Remote Sensing of the Oceans

Remote sensory systems are essential in oceanic studies. Because the oceans are whole systems, satellite-based remote sensors have become the tool of choice (although airborne sensors and shipboard systems are also used). Remote sensors detect temperature changes, currents, sea levels, and topography. Such technologies continue to evolve, providing clearer and more detailed images (and greater volumes of data). In light of global climate change concerns and environmental protection, wide-ranging remote sensors are integral for oceanic research.

Studying the Oceans

Oceans cover about 65.7 percent of Earth, spanning about 334 square kilometers (129 million square miles) and having a volume of 1,370 million cubic km (329 million cubic mi). About 97 percent of Earth’s water is found in oceans, which reach an average depth of about 3.8 km (2.4 mi).

Oceans play a critical role in many natural processes, including generating precipitation and weather patterns, transferring sediment, supporting countless marine ecosystems, and delivering essential chemical elements and compounds (such as carbon and methane).

Scientists have long studied the ocean. In the twenty-first century, as evidence of global climate change continues to surface, scientists are paying increased attention to changes in the extensive series of processes and systems under the ocean surface. In this arena, scientists focus on key indicators of shifts in oceanic systems and processes. These indicators include coastline erosion, temperature changes, changes in ocean currents, and organic materials (such as ice and algae blooms). Researchers also carefully monitor the discharge of human-made compounds, such as oil and waste materials, into the oceans.

In addition to researching isolated geographic areas, scientists are increasingly focusing on the ocean system as a whole. Such pursuits require the application of satellite-based and airborne research technologies, which can compile data and images of broad oceanic regions. Satellite sensors, thermal infrared radiometers, various types of radar, scatterometers, altimeters, and underwater autonomous vehicles all increase scientists’ understanding of the oceans.

Remote Sensors and the Oceans

Remote sensors are systems that collect data and imagery of a given object or phenomenon from a distance. In studying oceans, such sensors are usually placed aboard aircraft or satellites. Remote sensors may detect thermal conditions, wave height, current speeds, water vaporization, and debris movements (including the movements of icebergs and sediment).

A wide range of remote sensor technologies exist, most of which rely on the object emitting some form of detectable energy (such as microwaves or solar radiation). For example, radar emits radio waves at its target, reading the wave feedback. Lidar uses concentrated beams of light (lasers) to assess a target.

Remote sensors usually fall into two general categories: passive and active. Passive sensors target a specific area and seek naturally occurring energy emissions, such as the scattering of sunlight. Most passive sensors, therefore, rely on daylight hours to function. Active sensors emit a form of radiation of their own, targeting a geographic area on the ocean. The sensor reacquires the data when the light, microwave, or other emission forms are absorbed or reflected.

Data and Image Types

Remote sensors may glean information about the spread of pollutants and erosion. Scientists use such technologies to study a key indicator of ocean pollution and erosion: the coastal plume, which appears along many coasts.

In a coastal plume, denser water pushes lighter, unpolluted water upward, along with plankton and other materials. Scientists frequently use thermal radiometers to detect coastal plumes, which use infrared to capture the heat signatures of the dense substances caught in the plume.

Another highly useful tool for scientists studying the oceanic system is a scatterometer. Scatterometers are active radar systems that usually are mounted on a satellite. The device transmits high-frequency microwave pulses at the ocean surface. The pulses that bounce back to the satellite are then measured. The different return waves provide information about the winds at the ocean surface, creating a detailed composite of wave sizes and other aspects of the ocean’s surface. Knowledge of these surface winds, combined with data from other instrumentation (such as the aforementioned thermal radiometers, radar, lidar, and other sensor systems), helps scientists understand the speed at which coastal plumes, algae blooms, and oceanic storm fronts move and provide information on surface water temperatures and wave sizes.

Microwave sensors also have several other beneficial applications to oceanic studies, as every object on the surface emits some type of microwave energy (although that energy is usually at a low level of radiation). Sea ice is one surface object that emits a detectable microwave signature that can be scanned from orbit, even through cloud cover and at any time of day. Passive microwave sensors (introduced in the early 1970s and have since evolved considerably) are found on many satellites. They track sea ice, revealing clues about surface temperatures, wind conditions, and currents. Passive microwave sensors cannot assess the temperature of a piece of ice. Still, they can be used to detect the physical and chemical properties of a block of sea ice, including its crystalline structure.

Synthetic Aperture Radar

Another useful remote sensor in this arena is synthetic aperture radar (SAR). Like other radar systems, SAR emits radio waves at a target and reads the echo to create an image of the target.

However, SAR, usually attached to an aircraft or a satellite, continuously sends such waves at the target as it moves overhead. The constant echoes create a more comprehensive, multidimensional image of the target. SARs are useful in studying coastal plumes, as they generate images of them and track their growth and movements.

SAR systems are useful in gathering comprehensive data about oceanic conditions. Wind velocities and wave height are essential for scientists seeking to monitor ocean currents, sedimentation transfer, and evaporation changes. Many SAR systems can even penetrate the ocean surface and provide detailed images of the ocean floor's topography. Such detailed information can help scientists generate computer models and more accurately predict changes.

Innovative Remote Sensors

In addition to using well-established remote sensors, scientists are exploring innovative sensor technologies. When combined with a series of other sensors, these sensors provide even greater clarity to the oceanic target at hand.

For example, polarimetric passive radiometers can detect electromagnetic waves from the source target. In concert with radiometers and other remote sensors, polarimetric passive radiometers can help scientists gain readings on ocean salinity (the volume of salt in ocean water), adding a new dimension to studying oceanic conditions.

Earth’s oceans are dynamic and constantly change in a wide range of areas. Knowing this, scientists must consider certain environmental conditions when studying oceanic phenomena and characteristics. For example, in the study of circulation (the water’s flow patterns), scientists must account for rising and falling water levels. Ideally, such studies are conducted when the ocean is at its lowest tidal level so that elements such as high waves can be avoided (such conditions can disguise or muddle examinations).

An innovative tool for calculating oceanic elevation is the interferometric radar altimeter. This type of remote sensor observes two sets of waves from the target (such as the energy between high waves). Examining this wave interference provides data on the ocean’s elevation and provides greater resolution to scientific measurements.

Remote sensors have proved effective at analyzing shallow-depth and surface conditions. A vexing pursuit, however, in the study of oceans (particularly in Arctic and Antarctic regions) is deep currents. Remote sensors have had problems penetrating the dense underwater environment, creating only low-resolution images that are largely unusable.

Scientists have applied another innovative remote sensory approach to this problem. In many areas of the Northern and Southern oceans, subsurface density can be used to provide clues about deep currents. In one study, researchers analyzed the relationship between subsurface density and surface elevation (a theoretical concept known as the gravest empirical mode [GEM]). The study’s authors argued that these sizable areas of density can be used to calculate salinity and temperatures (two key elements that play a role in ocean currents). Because of these findings, researchers may use satellite-based and aircraft-mounted remote sensors known as altimeters (which measure elevation) to isolate these columns of dense surface and subsurface water. Researchers may find another useful tool in studying the oceans’ dynamic systems from the data collected through this GEM-altimeter approach.

Furthermore, scientists are considering new techniques in the study of sea ice through the use of microwave sensors. As stated earlier, passive microwave sensors do not detect the temperature of a piece of ice. However, some researchers have proposed utilizing the weather to assist in this capacity. This approach entails the analysis of an ice block’s microwave emissions and brightness (as created by local weather conditions). Using a two-step mathematical algorithm, scientists are generating a catalog of global sea ice that may be used to track temperature changes on the ocean’s surface.

Implications and Future Prospects

The evolution of remote sensors has helped scientists better understand some of the oceanic system's most challenging aspects. Researchers have long used contact sensors (such as those mounted on buoys or dragged behind moving ships) to study ocean elevations, wave height, floor topography, and temperature changes.

Remote sensors used today add many more dimensions to these studies. For example, satellite-based radars, altimeters, and radiometers generate data on these conditions but on a much larger scale and with much greater resolution.

Adding to the benefits of remote sensors in the study of the oceans is that the ever-evolving technologies can be used in locations in which previous systems would acquire unreliable data only; also, remote sensors can now speed up thorough studies that used to take much longer. The remote and harsh environments of the Arctic and Antarctica (the northern and southern ocean regions) provide excellent examples of this fact.

In the twenty-first century, remote sensors can detect thermal changes, the formation of icebergs, density shifts, currents, and other important phenomena from orbit. Such developments in research on the northern and southern oceans are critical, as scientists who are concerned with global climate change tend to focus on ocean elevation and debris in and around these geographic areas.

Additionally, the remote sensors found, particularly on satellites, are easily accessed by participating scientists around the world. There exists a wide range of internationally managed satellites with onboard remote sensors trained on Earth’s oceans. Among these satellites are GOES, RapidScat, MODIS, and MERIS, part of a joint program of the National Aeronautics and Space Administration and the National Oceanic and Atmospheric Administration (NOAA). Government agencies such as NOAA and university-based researchers and graduate students can easily access such networks.

These scientists can share data and develop large-scale international studies through easy access and global communications networks. With the evolution of SAR, scatterometers, altimeters, and other remote sensors (and with the application of such innovative remote-sensing techniques as GEM), scientists will likely continue to generate voluminous and detailed data and images of Earth’s oceanic system. As the twenty-first century progresses, new generations of these remote-sending technologies are being developed and refined, providing increased insight into the world’s oceans. For example, existing satellite lidar technology is advancing and allowing scientists to collect more in-depth measurements of the ocean’s optical properties. Remote sensing technologies are being applied to measure the level of debris and plastic in the world’s oceans, helping in the fight to mitigate the effects of anthropogenic climate change.

Principal Terms

active sensor: a type of remote sensor that emits radiation at a target to study its composition and condition

gravest empirical mode: concept in which the relationship between oceanic subsurface density and surface elevation is examined

lidar: a type of remote sensor that operates similarly to radar but uses lasers instead of radio waves

Northern Ocean: Arctic portion of Earth’s oceanic system

passive sensor: a type of remote sensor that detects naturally emitted energy, such as reflected sunlight, from target sources

scatterometer: active radar that emits high-frequency microwave pulses at a target

Southern Ocean: Antarctic portion of Earth’s oceanic system

synthetic aperture radar: a type of radar that emits a high volume of radio waves at a target as it passes overhead, creating a multidimensional image of the target

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