The present article is an introductory paper in this special issue on synthetic aperture radar (SAR). A short review is presented on the recent trend and development of SAR and related techniques with selected topics, including the fields of applications, specifications of airborne and spaceborne SARs, and information contents in and interpretations of amplitude data, interferometric SAR (InSAR) data, and polarimetric SAR (PolSAR) data. The review is by no means extensive, and as such only brief summaries of of each selected topics and key references are provided. For further details, the readers are recommended to read the literature given in the references theirin.

Recent Trend and Advance of Synthetic Aperture Radar with Selected Topics

We present an empirical C-band Cross-Polarization Ocean (C-2PO) model for wind retrievals from synthetic aperture radar (SAR) data collected by the RADARSAT-2 satellite. The C-2PO model relates normalized radar cross section (NRCS) in cross polarization to wind speed at 10-m height. This wind retrieval model has the characteristic that it is independent of wind direction and radar incidence angle but is quite linear with respect to wind speed. To evaluate the accuracy of the proposed model, winds with a resolution on the scale of 1 km were retrieved from a dual-polarization SAR image of Hurricane Earl on 2 September 2010, using the C-2PO model and compared with CMOD5.N, the newest available C-band geophysical model function (GMF), and validated with collocated airborne stepped-frequency microwave radiometer measurements and National Data Buoy Center data. Results suggest that for winds up to 38 m s−1, C-2PO has a bias of −0.89 m s−1 and a root-meansquare error of 3.23 m s−1 compared to CMOD5.N, which has ...

Cross-Polarized Synthetic Aperture Radar: A New Potential Measurement Technique for Hurricanes

Wetlands are an important natural resource that requires monitoring. A key step in environmental monitoring is to map the locations and characteristics of the resource to better enable assessment of change over time. Synthetic Aperture Radar (SAR) systems are helpful in this way for wetland resources because their data can be used to map and monitor changes in surface water extent, saturated soils, flooded vegetation, and changes in wetland vegetation cover. We review a few techniques to demonstrate SAR capabilities for wetland monitoring, including the commonly used method of grey-level thresholding for mapping surface water and highlighting changes in extent, and approaches for polarimetric decompositions to map flooded vegetation and changes from one class of land cover to another. We use the Curvelet-based change detection and the Wishart-Chernoff Distance approaches to show how they substantially improve mapping of flooded vegetation and flagging areas of change, respectively. We recommend that the increasing availability SAR data and the proven ability of these data to map various components of wetlands mean SAR should be considered as a critical component of a wetland monitoring system.

A collection of SAR methodologies for monitoring wetlands.

In 2008, the Canadian Space Agency sponsored the Radarsat Hurricane Applications Project (RHAP), for researching new developments in the application of Radarsat-1 synthetic aperture radar (SAR) data and innovative mapping approaches to better understand the dynamics of tropical cyclone genesis, morphology, and movement. Although tropical cyclones can be detected by many remote sensors, SAR can yield high-resolution (subkilometer) and low-level storm information that cannot be seen below the clouds by other sensors. In addition to the wind field and tropical cyclone eye information, structures associated with atmospheric processes can also be detected by SAR. We have acquired 161 Radarsat-1 SAR images through RHAP between 2001 and 2007. Among these, 73 images show clear tropical cyclone eye structure. In addition, we also acquired 10 images from the European Space Agency's Envisat SAR between 2004 and 2010. Both Atlantic hurricanes and Pacific typhoons are included. In this study, we analyze these 83 (73 R...

https://www.mers.byu.edu/long/papers/BAMS2013_li.pdf

Tropical Cyclone Morphology from Spaceborne Synthetic Aperture Radar

This paper presents a first attempt to analyze C-band RADARSAT-2 measurements of the normalized radar cross sections (NRCS) in quad-polarization acquisition mode (HH, VV, HV, and VH) over the ocean. NRCS in copolarizations and cross-polarizations are found to be different; the latter is independent of radar incidence angles and wind directions, but is quite linear with respect to wind speeds. We also investigate the properties of the polarization ratio, denoted PR, and show that it is dependent on incidence angle and azimuth angle as well as wind speed. It also correlates well with wave steepness and significant wave height. Moreover, the polarization difference shows a linear relationship with wind speed. Two new analytical models are proposed to estimate PR; one is a function of incidence angle only, while the other has additional dependence on wind speed. Comparisons are presented with theoretical and empirical PR models from the literature; the new PR model which includes wind speed dependence is shown to best compare with observed RADARSAT-2 data. An assessment of this PR model is given using different CMOD algorithms and RADARSAT-2 images. Results show that the wind speeds retrieved from this PR model and CMOD5. N are in good agreement with buoy measurements (standard deviation, 1.37 m/s). This joint GMF-PR approach constitutes a promising hybrid model for wind speed retrievals from HH-polarized RADARSAT-2 images.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2010JC006522

Wind speed retrieval from RADARSAT-2 quad-polarization images using a new polarization ratio model

C-band ocean backscatter observations over operational weather buoys using RADARSAT-2 fine quad mode data have resulted in new empirical relationships for the C-band co-polarization ratio and the C-band cross-polarization (cross-pol) ocean backscatter. The cross-pol relationship is independent of incidence angle and wind direction, which simplifies wind speed retrieval from synthetic aperture radar imagery for sufficiently high wind speeds.

P W Vachon

Papers

C-Band Cross-Polarization Wind Speed Retrieval