Sang-Hoon Hong

Evaluation of TerraSAR-X Observations for Wetland InSAR Application

This paper assesses the potential of using spaceborne X-band synthetic aperture radar (SAR) data for monitoring water-level changes over wetlands. Our analysis is based on three sets of TerraSAR-X (TSX) observations acquired over South Floridas Everglades wetlands during an eight-month period in 2008. The first set was acquired in single HH polarization stripmap mode over our northern study area, consisting of managed wetlands and urban environments. The second set was acquired in dual-polarization stripmap mode over the western half of the same area, consisting mostly of managed wetlands. The third set was also acquired with dual-polarization stripmap mode over our southern study area, consisting of natural flow freshand salt-water wetlands in the southern Everglades. The first data set was used for a proof-of-concept study to verify that X-band data can generate coherent interferograms in wetland areas. Interferometric processing of this data set shows a high level of coherence (> 0.35) over both wetland and urban regions, maintaining interferometric phase in all three interferograms spanning 11 days. Surprisingly, phase is maintained over some of the wetlands even for interferograms spanning 33 days. The other two data sets were used to evaluate interferometric coherence of all four polarization modes and to determine dominant scattering mechanism in each wetland environment. Our results show high coherence values (> 0.4) in all polarization modes, with highest values in HH, then VV, and lowest in HV or VH. Interferograms calculated from multipolarization data show very similar fringe patterns regardless of the polarization type, suggesting that the phase information in all polarization data reflects water-level changes in wetlands and that volume scattering may be less important than commonly believed. We also used the two multipolarization data sets to conduct the Pauli decomposition, finding a strong dependence of scattering mechanism on vegetation type. The high interferometric coherence level of all polarization data suggests that a significant part of the X-band scattered signal interacts with lower sections of the vegetation (trunks and branches), because scattering from wind-affected canopies cannot support such a high coherence level. The high spatial resolution of TSX, combined with its 11-day repeat orbit, makes this X-band sensor surprisingly suitable for wetland interferometric SAR applications.

Polarimetric Features of Oyster Farm Observed by AIRSAR and JERS-1

The polarimetric features of an oyster farm in a coastal area are analyzed to verify the applicability of radar polarimetry and interferometry. L-band Airborne Synthetic Aperture Radar (AIRSAR) data and Japan Earth Resources Satellite (JERS-1) data are used to examine the unique structure of an oyster farm located in South Korea. A specific feature of the oyster farm is the presence of numerous arrays of structures of various orientations that consist of exercise-bar-shaped poles protruding above sea level. This paper demonstrates that tide level is strongly correlated with the double-bounce scattering power from the vertical pole structures. This phenomenon is also verified by laboratory measurements using a network analyzer. In the laboratory experiment, double-bounce scattering and total power showed increasing trends with increased height of the vertical poles. Single-bounce scattering is sensitive to the orientation of horizontal poles relative to antenna orientation. HH-polarization is the most effective technique for imaging oyster farms from L-band polarimetric AIRSAR data. The authors were able to use a three-component decomposition of the AIRSAR data to distinguish an exposed tidal flat from a submerged tidal flat. The characteristics of the exposed tidal flat are similar to those of the carbon sponge in the laboratory test, except that the double-bounce scattering power is slightly greater in the real-world example. The single-bounce scattering component in AIRSAR data is generally greater than that in laboratory measurements because of sea-surface conditions and oyster growth. When the horizontal pole was aligned normal to the radar look direction, single-bounce scattering was greater than the double-bounce scattering, even under water-covered conditions. While a difference in tide height of 10 cm contributed approximately 3.0 dB in the laboratory experiment, a difference in tide height of 20 cm contributed to only approximately 1.7 dB in the JERS-1 SAR image intensity. JERS-1 SAR image intensity for areas dominated by double- and single-bounce scattering was 0.78 and 0.56, respectively. Results confirm that polarimetric SAR data are useful in selecting areas dominated by double-bounce scattering in oyster farms

An application of L-band synthetic aperture radar to tide height measurement

A method for measuring the tide height near the coast from L-band synthetic aperture radar (SAR) data is presented. Twenty-one coherent interferograms have been successfully constructed from Japanese Earth Resources Satellite 1 (JERS-1) SAR data obtained over oyster sea-farming structures. A coherence analysis of the 21 interferometric pairs showed that a perpendicular baseline of less than 3 km, with a temporal baseline within 500 days, are required to obtain a coherent pair, with a coherence higher than 0.25, in the study area. The coherent phases preserved in the interferograms showed a close relation with the sea level. The problem of phase unwrapping to restore an absolute tide height was overcome by introducing normalized image intensities. The radar measurements estimated by the proposed method were verified using tide gauge data, and comparison of the two datasets yielded a correlation coefficient R/sup 2/ of 0.91, with a root mean square error of 5.76 cm. The results demonstrate that radar interferometry can be applied for a tide height measurement near the coast given sufficient structures that return off-nadir radar pulses to the antenna. The multipolarized L-band SAR system will provide better results, using only double-bounced signals, in the future.

Evaluation of Polarimetric SAR Decomposition for Classifying Wetland Vegetation Types

The Florida Everglades is the largest subtropical wetland system in the United States and, as with subtropical and tropical wetlands elsewhere, has been threatened by severe environmental stresses. It is very important to monitor such wetlands to inform management on the status of these fragile ecosystems. This study aims to examine the applicability of TerraSAR-X quadruple polarimetric (quad-pol) synthetic aperture radar (PolSAR) data for classifying wetland vegetation in the Everglades. We processed quad-pol data using the Hong & Wdowinski four-component decomposition, which accounts for double bounce scattering in the cross-polarization signal. The calculated decomposition images consist of four scattering mechanisms (single, co- and cross-pol double, and volume scattering). We applied an object-oriented image analysis approach to classify vegetation types with the decomposition results. We also used a high-resolution multispectral optical RapidEye image to compare statistics and classification results with Synthetic Aperture Radar (SAR) observations. The calculated classification accuracy was higher than 85%, suggesting that the TerraSAR-X quad-pol SAR signal had a high potential for distinguishing different vegetation types. Scattering components from SAR acquisition were particularly advantageous for classifying mangroves along tidal channels. We conclude that the typical scattering behaviors from model-based decomposition are useful for discriminating among different wetland vegetation types.

Extraction of ground control points (GCPs) from synthetic aperture radar images and SRTM DEM

Qualified ground control points (GCPs) are crucial in the geolocation of a remotely sensed image. If a region has no published map, the geographical coordinates of GCPs must be obtained indirectly. Although these can be re‐constructed from a synthetic aperture radar (SAR) image using sensor position and velocity, this conventional method does not provide accurate GCPs owing to the variable elevations of actual topography. We propose an inverse geolocation method for GCPs by SAR simulation. This improves the accuracy of extracted GCPs by accommodating topographic effects, and requires a high‐resolution digital elevation model (DEM) and SAR with precise orbit data. The errors in the northing derive mainly from the imaging pulse time, and were improved by applying an azimuth time correction. In this study ERS SAR data with precise Delft orbit and shuttle radar topography mission (SRTM) DEM were used. Mean values for the planimetric distance error in the GCPs were 4.1 and 5.4 m with standard deviations of 10.6 and 16.9 m in northing and easting, respectively. The obtained GCPs were applied to an IRS image for geo‐rectification, and the result was mean image positional errors of 3.6 and 2.7 m with standard deviations of 8.4 and 15.4 m in northing and easting, respectively. The root mean square errors are 9.0 and 15.3 m in northing and easting, respectively.

Multitemporal Multitrack Monitoring of Wetland Water Levels in the Florida Everglades Using ALOS PALSAR Data With Interferometric Processing

We present an improved wetland interferometric synthetic aperture radar (InSAR) technique that uses multitrack SAR data and ground-based stage (water level) data to calculate a time series of high spatial resolution water level maps throughout wide wetland areas. The technique was applied to a wetland area in the northern Everglades, Florida, using a four-year-long Advanced Land Observing Satellite (ALOS) Phased Array L-band Synthetic Aperture Rada (PALSAR) data set acquired during 2007-2011. Although the temporal resolution of ALOS PALSAR interferograms is low (multiples of the satellites 46-day revisit cycle), the multitrack algorithm combines results from the four tracks and significantly improves the observation frequency up to seven days in the best case. A quality control analysis indicates that the average root-mean-square error of the differences between the InSAR- and stage-based water levels is 4.2 cm. The end products of absolute water level time series with improved temporal and high spatial resolutions can be used as excellent constraints for high spatial resolution wetland flow models and other water resource applications.

Evaluation of the quad-polarimetric Radarsat-2 observations for the wetland InSAR application

Wetland interferometric synthetic aperture radar (InSAR) has been successfully applied to observe phase variations related to water level changes in aquatic environments with emergent vegetation. In this study, we evaluate the quadrant polarimetric Radarsat-2 (C-band) observations for the wetland InSAR applications by testing two datasets acquired over south Floridas Everglades wetlands. The first set was acquired over our northern study area, consisting of mostly managed wetlands and agricultural environments. The second set was acquired over our southern study area, consisting of naturally flowing fresh- and salt-water wetlands in the southern Everglades. In both sets, observations were acquired every repeat orbit (24 days) to generate interferograms with short temporal baselines that can maintain high coherence levels. Our results showed high coherence values in all polarization modes (from 0.26 to 0.42), with highest values in HH, then VV, and lowest in HV or VH. Surprisingly, all the quadrant polarimetric interferograms showed very similar fringe patterns regardless of the polarization type, suggesting that water level changes can be detected in all polarizations. Furthermore, the observations implied that double bounce is the dominant scattering mechanism, even in cross polarization (HV and VH), and not volume scattering as commonly assumed.

Polarimetric Decompositions of Temperate Wetlands at C-Band

C-band SAR is well established as a useful sensor for water resources applications. It is commonly accepted that the backscatter from wetlands that consist of many emergent stems over open water (swamps and marshes) is dominated by a double-bounce scattering mechanism. However, recent observations with fully polarimetric data from Radarsat-2 over the extensive wetlands of the Everglades and numerous small wetlands in Ontario appear to be inconsistent with this interpretation of the backscatter physics. In this paper, we use several forms of polarimetric analysis and decomposition. All of these indicate that the backscatter from small marshes and swamps in Ontario is dominated by polarimetric characteristics normally attributed to the odd-bounce mechanism. This anomalous result might be explained as a consequence of changes in the double-bounce reflectance properties of vegetation as a function of the incidence angle. However, detailed electromagnetic backscatter modeling will be needed to provide a more complete and reliable understanding of the details of backscattering from wetlands with emergent vegetation. Additional observational and theoretical work will be required to document and understand the unusual results we report here. If these results are substantiated, the SAR community must re-interpret the generally accepted meanings of the popular decomposition variables, and introduce new terminology to describe them. This would lead to an improved understanding of the backscatter physics and better use of polarimetric SAR for wetland management applications.

Extraction of ground control points (GCPs) from synthetic aperture radar image using DEM

Synthetic aperture radar (SAR) data have serious geometric distortions caused by inherent side looking geometry and are usually corrupted by speckle noises so that it is difficult to identify ground control points (GCPs). The location of the pixel in a given SAR image can be derived from the knowledge of sensor position and velocity. We propose a GCP extraction algorithm that has an improved capability. The algorithm is based on the geocoding process of the radar image and requires precise orbit information. Whereas conventional geolocation algorithms depend on the knowledge of the target height relative to an assumed Earth ellipsoid model, more precise positioning can be achieved if utilizing an available digital elevation model (e.g. USGS GTOPO-30 DEM, SRTM DEM, etc.). The ERS SAR data with precise Delft orbit information and the national digital elevation model (DEM) were used. We analyze the accuracy of the results from our algorithm by using digital map. In case of using SRTM DEM, the mean values of the planimetric distance error are 9.1 m and 14.7 m with standard deviations of 6.4 m and 9.3 m in northing and easting direction, respectively. The errors in northing direction depend mainly on the imaging pulse time, and those in easting direction are related to the accuracy of used digital elevation model. The GCPs extracted were used for georectification of optical images as well as SAR data.

Evaluation of RADARSAT-2 Acquisition Modes for Wetland Monitoring Applications

Abstract. Interferometric synthetic aperture radar (InSAR) techniques can monitor water-level changes in wetlands with suitable coherence. RADARSAT-2 has many beam modes with varying system parameters to satisfy a wide range of applications. During data acquisition the SAR signals are digitized using eight-bit analog-to-digital converters followed by block adaptive quantization (BAQ) coding. Most RADARSAT-2 beam modes use three-bit BAQ but some modes use two-bit BAQ to accommodate larger data sets, including the wide multilook fine and wide ultrafine modes. These modes are attractive for surface monitoring applications due to good resolution over a wide swath. The two-bit BAQ can have signal saturation due to the smaller dynamic range and an increased phase noise. The Everglades National Park (ENP) has been used for numerous InSAR investigations of water level monitoring. This study describes the results of an evaluation of RADARSAT-2 products from different modes for the monitoring of water-level changes and flooded vegetation in ENP. The objective was to evaluate products from a variety of beam modes for wetland monitoring applications and subsequent unwrapping of the interferograms for water level estimation. The results show that wide-swath high-resolution modes are suitable for InSAR applications due to adequate coherence and high backscatter intensity. Résumé. Les techniques InSAR peuvent suivre l’évolution du niveau de l’eau dans les zones humides avec une cohérence convenable. RADARSAT-2 a de nombreux modes de faisceaux avec différents paramètres du système pour répondre à une vaste gamme d’applications. Lors de l’acquisition des données, les signaux SAR sont numérisés à l’aide de convertisseurs analogiques/numériques de 8 bits suivis par un codage de la quantification adaptive de blocs (BAQ). La plupart des modes de faisceaux de RADARSAT-2 utilisent une BAQ de 3 bits, alors que certains modes utilisent une BAQ de 2 bits pour l’acquisition de plus grands ensembles de données, y compris les modes de faisceaux larges multivisées à résolution fine et de faisceaux larges à résolution ultra-fine. Ces modes sont attrayants pour les applications de surveillance de surface grâce à une bonne résolution sur une large fauchée. La BAQ à 2 bits peut montrer une saturation du signal en raison de sa gamme dynamique plus petite et d’un bruit de phase augmenté. Le Parc national des Everglades (ENP) a été utilisé pour de nombreuses études InSAR de la surveillance du niveau de l’eau. Cette étude décrit les résultats d’une évaluation des produits de RADARSAT-2 à partir de différents modes pour le suivi des changements du niveau de l’eau et de la végétation inondée dans l’ENP. L’objectif était d’évaluer les résultats à partir d’une variété de modes de faisceaux pour les applications de surveillance de zones humides et le déroulement ultérieur des interférogrammes pour l’estimation du niveau de l’eau. Les résultats montrent que les modes de faisceaux larges à haute résolution sont adaptés aux applications InSAR en raison de la cohérence adéquate et de la forte intensité de rétrodiffusion.