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SUBJECTS
Science Information By:
Ocean Monitoring Workstation Products
There are currently five products that can be generated from the OMW.
Vessel Detection (location, heading and speed),
Frontal Features (ocean current boundaries, eddies, weather fronts)
Ocean Wave Information (length, direction and height)
Slicks (regions of low radar backscatter)
Winds (speed and direction)
Each of these products produces a data file with information about the variables computed.
Vessel Detection
Detection of targets (ships) is limited by the spatial resolution of the sensor, the background clutter levels and the nature of the speckle noise in the image. Identifying bright point targets and associated linear features (wakes) is complicated by speckle noise, false targets and different wake signatures. Wakes manifest themselves in several forms in SAR images including Kelvin waves (bright), dark turbulent wakes, stern wakes and internal waves generated by the ship's passage.
The OMW employs a two step process for ship and wake detection. The system first carries out a target detection procedure to identify candidate vessels, followed by a procedure for detecting and analyzing wake signatures.
The target analysis employs two algorithms. The first, N sigma, divides the image into smaller areas to minimize large scale backscatter variations. The local mean and standard deviation is calculated to determine a target threshold above which a target is considered real. The second algorithm is a Constant False Alarm Rate approach using a K-distribution to model the image intensity probability function.
The wake analysis is carried out in the Radon transform domain where linear features in the original image appear as peaks or troughs. A method known as the Dempster-Schafer algorithm makes use of belief functions to assist in determining if a peak or trough is related to a wake feature. In cases where the wake is identifiable, the vessel speed and heading is also computed based on the offset of the ship location from the apex of the wake in the along track satellite direction.
Frontal Features
This product shows features related to oceanic and atmospheric fronts as well as eddies that are extracted from the SAR images. The processing uses image morphology to isolate significant features and these are then classified by a comparison to averaged and normalized profiles of pixel values across the detected feature to ideal profiles for each type of feature. Results are expressed in terms of strings of latitude and longitude points that constitute individual features.
Ocean Wave Information
This is a spectrum from a sub-image extracted from the larger SAR image. The spectrum is a measure of the image pixel value variance. As such it is not an ocean wave spectrum, and is not scaled to represent a power spectral density for the ocean surface wave field. In order to do so, a complete model for the radar backscatter from the ocean surface must exist., which describes the modulation in backscatter associated with the ocean surface wave field. This model must be invertible to be solved for the ocean surface wave spectrum. Due to the non-linear imaging mechanisms that contribute to the wave patterns observed in SAR images, such a model does not exist.
Coupling the estimate of the wind, with the estimate of the azimuth cut-off wave number permits a determination of the significant wave height to within approximately 0.5m.
There are no graphical products generated for the wave information derived from the SAR image.
Slicks
This extracts regions of exceptionally low backscatter from the image which can be caused by biological films or oil spills. The output consists of information both as a map and a table of the location, shape and spatial extent of such features.
Winds
The wind product relies on calibrated SAR data to extract radar cross section values and estimate wind speed. Under suitable circumstances, the wind direction may also be estimated use low frequency variations in backscatter over the ocean. The output consists of a graphical display and a table of wind speeds and directions on a grid superimposed on the image.
