ERS-ENVISAT Cross-interferometry for Coastal DEM Construction

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1 ERS-ENVISAT Cross-interferometry for Coastal DEM Construction Sang-Hoon Hong and Joong-Sun Won Department of Earth System Sciences, Yonsei University, 134 Shinchon-dong, Seodaemun-gu, , Seoul, Korea Phone: , Fax: , ABSTRACT Digital elevation models (DEMs) of a coastal area in a time series are very useful to monitor annual and seasonal coastal morphologic changes. Topographic features in coastal regions change significantly within a relatively short period compared with landmass. However, it is difficult to obtain coherent interferometric SAR pairs over coastal areas. ERS-ENVISAT 30-minute tandem pair is useful to observe coastal regions. We studied an ERS-ENVISAT crossinterferometric pair with a perpendicular baseline of 1.4 km and a height ambiguity of 6 m. The small height ambiguity is favorable to construct a DEM in low relief coastal areas. Accurate coregistration was required because of the differences of the pulse repetition frequency and range sampling rate between the two sensors. Resampling of ENVISAT ASAR data was performed to have equivalent pixel spacing to the ERS SAR data. Range and azimuth offset were estimated to a sub-pixel accuracy using image intensity cross correlation. A larger window chip size than a normal case was used because it was difficult to distinguish typical features. Difference of 540 Hz Doppler centroid was estimated, and the cross-interferogram was highly coherent. As range bin increased, the difference of Doppler centroid also increased. The differences of Doppler centroid were about 450 Hz and 600 Hz, respectively in near and far range. It resulted in lower coherence in far range than in near range. Coherence was improved by applying azimuth and range common band filtering. Coherences over wetland in near and far range were about 0.8 and 0.5, respectively. ERS- ENVISAT cross-interferogram usually lost information in urban area. Accuracy of the DEM constructed by the ERS- ENVISAT 30-minute pair in a coastal area is to be evaluated. 1. INTRODUCTION Coastal areas are the buffer zone between landmass and ocean. They are invaluable from geologic, environmental and ecological aspects. They change rapidly than landmass, and characterized by extremely low slopes. A high precision digital elevation models (DEMs) of coastal areas in a time series are necessary to estimate sediment budget or monitor changes annually or seasonally. A small height ambiguity by ERS-ENVISAT cross-interferometry with a large perpendicular baseline about two kilometers might be useful for DEM construction in coastal area. The first results of the inland DEMs construction from ERS-ENVISAT cross-interferometry were reported Adam and Colesanti et al. [1], [2]. Recently, a permanent scatterer approach using ERS-ENVISAT cross-interferometry is more focused [3], [4]. Since, however, it is too difficult to find the permanent scatterers in the coastal areas, a permanent scatterer interferometric SAR (PS-InSAR) technique can be hardly applied to monitoring coastal changes. The objective of study is to review characteristics of ERS-ENVISAT cross-interferometric pair in the coastal area. 2. TEST SITE The test site was selected from coastal areas in the southeastern Louisiana. A normal baseline was 1.4 km with a height ambiguity of 6.4 meters. The land cover map and shuttle radar topography mission (SRTM) data of around the New Orleans is shown in figure 1. Pink area is urban residence area, the city of New Orleans, green and yellow present deciduous forest and pasture. And sky blue is the wetland that we are interested in. Although SRTM data is available in this area, they are inaccurate and a large number of void data in coastal area. And a time series analysis is required for monitoring coastal changes. Therefore, cross-interferometry with 30 minutes temporal baseline must be one and only practical approach in coastal monitoring.

2 Forest & pasture Lake New Orleans city Wetlands Fig. 1. The land cover map (left) from the U.S. Geological Survey National Land Cover Database and a SRTM DEM (right) from the JPL NASA the New Orleans. 3. INTERFEROGRAMS AND DOPPLER CENTROIDS We generated interferograms using ENVISAT-ENVISAT pair, ERS-ERS pair and ERS-ENVISAT tandem crossinterferometric pair. ENVISAT-ENVISAT pair (106 days) and ERS-ERS pair (141 days) have large temporal baselines. In cross-interferometric processing, an accurate coregistration was required because of the differences of the pulse repetition frequency and range sampling rate between the two sensors. Resampling of ENVISAT data was first performed to have equivalent pixel spacing to the ERS data. Range and azimuth offset were estimated to a sub-pixel accuracy using image intensity cross correlation. A larger window chip size than a general case was used because it was difficult to distinguish typical features. Coherences and interferograms are shown in Figure 2. In ENVISAT-ENVISAT pair and ERS-ERS pair, while coherence is maintained in urban area, wetland lost coherence. The coastal wetland is imaged by coherent phase in ERS-ENVISAT tandem cross-interferometric pair. Therefore, it is not practical to apply repeat-pass spaceborne interferometry with a large temporal baseline to coastal areas due to serious temporal decorrelation and short baseline. After some ERS-2 gyroscope mode mission, no significant degradation in attitude and Doppler centroid stability was reported recently. For 95 % of the data, the Doppler centroid frequency is within the range of ±4500 Hz [5]. Its range will cause careful application in radar interferometry. Figure 3 presents Doppler centroids in azimuth spectrums of interferometric pairs. The difference of the Doppler centroid in ENVISAT-ENVISAT pair was about only 10 Hz. ERS- ERS pairs, however, resulted in 260 Hz and showed unstable ERS-2 gyroscope. It was worsen in ERS-ENVISAT crossinterferometric pair with 540 Hz difference. The plots of the Doppler centroid of ERS and ENVISAT data against range bin number are shown in figure 4. The Doppler centroid of ERS2 SAR increased and that of ENVISAT ASAR decreased as the range increased. The dash line shows the modeling of Doppler centroid and the solid line presents observed Doppler centroid. We have tested using two frame scenes, and they showed the similar results. The differences between modeled and observed were larger in ERS data than those in ENVISAT data. Then, we calculated difference of Doppler centroids between master and slave of the interferometric pairs. Figure 5 shows the variation of the difference of Doppler centroids. In ENVISAT-ENVISAT pair, the difference was almost zero over entire range bins. The difference in ERS-ERS pair gently increased. Slope of the difference, however, was steep in ERS-ENVISAT cross-interferometric pairs. The differences of Doppler centroid were about 450 Hz and 600 Hz, respectively in near and far range. The result means that qualities of crossinterferogram at near and far ranges might be different.

3 106 days 30 minutes 141 days Fig. 2. The coherence map (top) and interferogram (bottom) of ENVISAT-ENVISAT pair (left), ERS-ERS pair (center) and ERS-ENVISAT cross-interferometric pair (right). Fig. 3. Azimuth spectrum of ENVISAT-ENVISAT pair (left), ERS-ERS pair (center) and ERS-ENVISAT pair (right). Fig. 4. Variation of Doppler centroids of the two pairs with range. Fig. 5. DC variation with range

4 4. COHERENCE Figure 6 shows the coherence map of two cross-interferometric pairs. Coherence at near is clearly better than at far range. They show that coherence is affected by the difference of Doppler centroids in terms of range. Coherences of figure 6 (top) were about 0.2 and 0.5 in coastal areas and urban areas, respectively. Coherence of another pair in coastal areas was higher than 0.7 specifically in central parts and better than the previous pair in overall. In this pair, coherence at central parts was higher than near and far range. The reason of low coherence at near range may be caused by a systematic problem or different surface characteristics. A A B B A A C C C C Fig. 6. Coherence maps (left) and coherence profiles (right) of two ERS-ENVISAT cross-interferometric pairs. Fig. 7. Coherence histogram of entire area (left) and profiles at B-B and C-C lines in figure 6 (center and right). To improve coherence, we applied common band filtering in azimuth and range direction. The coherence was greatly improved by azimuth common-band filtering (Figure 7). But improvement by range filtering was not so significant. Figure 7 (center), (right) presents the improvement of the plot of coherence along B-B and C-C lines. Especially the coherence in wetland of figure 7 (right) is about from 0.8 to 1.0 and is extremely well. The plot of ENVISAT-ENVISAT pair with green line is different from one of cross-interferometric pair. The coherence of urban area is higher than that of coastal area relatively. This is related to the random phase screen with urban area in cross-interferometric pair.

5 The coherence histogram on the urban and coastal area is shown in figure 8. The coherence at coastal areas is much lower than that of urban areas in ENVISAT-ENVISAT and ERS-ERS pairs. As previous researches pointed out coherence at coastal areas in ERS-ENVISAT cross-interferometric pair is higher than in urban area. The coherence was about from 0.6 to 0.8 in coastal areas. The coherence of urban area was about 0.4, it is related to the random phase screen on the urbanized area. But the New Orleans city has less man-made buildings or structures than other major cities. So observed coherence in urban area was not that bad. Magnified cross-interferogram around coastal areas and the unwrapped profiles along two lines is shown in figure 9. Fig. 8. Coherence histogram of coastal area and urban area with ENVISAT-ENVISAT pair (left), ERS-ERS pair (center) and ERS-ENVISAT cross-interferometric pair (right). B B A A Fig. 9. Cross-interferogram in coastal areas (left) and unwrapped profiles along A-A and B-B lines (right). 5. CONCLUSION Test of radar cross-interferometry in the coastal area was partially successful. Cross-interferometry may be a solution for coastal change monitoring. The difference of Doppler centroids of cross-interferometric pair varies with range. The difference in ERS-ERS pair gently increased. Slope of the difference, however, was steep in ERS-ENVISAT crossinterferometric pairs. The differences of Doppler centroid were about 450 Hz and 600 Hz in near and far range, respectively. We need to study further to offset it. The coherence of the coastal area is about from 0.6 to 0.8, it is sufficient to construct coastal DEM. Coherence of another frame was higher than 0.8. Accuracy of the DEM constructed by the ERS-ENVISAT 30-minute pair in a coastal area is to be evaluated.

6 Acknowledgment The ERS and ENVISAT SAR data were provided by Rosenstiel School of Marine and Atmospheric Science, at University of Miami and Science Applications International Corporation at USGS/EROS Data Center, and authors deeply thank to their data support. References [1] Adam N., First cross interferogram using the radar sensors ENVISAT/ASAR and ERS-2, online at: [2] Colesanti C., De Zan F., Ferretti A., Prati C., and Rocca F., Generation of DEM with sub-metric vertical accuracy from 30 ERS-ENVISAT pairs, Proc. FRINGE 2003 Workshop, Frascati, Italy, 1-5, December, CD-ROM version, [3] Arrigoni M., Colesanti C., Ferretti A., Perissin D., Prati C., and Rocca F., Identification of the location phase screen of ERS-ENVISAT permanent scatterers, Proc. FRINGE 2003 Workshop, Frascati, Italy, 1-5, December, CD- ROM version, [4] Blanco, P., Mallorqui, J.J., Navarrete, D., Duque, S., Prats, P., Romero, R., Dominguez, J., and Carrasco, D., Application of the coherent pixels technique to the generation of deformation maps with ERS and ENVISAT data, Proc. IGARSS 05, Seoul, Korea, 25-29, July, Vol 3., , [5] Miranda, N., Rosich, B., Santella, C., and Grion, M., Review of the impact of ERS-2 piloting modes on the sar doppler stability, Proc. FRINGE 2003 Workshop, Frascati, Italy, 1-5, December, CD-ROM version, [6] Gatelli, F., Guarnieri A. M., Parizzi F., Pasquali P., Prati C., and Rocca F., The Wavenumber Shift in SAR Interferometry, IEEE Trans. Geosci. Remote Sensing, Vol. 32, , [7] Monti Guarnieri A., Prati C., ERS-ENVISAT Combination for Interferometry and Super-resolution, ERS- ENVISAT Symposium, Gothenburg, Sweden, 16-20, October, 2000.