Satellite sensors provide a wide area surveillance capability and radar, operating between 1 GHz and 10 GHz, is not constrained by cloud cover and is not affected by a lack of sunlight. Satellite borne Synthetic Aperture Radar (SAR) is capable of high resolution. For example, images in the ScanSAR Narrow mode of RADARSAT-1 and -2 have a swath width of about 300 km and a resolution cell size of about 75 m.
The wind speed and direction relative to the SAR affect the ocean surface radar backscatter cross section; models, such as the CMOD family, exist to describe the functionality and are included in the Space-Eye wind retrieval application. If the wind direction is given, these models can usually be inverted uniquely to yield a wind speed from a radiometrically calibrated SAR image. The wind direction can be obtained by detecting quasi-periodic structures in the radar image; these streaks are often associated with boundary layer rolls. There is a fundamental 180 degree ambiguity in the wind direction that must be resolved either visually from shadowing or from some other source, such as ASCAT. Therefore an external direction must be entered and this is used to determine the sense of the wind vectors after extraction: see the retrieved wind field product depicted below. For ScanSAR narrow or Wide images, there are typically many wind direction extractions.
A land mask in ESRI polygon shape file format is often required. This can be supplied for limited areas.
In addition, ocean waves modify the radar backscatter coefficient and information about the direction of the sea and the significant wave height can be extracted.
A disadvantage of satellite borne sensors, including RADARSAT, Envisat, etc., is often the lack of persistent surveillance because, with each satellite, a small area of the earth's surface at low and mid-latitudes can only be imaged once every few days. However, the high resolution of SAR can be more important for some naval and commercial applications.
Space-Eye is a software application to retrieve or extract a wind field product (the vectors representing direction and speed) from a SAR image. Accepted CMOD models (the default is CMOD4) are employed. Currently the satellites RADARSAT-1, RADARSAT-2, Envisat, PALSAR and Sentinel-1 are supported by the Space-Eye application; the code for TerraSARX is under development.
The wind speed is found with a resolution of better than 1 km, which is higher than is typical of satellite borne scatterometers.
The Space-Eye wind extraction application is designed to function using commonly available commercial imagery with the emphasis on wide swaths. For example, RADARSAT imagery can be obtained from MDA Geospatial Services or produced by the Scanex processor. Space-Eye is currently in operational use. The data can be saved as a ".csv" file that can be loaded into other programs such as Excel and ArcGIS.
The cost of the Space-Eye wind retrieval application starts at $3,000 USD and depends on the type of license and the number of satellites and seats.
The extracted SAR wind field product shown below is from RADARSAT-2 (Los Angeles). Notice that the wind directions vary over the plot and are consistent with shadowing by the islands. Another example of a Black Sea wind retrieval product using a Scanex processed RADARSAT-1 image is shown in SAR Wind-Field Product of the Black Sea. An example of an Envisat wind retrieval is shown in SAR Wind-Field Product of Hawaii from Envisat (Data provided by the European Space Agency).
Space-Eye wind retrieval product derived from RADARSAT-2 image off Los Angeles.
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