John Wolfe

C-Band Cross-Polarization Wind Speed Retrieval

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.

Wind direction estimation from SAR images of the ocean using wavelet analysis

We present a method for the automatic estimation of wind directions from synthetic aperture radar (SAR) images of the ocean. The method is based on a wavelet analysis and assumes that the wind direction aligns with boundary-layer atmospheric roll vortices, which often appear as streaks at kilometre scales in SAR images of the ocean, and measures the orientation of the streaks. Unlike estimation methods that use the discrete Fourier transform (DFT), the streaks in SAR images are described quantitatively as a natural output of this method. Furthermore, more optimal wind directions are obtained by comparing the directional orientation of the streaks at different spatial scales. Sub-scenes in which the streaks are too weak to determine wind direction do not return a direction, as governed by a user-selected threshold. Wind directions for these sub-scenes are based on those in neighbouring sub-scenes by using an adaptive smoothing technique. Quality control involves tuning the threshold level. We apply the method to two examples of RADARSAT-1 SAR images. The results are compared with those of a DFT-based wind direction analysis, and it is shown that a robust wind direction field is obtained. Mesoscale wind structures can be described by using a finer computing grid. The estimated wind directions still include a 180° direction ambiguity.

Requirement on Antenna Cross-Polarization Isolation for the Operational Use of C-Band SAR Constellations in Maritime Surveillance

The issue of antenna cross-polarization isolation has been previously discussed for the design of fully polarimetric synthetic aperture radars (SARs). Dual-polarized antennas with cross-polarization isolation that is better than -30 dB are desirable for more convenient polarimetric data calibration since measurements of antenna crosstalk (magnitude and phase) variations with incidence angle are not required. For an antenna with significant polarization crosstalk, it is still possible to retrieve pure polarization measurements of HH, HV, VH, and VV provided that the four corresponding received voltages are measured. However, it is not possible to recover from cross-polarization contamination for single- or dual-polarization measurements. Therefore, it is important to set up a minimum requirement on dual-polarized antenna isolation so that single- and dual-polarization applications are not unduly affected. In this letter, the minimum requirement on cross-polarization antenna isolation is investigated for operational use of C-band SARs in maritime surveillance applications. Calibrated polarimetric RADARSAT-2 data are used to simulate single- and dual-polarization data with cross-polarization contamination for a dual-polarized antenna with cross-polarization isolation ranging from -20 to -35 dB. It is shown that the cross-polarization HV (or VH) channel can be significantly affected, particularly at steep incidence angles. As a result, key applications that require the use of pure HV, such as ship detection and wind-speed measurements, are significantly affected. A requirement for a minimum of -30-dB antenna isolation is established. Antennas with cross-polarization isolation better than -35 dB are desirable for reliable exploitation of HV data at steep incidence angles.

Validation of RADARSAT-1 vessel signatures with AISLive data

Automatic identification system (AIS) data have the potential to contribute significantly to the development of automated algorithms for ship signature identification in remotely sensed imagery. Datasets composed of RADARSAT-1 imagery collected over a 3-month period and the corresponding AIS data from AISLive were compiled. SAR-derived ship length and radar cross section features were validated against AIS data, thus demonstrating the utility of AIS as a source of ground truth.

Analysis of Sentinel-1 marine applications potential

Ship detectability modelling has been carried out for Sentinel-1, including development of a ship detectability tool that applies to Sentinel-1, RADARSAT-1, RADARSAT-2, and Envisat ASAR image data. Capabilities and limitations of the tools predictions are discussed. Also considered is Sentinel-1s expected capability for other maritime applications, including iceberg detection, wind retrieval, oil spill detection, sea ice surveillance, and ship wake detection. In general, the Interferometric Wide Swath mode with its high spatial resolution and 250 km swath will be a useful mode for surveillance of the littoral zone.

Toward Automated Identification of Sea Surface Temperature Front Signatures in Radarsat-2 Images

AbstractThe Canadian Forces Meteorology and Oceanography Center produces a near-daily ocean feature analysis, based on sea surface temperature (SST) images collected by spaceborne radiometers, to keep the fleet informed of the location of tactically important ocean features. Ubiquitous cloud cover hampers these data. In this paper, a methodology for the identification of SST front signatures in cloud-independent synthetic aperture radar (SAR) images is described. Accurate identification of ocean features in SAR images, although attainable to an experienced analyst, is a difficult process to automate. As a first attempt, the authors aimed to discriminate between signatures of SST fronts and those caused by all other processes. Candidate SST front signatures were identified in Radarsat-2 images using a Canny edge detector. A feature vector of textural and contextual measures was constructed for each candidate edge, and edges were validated by comparison with coincident SST images. Each candidate was classif...

Comparison of C-band wind retrieval model functions with airborne multipolarization SAR data

C-band ocean wind retrieval model functions driven by buoy or scatterometer wind vectors are favourably compared with well-calibrated multipolarization data acquired by an airborne synthetic aperture radar (SAR). It is apparent that currently available hybrid C-band models will provide a good starting point for RADARSAT-2 SAR data utilization for marine applications and a basis for improved C-band ocean wind retrieval model functions.

Automated Discrimination of Certain Brightness Fronts in RADARSAT-2 Images of the Ocean Surface

AbstractAutomated classification of the signatures of atmospheric and oceanic processes in synthetic aperture radar (SAR) images of the ocean surface has been a difficult problem, partly because different processes can produce signatures that are very similar in appearance. For example, brightness fronts that are the signatures of horizontal wind shear caused by atmospheric processes that occur independently of properties of the ocean (WIN herein) often appear very similar to brightness fronts that are signatures of sea surface temperature (SST) fronts (SST herein). Using Moderate Resolution Imaging Spectroradiometer (MODIS)-derived SST for validation, 302 SAR SST and 193 SAR WIN signatures were collected from over 250 RADARSAT-2 images of the Gulf Stream region using a Canny edge detector. A vector consisting of textural and contextual features was extracted from each signature and used to train and test logistic regression, maximum likelihood, and binary tree classifiers. Following methods proven effect...

Oil detection in RADARSAT-2 quad-polarization imagery: implications for ScanSAR performance

Environment Canadas Integrated Satellite Tracking of Pollution (ISTOP) program uses RADARSAT-2 data to vector pollution surveillance assets to areas where oil discharges/spills are suspected in support of enforcement and/or cleanup efforts. RADARSAT-2s new imaging capabilities and ground system promises significant improvements in ISTOPs ability to detect and report on oil pollution. Of specific interest is the potential of dual polarization ScanSAR data acquired with VV polarization to improve the detection of oil pollution compared to data acquired with HH polarization, and with VH polarization to concurrently detect ship targets. A series of 101 RADARSAT-2 fine quad images were acquired over Coal Oil Point, near Santa Barbara, California where a seep field naturally releases hydrocarbons. The oil and gas releases in this region are visible on the sea surface and have been well documented allowing for the remote sensing of a constant source of oil at a fixed location. Although the make-up of the oil seep field could be different from that of oil spills, it provides a representative target that can be routinely imaged under a variety of wind conditions. Results derived from the fine quad imagery with a lower noise floor were adjusted to mimic the noise floor limitations of ScanSAR. In this study it was found that VV performed better than HH for oil detection, especially at higher incidence angles.

Ship signatures in synthetic aperture radar imagery

Ship signatures in synthetic aperture radar (SAR) imagery have been matched to Automatic Identification System (AIS) data to yield a large database of known ships for ship signature analysis. This paper focuses on ship radar cross section and ship length derived from the ship signature length. Cross- polarization is an attractive option for ship detection.