The oceans not only provide valuable food and biophysical
resources, they also serve as transportation routes, are crucially
important in weather system formation and CO2 storage, and are an
important link in the Earth's hydrological balance. Understanding
ocean dynamics is important for fish stock assessment, ship
routing, predicting global circulation consequences of phenomena
such as El Niño, forecasting and monitoring storms so as to
reduce the impact of disaster on marine navigation, offshore
exploration, and coastal settlements. Studies of ocean dynamics
include wind and wave retrieval (direction, speed, height),
mesoscale feature identification, bathymetry, water temperature,
and ocean productivity.
Ocean circulation patterns can be determined by the examination of mesoscale features such as eddies, and surface gravity waves. This knowledge is used in global climate modelling, pollution monitoring, navigation and forecasting for offshore operations.
Some of the key areas of interest will include the following:
Wide area coverage is useful for monitoring and surveillance applications including ship traffic, fisheries monitoring, oil spill mapping, and ocean circulation mapping. Intermediate area coverage is useful for monitoring ship traffic, near-shore fisheries activities, oil spill mapping, and inter-tidal feature mapping. Small area coverage is useful for harbour traffic monitoring, aquaculture site location and small spill mapping.
For general sea-state information (waves, currents, winds), the data is usually time-sensitive; meaning that the information is only valuable if it is received while the conditions exist. ASAR data is expected to play a key role in the study of wave characteristics.
Certain wind speed conditions are necessary in order for the SAR
to receive signal information from the ocean surface. At very low
wind speeds (2 to 3m/s) the SAR is not sensitive enough to detect
the ocean "clutter" and at very high wind speeds (greater than 14
m/s) the ocean clutter masks whatever surface features may be
present. The principal scattering mechanism for ocean surface
imaging is Bragg scattering, whereby the short waves on the ocean
surface create spatially varying surface patterns. The backscatter
intensity is a function of the incidence angle and radar
wavelength, as well as the sea state conditions at the time of
imaging. The surface waves that lead to Bragg scattering are
roughly equivalent to the wavelength used by ASAR and RADARSAT (5.3
cm). These short waves are generally formed in response to the wind
stress at the upper ocean layer. Modulation in the short (surface)
waves may be caused by long gravity waves, variable wind speed, and
surface currents associated with upper ocean processes such as
eddies, fronts and internal waves. These variations result in
spatially variable surface roughness patterns which are detectable
on SAR imagery
Atmospheric Waves (Copyright 1994, European Space Agency)
The SAR data for this image was taken by the European Space Agency's ERS-1 satellite on August 17, 1994. The scene shows the southern coast of Melville Island's Dundas Peninsula (in the Parry Islands of northern Canada), with north pointing about 30 degrees to the right.
Internal waves form at the interfaces between layers of different water density, which are associated with velocity shears (i.e., where the water above and below the interface is either moving in opposite directions or in the same direction at different speeds). Oscillations can occur if the water is displaced vertically resulting in internal waves. Internal waves in general occur on a variety of scales and are widespread phenomena in the oceans. The most important are those associated with tidal oscillations along continental margins. The internal waves are large enough to be detected by satellite imagery. In the image shown below, the internal waves, are manifested on the ocean surface as a repeating curvilinear pattern of dark and light banding, a few kilometres east of the Strait of Gibraltar, where the Atlantic Ocean and Mediterranean Sea meet. Significant amounts of water move into the Mediterranean from the Atlantic during high tide and/or storm surges.
ASAR scene of internal waves: Strait of Gibraltar (ESA 2002)
One ASAR study being proposed by Dr. Olga Lavrova, a senior scientist at the Space Research Institute Russian Academy of Sciences in Russia, plans to investigate circulation processes in the ocean and atmosphere (transformation of speed field, energy and momentum transfer) for the case of a stratified flow running against natural obstacles. The study will employ theoretical and experimental investigation of the spatial and temporal structure and dynamics of waves, vortexes, and vortex streets that emerge behind small islands, capes, rocks, and underwater rapids in the presence of currents in the ocean, and due to air flows on the shore and islands in the atmosphere. In addition to the study of forms and parameters of these lee structures, their relation to the speed of the run-against flow, the stratification of the media and the morphometry of the obstacle will be considered. This paves the way for the estimation of flow speed and media density stratification from space.
There is increasing interest in the maritime community in high-precision nowcasting of ocean fronts, eddies and current shears. Important application areas could be: piloting of large transport ships, fisheries and fish farming, sea floor operations and autonomous underwater vehicles, acoustic sensors and acoustic communication. Also, ASAR imagery, together with data from other ENVISAT instruments such as MERIS and AATSR, will significantly enhance the nowcasting of ocean features in coastal waters.
Open ocean applications include the study of large-scale ocean features manifested at the ocean surface by the interaction of wind-driven currents with the marine boundary layer. The principle scattering mechanism for ocean surface imaging is Bragg scattering, whereby the short waves create spatially varying surface patterns. The backscatter intensity is a function of the incidence angle and radar/wavelength, as well as the wind and wave condition at the time of imaging. For RADARSAT (5.3 cm wavelength), the surface waves that lead to Bragg scattering are roughly equivalent to its wavelength. These short waves are generally formed in response to the wind stress at the marine boundary layer. Modulation in the short waves may be caused by long gravity waves, variable wind speed, and surface currents associated with upper ocean processes such as eddies, fronts, and internal waves. These variations result in spatially variable surface roughness pattern which is imaged by the SAR.
Coastlines are environmentally sensitive interfaces between the ocean and the land, and respond to changes brought about by economic development and changing land-use patterns. Often coastlines are biologically diverse inter-tidal zones and can also be highly urbanised. With over 60% of the world's population living close to the ocean, the coastal zone is a region subject to increasing stress from human activity. Government agencies concerned with the impact of human activities in this region need new data sources with which to monitor such diverse changes as coastal erosion, loss of natural habitat, urbanisation, effluents and offshore pollution. Many of the dynamics of the open ocean and changes in the coastal region can be mapped and monitored using remote sensing techniques.
Coastal zone monitoring implies observation of the interaction of oceanographic and atmospheric phenomena with human activities in the near-shore region. The key issues include the delineation of the coastline, defining areas of erosion and sedimentation, mapping the inter-tidal vegetation, and identifying areas of human settlement and accompanying activities. The coastal zone is an environmentally sensitive region subject to increasing stress from economic development, and government agencies concerned with the impact of human activities in the near-shore region are looking for new data sources with which to monitor this region.
An excellent coastal zone application of radar is aquaculture site monitoring. These man-made structures provide higher signal returns than the surrounding water.
ASAR data will certainly be used within the range of activities in the coastal zone. Examples of current use of SAR data in the coastal zone include: topographic maps of tidal flats, sea bed topography, sediment distribution in The Netherlands, an inter-tidal digital terrain model of the Wash in the UK, and coastal erosion in French Guiana.
The availability of multi-polarised data and data at different incidence angles, or at a specific incidence angle, should improve the accuracy and quality of products for many applications. The Wide Swath (WS) and Global Monitoring (GM) Modes will provide data that is not currently available, for applications requiring large area coverage.
For example, a new conceptual scheme in coastal research being proposed by Dr. Francis Gohin, a Physical Oceanographer at IFREMER in France, is to deploy optical instruments, combined with airborne and spaceborne spectral and SAR imagers like ASAR, to provide an up-to-date means of observing the narrow bands of red tides. By integrating colour data obtained from aircraft and satellites in classical data sets, a 3-D numerical model will provide estimation of the chlorophyll content and the suspended matter concentration on the continental shelf of the Bay of Biscay. Remote sensing methods can be used in the validation of such models. In return, these models help to include passive remote sensing data, poorly sampled in time because of clouds, in a regular set of simulations.
In addition, a study being proposed by researcher Samuray Elitas M.Sc. of the TUBITAK Marmara Research centre in Turkey, envisions ASAR data being used to analyse coastal regions there. As ASAR and multicolour MERIS images for the project area arrive, the most recent and/or simultaneous pollution mapping of the Marmara Sea will be evaluated by relating other geographical information data like bathymetrical information and land usage information. Consequently, geographical information systems for the Marmara marine environment, supported by ASAR and MERIS images will be established as a whole database.
The effects of bathymetry are visible in near-shore regions under light wind conditions. Small incidence angles are better suited to imaging inter-tidal features such as mudflats, shoals and sandbars.
Oil spills can destroy marine life as well as damage habitat for land animals and humans. The majority of marine oil spills result from ships emptying their billage tanks before or after entering port. Large area oil spills result from tanker ruptures or collisions with reefs, rocky shoals, or other ships. These spills are usually spectacular in the extent of their environmental damage and generate wide spread media coverage. Routine surveillance of shipping routes and coastal areas is necessary to enforce maritime pollution laws and identify offenders.
Oil slicks and natural surfactants are imaged through the localised suppression of Bragg scale waves. Under calm conditions, natural surfactants may form over large areas of the ocean, along current boundaries, and in areas of upwelling. The accumulation of natural surfactants at these boundaries can delineate the general circulation pattern and are visible on the radar image as curvilinear features with a darker tone than the surrounding ocean. Oil spills also have a darker tone with respect to the surrounding ocean background. The detection of an oil spill is strongly dependent upon the wind speed. At wind speeds greater than 10 m/s, the slick will be broken up and dispersed, making it difficult to detect. Another factor that can play a role in the successful detection of an oil spill is the difficulty in distinguishing between a natural surfactant and an oil spill. Multi-temporal data and ancillary information can help to discriminate between the two phenomena. Wind shadows near land, regions of low wind speed, and grease ice can also be mistaken for oil spills and ancillary data (or an experienced user) is necessary to distinguish between these features and a spill.
Small incidence angles are optimum for oil spill detection. Detection will also depend on the spill size, sea state conditions and image resolution.
At the Norwegian Computing Centre, senior research scientist Anne Solberg proposes to modify algorithms for automatic detection of oil spills originally developed for ERS SAR images, for use with ASAR images. Methods have been developed for automatic detection of oil spills in ERS images as part of the Norwegian oil spill project and the European Union project ENVISYS. ASAR Wide Swath data will have a different pixel size and a different radiometric resolution than the ERS SAR images, and these will be incorporated in the detection and classification algorithms. The new project will use ASAR data from four test sites with a high probability of observing oil slicks: the North Sea, the English Channel, and two sites in the Mediterranean.
With larger incidence angles, the ocean background clutter effects are reduced, improving the detection of ships, coastline and ice edges. For example, a ship is a bright point target against the ocean background clutter and can be detected using image thresholding techniques. However, as the ocean clutter increases with increasing wind speeds, ship detection becomes more difficult. At wind speeds greater than 10 m/s it is difficult to detect small fishing vessels. This relationship with wind speed is a critical factor for ship detection as well as oil spill mapping and feature detection. As the wind speeds increase, the radar cross-section of the ocean increases, reducing the contract between the feature of interest and the surrounding ocean.
Ship detection is a good example of the operational role of radar. A wide range of ship sizes may be detected under a variety of sea-state conditions. Radar can infer ship size, and if a wake is present, its speed and direction of travel. It should be noted that an HH polarisation is less sensitive to wake detection and, in studies to date, wakes are infrequently detected. Potential users of this information include agencies who monitor ship traffic, authorities responsible for sovereignty and fisheries surveillance, as well as customs and excise agencies charged with stopping illegal smuggling activities.
Ship Wake. ESA image courtesy of the Alaska SAR Facility (copyright ESA)
Large incidence angles are optimum for ship target detection. Detection depends on ship size and type, heading with respect to look angles, and sea state conditions at the time of imaging.