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: [email protected]
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.
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.
II.
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 Cband 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
____________________________________________________________
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 HVpolarization was more sensitive to moisture content and resulted in maximum variation of about 8 dB, while C-band HHpolarization 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 2. A coherence map of ERS-1/2 tandem pair in the Youngjeongdo tidal flat.
Figure 3. Profiles of coherences along “1”, “4”, and “7” in Figure 2.
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).
Figure 5. Examples of profiles showing relation between radar coherence and the reflectances of Landsat TM band 4 and 5.
IV.
LANDSAT TM AND RADAR COHERENCE
We have also investigated the relation between the coherence of interferometric phase and Landsat TM data. The Landsat TM
image was acquired under similar tidal conditions. In the tidal flat, there was higher correlation in mid- and upper-flats but lower
correlation in low tidal flats. Figure 5 (a) and (b) are profiles showing relations between the coherence of an ERS-1/2 tandem pair
and the Landsat TM band 4- and 5-images. All data plotted in Figure 5 were normalized using each data set’s mean and standard
deviation. Correlation coefficients between the coherence and the TM 4 and TM 5 are respectively 0.73 and 0.71 in the profile AA’.
In the profile BB’, correlation coefficients were 0.70 and 0.69, respectively. The patterns were very similar to each other, and
specifically reflectance of TM band 4 agreed relatively well with the ERS-1/2 tandem pair coherence.
The similarity can be explained by the effect of tidal flat surface water. TM band 4 and 5 are sensitive to tidal flat surface
conditions specifically interstitial and remnant surface water. The tidal flat conditions that can construct highly coherent
interferometric pair are similar to that causing high reflectance of TM band 4 and 5. Therefore one might be possible to use the
Landsat TM bands 4 and 5 for planning spaceborne interferometric SAR data acquisition in tidal flats.
V.
CONCLUSIONS
We investigated the ERS InSAR technique for constructing tidal flat DEMs. We partially succeeded in obtaining coherent ERS1/2 tandem pairs in Korean tidal flats and constructing DEM. The coherence was about 0.6 in tidal flats under flood tide conditions.
The accuracy of the resulting DEM is yet to be evaluated. The coherence of the interferogram was generally agreed with the
reflectances of Landsat TM bands 4 and 5 with a correlation coefficient of about 0.7.
Using sophisticated future spaceborne SAR systems such as a cartwheel [10] or ENVISAT ASAR (ERS-2 and ENVISAT
ASAR 30-minute tandem pair), high precision tidal flat DEMs can possibly be constructed if data acquisition plans are properly
made.
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