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ALOS PALSAR Interferometric Synthetic Aperture Radar (InSAR)

Zhong Lu, U.S. Geological Survey (USGS), Vancouver, WA 98683, phone: (360) 993-8911, email: lu@usgs.gov

The Japan Aerospace Exploration Agency’s (JAXA) Advanced Land Observing Satellite (ALOS) was successfully launched on January 24, 2006. ALOS, a follow-on mission for the Japanese Earth Resources Satellite-1 (JERS-1), carries three sensors: 1) the Panchromatic Remote-Sensing Instrument for Stereo Mapping (PRISM) for digital elevation mapping, 2) the Advanced Visible and Near Infrared Radiometer type 2 (AVNIR-2) for land cover characterization, and 3) the Phased Array type L-band Synthetic Aperture Radar (PALSAR) for day-and-night and all-weather observation. ALOS orbits at an altitude of 691.65 km (at the Equator) with a 46-day repeat cycle. The PALSAR performs in all aspects better than the JERS-1 SAR (Shimada, et al., 2007). PALSAR can operate at four primary modes with diverse polarizations and offnadir angles: (a) highresolution single-polarization (FBS) mode, (b) high-resolution, dual-polarization (FBD) mode, (c) fully-polarimetric (PLR) mode, and (d) ScanSAR mode (Table 1). The center frequency of PALSAR is 1270 MHz, resulting in a wavelength of 23.62 cm. Because of the distinct difference in radar wavelength (23.62 cm versus 23.53 cm) and imaging geometry, it is generally not feasible to combine ALOS and JERS-1 SAR images to generate a crossplatform interferogram.

The critical baseline for a PALSAR interferogram, in the default FBS mode, can reach 13 km over flat areas (Figure 1). This is due to the larger chirp bandwidth and longer wavelength of L-band, compared to C-band, sensors such as the European Space Agency’s (ESA) European Remote Sensing-1/-2 and Envisat, and the Canadian Space Agency’s (CSA) Radarsat -1 (standard beam modes). In addition, PALSAR is yaw-steered, and Doppler centroids of PALSAR images fall within a few hundred hertz of zero Doppler. The orbital tube is controlled within one km or less; therefore, PALSAR images from the same imaging geometry can be combined to produce coherent interferograms. This includes the combination of PALSAR images at different spatial resolutions, between FBS and FBD modes, for interferometric applications. The larger critical baseline for a PALSAR interferogram causes it to be very sensitive to topographic relief. Accordingly, a high accuracy Digital Elevation Model (DEM) is needed to remove the topographic contribution in the original interferogram in order to generate a deformation map. For a PALSAR interferogram with a large baseline over high topographic relief areas, terraininduced, localized range offsets need to be considered in order to precisely register two PALSAR images so as to preserve coherence. In essence, the polynomials that measure the offset fields between the PALSAR images should be a function of range, azimuth, and elevation. Finally, burst synchronization between repeat-pass PALSAR ScanSAR images has not been optimized, so ScanSAR interferometry with ALOS PALSAR data is not yet feasible. More information on PALSAR interferometry can be obtained from a technical note by Sandwell and Wei (2006).

The next few years will witness more exciting technical and scientific breakthroughs in many aspects of utilization of ALOS L-band PALSAR imagery. First, L-band PALSAR will enable InSAR deformation mapping at global scales where C-band InSAR can be plagued by loss of coherent signal due to vegetation. Second, fully-polarized ALOS PALSAR will allow better characterization of wetland and vegetation structure and ground features. Third, the combination of polarimetric and interferometric analysis (called Pol-InSAR) will offer a new capability for landscape mapping and monitoring. ALOS Pol-InSAR will enable optimization procedures that maximize the interferometric coherence and target decomposition approaches to the separation of radar backscattering returns from the canopy top, from the bulk volume of the vegetation, and from the ground surface. The difference in interferometric phase measurement then leads to the difference in height between the physical scatterers that possess these mechanisms. Accordingly, ALOS Pol-InSAR imagery will enable significant advances in many fields of application including: a) land cover mapping and wetland mapping, (Figure 2) particularly over regions where weather conditions hinder optical remote sensing, b) inferring soil moisture with a horizontal resolution (several meters) that is not attainable otherwise, and c) mapping forest height and biomass with the generation of “bare-earth” DEMs, and much more. Along with other SAR and optical imagery, ALOS PALSAR will address and provide solutions to many scientific questions related to natural hazard monitoring and natural resource management.

Acknowledgments

ALOS, Envisat, and Radarsat adarsat-1 SAR images are copyrighted JAXA, ESA, and CSA, respectively, and were provided by the Alaska Satellite Facility (ASF) and ESA. This work was supported by funding from the NASA Earth Surface and Interior Program, the USGS Volcano Hazards Program, and the USGS Land Remote Sensing Program. Technical reviews by O. Kwoun and C. Wicks are greatly appreciated.

References

Lu, Z. and O. Kwoun, Radarsat-1 and ERS Interferometric Analysis Over Southeastern Coastal Louisiana: Implication for Mapping Water-Level Changes Beneath Swamp Forests, IEEE Transactions on Geoscience and Remote Sensing, submitted, 2007.

Sandwell, D. and M. Wei, ALOS Interferometry, http://topex.ucsd.edu/alos/, 2006.

Shimada, M., and others, PALSAR CALVAL Summary and Update 2007, IEEE International Geoscience and Remote Sensing Symposium, Barcelona, Spain, 2007.

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