C-band VV-polarization would cause the least temporal decorrelation in SAR interferometric pairs.

The maximum difference of backscattering intensity is expected to be less than 17 dB in the tidal flat. The actual maximum difference of sigma nought in a SAR image was larger than this values. Tidal conditions and remnant surface water is important additional parameters controlling parameter in the tidal flats as [6] pointed out. Surface are usually covered with scattered water of at least a few centimeter deep considerably long hours after the bottom surface is fully exposed under the ebb tide conditions, which might seriously affect radar backscattering in especially mud or mixed flats on the ebb tide.

III. ERS-1/2 TANDEM PAIRS AND COHERENCE The spaceborne repeat-pass radar interferometric technique has never been fully investigated in tidal flats, although airborne

single-pass interferometry was proven to be effective [3]. If a spaceborne SAR system is possible to generate an intertidal flat DEM properly, it would be more useful than the waterline method. Since the data acquisition of SAR is independent of cloud conditions, a SAR interferometric pair can be obtained in a short period. However, one main obstacle to spaceborne SAR interferometry is the low probability of data acquisition under favourable tidal conditions. Here we focus on investigating the application feasibility of the spaceborne radar interferometry to intertidal flat DEM generation and conditions favourable to future SAR systems such as cartwheel proposed by [8].

Coherence of the interferomgram in a tidal flat is an important criterion for the feasibility. The main task will be to find elements controlling coherence in tidal flats. Tidal conditions must be one major element influencing to coherence. However, there are additional elements including surface water cover, surface roughness, sand ripples, etc. The 1-2 cm deep surface water remains for a considerable time even after the bottom surface is fully exposed, which seriously affect backscattering. This problem is more serious in mixed or mud flats than in sand flats. Surface roughness, especially r.m.s. height, is a critical parameter to backscattering, and consequently the change in surface roughness would result in incoherent interferometric phases. Sand ripples are subject to change by tide. The effects of the last two are significant in sand flats rather than mud flats.

Figure 2 is a coherence map of ERS-1/2 tandem pair (95/12/21-95/12/22) over the Youngjeong-do area. The coherence was high maintaining about 0.6 along three profiles in Figure 3. The tide conditions were both flood tide conditions: 3.34 m under flood tide for master data; and 2.45 m under flood tide for slave data. In fact, we have tested several ERS and JERS-1 interferometric pairs, and found that it is very important to acquire data sets under flood tide conditions. Interferometric pairs acquired under ebb tide conditions generally resulted in much less coherent than those under flood tide conditions. When we use a future single-pass or near single-pass spaceborne interferometric SAR systems, it is strongly recommended to observe under flood tide conditions.

Figure 3. Profiles of coherences along “1”, “4”, and “7” in Figure 2.

Figure 2. A coherence map of ERS-1/2 tandem pair in the Youngjeong-do tidal flat.

From the ERS-1/2 tandem pair, we succeeded in unwrapping the interferometric phase and constructing a DEM in tidal flats as shown in Figure 4. Unfortunately we do not have ground truth data in this area since the ERS-1/2 tandem pair was obtained in 1995, and consequently we cannot evaluate the accuracy of the constructed tidal flat DEM. The general trend of tidal flat topography is well matched. The maximum elevation range in the tidal flat is about 5 m and the DEM profile in Figure 4(c) shows the values clearly. Inter-tidal creeks are also well seen in the DEM profile (Figure 4(c)). It is too early stage of the research to evaluate the accuracy of the obtained DEM. The results, however, demonstrate that it is possible to construct DEM by applying spaceborne InSAR systems in tidal flats if data sets are properly acquired.

Recently ERS-2 and ENVISAT 30-minute tandem pairs become available although it is yet to be operational. The ENVISAT ASAR tandem data pair has a large baseline (about 15 km) that would improve the accuracy of DEM in tidal flats. Since tidal flats usually have gentle slopes and a minimized volume scattering due to high moisture content, tidal flats might be one of the best sites for a high precision InSAR DEM test.

(a) (b) (c)

Figure 4. The interferogram of an ERS-1/2 tandem pair (a) and a DEM after phase unwrapping (b) in which the DEM f Landmass was masked. (c) A DEM profile along the A-A’ in (b).

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IV.

V.

LANDSAT TM AND RADAR COHERENCE ave also investigated the relation between the coherence of interferometric phase and Landsat TM data. The Landsat TM as acquired under similar tidal conditions. In the tidal flat, there was higher correlation in mid- and upper-flats but lower on in low tidal flats. Figure 5 (a) and (b) are profiles showing relations between the coherence of an ERS-1/2 tandem pair Landsat TM band 4- and 5-images. All data plotted in Figure 5 were normalized using each data set’s mean and standard n. Correlation coefficients between the coherence and the TM 4 and TM 5 are respectively 0.73 and 0.71 in the profile AA’. rofile BB’, correlation coefficients were 0.70 and 0.69, respectively. The patterns were very similar to each other, and lly reflectance of TM band 4 agreed relatively well with the ERS-1/2 tandem pair coherence.

similarity can be explained by the effect of tidal flat surface water. TM band 4 and 5 are sensitive to tidal flat surface ns specifically interstitial and remnant surface water. The tidal flat conditions that can construct highly coherent metric pair are similar to that causing high reflectance of TM band 4 and 5. Therefore one might be possible to use the

TM bands 4 and 5 for planning spaceborne interferometric SAR data acquisition in tidal flats.

CONCLUSIONS nvestigated the ERS InSAR technique for constructing tidal flat DEMs. We partially succeeded in obtaining coherent ERS-em pairs in Korean tidal flats and constructing DEM. The coherence was about 0.6 in tidal flats under flood tide conditions. uracy of the resulting DEM is yet to be evaluated. The coherence of the interferogram was generally agreed with the ces of Landsat TM bands 4 and 5 with a correlation coefficient of about 0.7.

g sophisticated future spaceborne SAR systems such as a cartwheel [10] or ENVISAT ASAR (ERS-2 and ENVISAT 0-minute tandem pair), high precision tidal flat DEMs can possibly be constructed if data acquisition plans are properly

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