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Chapter 4: Robots, Satellites, and Observatories

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In Douglas Adams’s five-part trilogy The Hitchhiker’s Guide to the Galaxy (1995), earthling Arthur Dent narrowly escapes Earth’s destruction thanks to an extraterrestrial Good Samaritan who is field-testing a not-so-dependable computerized guide to the galaxy. Their adventures, full of strange technological inventions, bear modest similarities to the technology-enhanced lives of oceanographers.

The application of electronic sensors for observing, measuring, and adventuring into the world ocean has provided a view of the ocean as never before (minus the leaps through hyperspace). These sensors—attached to ships, subs, satellites, robots, and even animals—have dramatically changed our ability to investigate and understand the ocean. Our 21st-century view of the ocean feels like a trip to a galaxy far, far away.

The ever-expanding fleet of sensor-outfitted platforms that fly above, glide on top, or dive below the surface of the ocean belongs to a broad category of tools known as ocean observing systems. Here we include any platform or sensor that permits observations and measurements at a distance. Independent and self-navigating platforms, such as satellites and autonomous underwater vehicles (AUVs), fit the definition to a tee. Tools as simple as drifters and as complex as seafloor observatories may also fall into this category. Some oceanographers might include conductivity-temperature-depth instruments, or CTDs, in this category (covered in Chapter 13). Most importantly, the tools of ocean observing systems share a common goal: to expand the reach of human senses over broad scales of space and time (e.g., Lee et al. 2017).

As oceanographers increase their ability to observe all parts of the ocean and connect measurements across space and time, a new and vital understanding of Earth and its ocean system will emerge. Our ability to survive in the coming centuries may well depend on the robotic eyes and ears of the sea. Like Arthur Dent, we may return to a restored Earth thanks in part to these platforms. That day, when it comes, may bring new meaning to “thanks for all the fish.”

4.1 Bottles, Drifters, and Floats

If you’ve ever seen a plastic bottle adrift on the surface of a lake or the ocean, then you have a pretty good understanding of one of the earliest tools for tracking ocean currents. The drift bottle method relies on a return-to-sender card sealed inside a buoyant container—traditionally a glass bottle—that is tossed from a ship or shore. The card requests that the finder return the card with information on the date and location where the bottle was found. By comparing the starting point to the final destination, the path of the drift bottle may be inferred. Drift bottles have served as indicators of surface current speed and direction since at least 1802 and remain in use today (e.g., Stommel 1965; Storch et al. 2020). At Woods Hole Oceanographic Institute (WHOI), oceanographer Dean “Bump” Bumpus (1912–2002) oversaw the release of hundreds of thousands of drift bottles during his 40-year career (Lipsett 2014; Woods Hole 2021). 

While drift bottles are not autonomous, their use inspired two types of passive float technologies in use today: drifters and floats. Drifters (a.k.a. surface drifting buoys) provide information on surface currents, and floats track currents at deeper depths. As WHOI physical oceanographer Bruce Warren (1937–2010) puts it, “Drifters float and floats sink” (e.g., Lumpkin et al. 2017). Drifters and floats represent what are known as Lagrangian platforms—devices that move with the currents—named after Italian mathematician Joseph-Louis Lagrange (1736–1813), who developed the mathematics of fluid flows (e.g., Knauss and Garfield 2017).

In the 1950s English oceanographer John Swallow (1923–1994) developed a float that could maintain a constant depth and transmit an acoustic signal so that it could be tracked by a ship or a receiving station (Swallow 1955). These acoustically transmitting floats—known as Swallow floats—provided some of the first hints at the complex, eddy-like nature of the ocean (e.g., Crease 1962).

 In 1979 the National Oceanic and Atmospheric Administration (NOAA), along with the World Meteorological Organization and Intergovernmental Oceanographic Commission, established the Global Drifter Program (GDP; Niiler 2001). The most common drifter in use today, the Surface Velocity Program (SVP) drifter, consists of a small, spherical surface float attached to a nearly 23-foot-long (7 m) cloth cylinder with holes in it (to reduce drag), the so-called holey sock drogue. As of August 2022, some 1,244 drifters have been deployed by 26 countries. Each transmits position and temperature data to a satellite for upload to NOAA’s GDP database and website. 

Swallow floats and SVP drifters set the stage for development of autonomous profiling floats, underwater robots that collect, store, and report data via satellite. The crown jewel of autonomous profiling floats is the Array for Real-Time Geostrophic Oceanography, known simply as Argo. 

Put into service in 1999, the Argo floats take their name from the legendary ship Argo. Commanded by the Greek hero Jason, Argo, with his heroic crew, the Argonauts, set sail to find the Golden Fleece, the fur of a winged golden ram, as told in Greek mythology (Colavito 2014). The Argo floats have been hailed as “one of the scientific triumphs of our age” (Gillis 2014). Fittingly, the Argo floats serve under the watchful eye of the ocean-observing  Jason satellites, named for the Greek mythological hero first mentioned by Homer in 800 BCE (Colavito 2014). Together they provide simultaneous ocean measurements above and below the surface. 

An individual Argo float resembles a tall and skinny version of R2-D2, the feisty robot in the Star Wars movies. They move up and down in the water column by regulating their buoyancy, the balance between rising or sinking in the water column (see Chapter 13). As anyone who has used a floatie knows, your ability to float on the surface depends on the volume of air inside the floatie. Let the air out and you sink. Fill the float with air and you float. An Argo float works in a similar fashion, except instead of air, the float uses oil in an internal reservoir. When pumped into a flexible bladder at the bottom of the float, the oil occupies a greater volume and makes the float more buoyant. When the oil is removed, the bladder shrinks and the float sinks. By carefully controlling the bladder, the float can sink, rise, or stay at a constant depth.

Argo floats can measure, store, and transmit observations on a number of physical, chemical, and biological properties, including temperature, salinity, pH, oxygen, and nitrate. As of August 2022, a flotilla of 3,930 floats report data from the surface to depths of 6,562 feet (2,000 m) across the entire world ocean, including the Southern and Arctic Oceans. Over the span of their more-than-two-decade lifetime, the Argo floats have generated more than two million profiles of the upper ocean, nearly four times the number of profiles available prior to their implementation. More than 2,000 scientific papers have used Argo data to support their findings (e.g., Jayne et al. 2017). 

In recent years a new generation of Argo floats—Deep Argo—are being deployed to obtain data over the entirety of the world ocean. Argo floats with nicknames such as Deep SOLO and Deep APEX developed in the United States will reach depths up to 19,685 feet (6,000 m). Deep NINJA and Deep ARVOR, under development by Japan and France, respectively, will operate at depths to 13,123 feet (4,000 m; Jayne et al. 2017). Oceanographers have also developed a new suite of sensors for measurements of optical and biogeochemical properties, a fleet of floats called Biogeochemical Argo (e.g., D’Ortenzio et al. 2020). These new Argo floats promise to shed light on the role of the deep ocean in climate change and improve our understanding of its physics, chemistry, and biology (Roemmich et al. 2019).

While all the above drifters and floats are traditionally deployed from ships, aircraft can launch floats too. The Air-Launched Autonomous Micro-Observer (ALAMO) can profile the ocean to depths of more than 3,900 feet (1,200 m). ALAMO floats were developed in response to a need for obtaining observations in places difficult to reach, such as ice-covered waters and the interior of hurricanes (e.g., Jayne and Bogue 2017; Goni et al. 2017). Deployments of ALAMO floats during the 2017 and 2018 Atlantic hurricane seasons yielded new insights into the response of the upper ocean to the passage of a hurricane and provided ground-truth data for hurricane forecast models (Sanabia and Jayne 2020). Ultimately, the better our understanding of factors that influence hurricane intensity, the more likely hurricane forecasts can provide accurate warnings and save property and human lives.

4.2 Gliders

Torpedo-shaped, with wings and a tail rudder, gliders “fly” through the water column like an underwater airplane. But gliders have no propellers or engines. Instead, gliders rely on buoyancy changes to propel them through the water. When they lose buoyancy, the nose of the glider becomes heavier and it sinks. But instead of sinking vertically, its wings cause it to move forward at an angle. Similarly, as the glider gains buoyancy and the nose lifts, the wings cause it to move forward in the water column at an angle. The resultant flight pattern resembles a sawtooth. While slow moving relative to propelled vehicles, the efficiency of their movement and their low cost allow them to carry out missions of a month or more (e.g., Schofield et al. 2007).

Many types of gliders exist, but the best known is the Slocum glider, named after Canadian-American adventurer Joshua Slocum (1844–1909), the first person to sail alone around the world (1895–1898). American engineer Douglas Webb (b. 1929) originally conceived of the glider in 1978 and then jointly developed it in the 1980s and 1990s with American oceanographer Henry Stommel (1920–1992; Nehring 2021). Like its namesake, the Slocum glider evokes a spirit of exploration and determination—but for robots.

The latest generation of gliders—wave gliders—use the energy of ocean waves for propulsion. The above-water portion of the platform—the float—resembles a surfboard, albeit one outfitted with an antenna and solar panel. Below the surface, a tether connects to an underwater sled—the sub—which features multiple fins arranged like Venetian blinds. These fins generate thrust when the float rises and falls in the ocean swell. Both float and sub may be outfitted with sensors (e.g., Pagniello et al. 2019; Grare et al. 2021). In 2013 a wave glider named Benjamin Franklin completed a 455-day journey of 7,939 nautical miles (14,703 km) from San Francisco Bay to Lady Musgrave Island off the coast of Australia. The feat earned the platform a Guinness World Record for the “longest journey of an unmanned autonomous surface vehicle” (Guinness World Records 2013).

4.3 Towfish

Originally deployed in the 1960s in sonar systems for mapping the seafloor and locating sunken craft, the towfish, a winged platform pulled by a cable behind a moving vessel, remains an important tool in modern oceanographic research. The platform’s resemblance to a torpedo and submariners’ use of “fish” as a slang for torpedoes (e.g., Newpower 2006) likely led to adoption of the term for oceanographic purposes (e.g., Flemming 1976). 

The simplest towfish resemble winged torpedoes. Other towfish look like sleds with wings attached. The wings allow the vehicles to “fly” up, down, or sideways while carrying instruments to measure various ocean properties. A pump may even be attached to deliver water samples to the surface. Some towfish carry nets to collect plankton, the microscopic organisms that drift with the currents in the ocean. Remotely operated towfish vehicles feature automated control functions. The vehicle can maintain a fixed depth or fly in what oceanographers refer to as the tow-yo pattern, a combination of towing and yo-yoing. Geologists were among the first to deploy towfish as part of side-scan sonar systems, which use sound pulses directed toward the seafloor at low angles to produce high-resolution images. Side-scan sonar has been instrumental in locating downed aircraft and sunken ships and providing highly detailed views of the seafloor (e.g., Schlee et al. 1995; Wu et al.  2021).

4.4 Remotely Operated Vehicles

This next piece of technology has become so popular that you can find it for sale on Amazon. Remotely operated vehicles (ROVs) owe their inspiration to the human desire to find shipwrecks. While we commonly think of ROVs as a new technology, their origins date back to the 1950s, when French diver-photographer Dimitri Rebikoff (1921–1997) launched the first ROV, the Poodle. On its first mission in 1954, the ROV Poodle found ancient Phoenician shipwrecks more than 540 feet (165 m) deep (Marx 1990).

Of course, the most famous ROV discovery came in 2013 when the American ROV Odysseus located the remains of the tourist submersible, Titan. On June 18, 2023, the Titan imploded with five passengers aboard on a visit to the passenger ship Titanic at 12,500 feet deep (3.8 km) in the North Atlantic (Mongilio 2023). Titanic, as you know, collided with an iceberg in 1912 and sank, killing 1,500 people. More than 70 years later, an American-French expedition under the guidance of WHOI oceanographer Robert Ballard located the vessel using an ROV (e.g., Ballard and Drew 2021).

Modern ROVs serve a broad range of military, industrial, scientific, and commercial needs. They come in a variety of sizes, shapes, and capabilities (e.g., Moore et al. 2010). The smallest ones are barely the size of a toaster, while the largest ones are bigger than a compact SUV. Most “swim” using a system of propellers controlled via commands sent through a fiber-optic cable called a tether (Christ and Wernli 2014). The ROV operator, the pilot, may use a joystick on a control console to maneuver the ROV. If you’re good with video games, you would probably make a good ROV pilot, especially if you know a little oceanography (hint, hint). 

Some ROVs have wheels or treads like a tank that permit them to crawl over the seafloor, but most resemble snowmobiles, with sleds stretching the length of the vehicle so that they can land gently on the seafloor. Depending on the application, an ROV may be outfitted with one or more manipulator arms, extendable, rotational appendages that accommodate a variety of specialized attachments. Grippers hold objects and grabbers can pick them up. Shovels can be attached for digging, tubes deployed for collecting sediments, suction devices used to collect animals, and any number of other devices for sampling and carrying out research (Sivčev et al. 2018). It’s quite the thrill to watch an ROV collect strange creatures never before seen by humans.

Many ROVs are equipped with cameras and banks of lights for high-definition photography and videography. Some even stream live over the internet. Oceanographers refer to this sensory technology as telepresence, a kind of virtual reality that lets viewers experience a place that is otherwise inaccessible to (most) humans. Telepresence allows land-based scientists (and you) to participate in missions, too. Experts from around the world watch the livestream and call in to offer scientific expertise and commentary on any of the features or organisms that the ROV encounters. Livestreams offer informative and occasionally goofy commentary on the geology, chemistry, physics, and biology of the seafloor. I confess to standing in line at the supermarket watching the live feed of an ROV camera probing the ocean depths from locations halfway across the world. Check it out next time they go live. (See https://oceanexplorer.noaa.gov/.)

4.5 Autonomous Underwater Vehicles

If ROVs had besties, they would be autonomous underwater vehicles, better known as AUVs, untethered craft that operate independently on pre-programmed missions. In fact, the US Navy lumps ROVs and AUVs into a single category: uncrewed underwater vehicles (UUVs). AUVs represent the ultimate expression of a robotic deep-sea explorer. Free of any cables, commanded by a computer brain, and potentially able to wander the ocean for unlimited periods of time, AUVs travel in a way that ROVs cannot. Most fly through the water column. Some crawl along the seafloor. Most impressive is their ability to maintain level flight and react to circumstances with limited decision-making capabilities.

Though robotic solo flights present a number of technological and engineering challenges, AUVs have begun to make their mark on 21st-century oceanographic research (e.g., Searle et al. 2018; Meyer et al. 2019). In a 2015 study off Guadalupe Island on the west coast of Baja California, researchers used AUVs to track the behavior of great white sharks (Carcharodon carcharias). Though the sharks regularly attacked the AUVs (they apparently don’t like being followed by a yellow torpedo), the work yielded new insights into the behavior of these magnificent animals (Kukulya et al. 2016). 

Researchers have also begun to use AUVs with adaptive tracking systems. In essence, the AUV detects and automatically tracks specific features, such as a particular temperature of water or concentration of plankton. Researchers working off Hawaii in 2018 employed two long-range AUVs to automatically track and sample an ocean eddy—a self-sustaining rotating loop of water tens to hundreds of miles wide. Eddies can generate conditions ideal for the growth of phytoplankton—microscopic plant drifters—which concentrate in a layer known as the deep chlorophyll maximum. By programming the AUVs to track the chlorophyll maximum, researchers were able to observe this subsurface feature in unprecedented detail. To an oceanographer, that’s impressive (Zhang et al. 2019; Zhang et al. 2021). 

The latest generation of AUVs promises to greatly expand their capabilities. Hovering AUVs maintain their position over a phenomenon of interest such as a hydrothermal vent. Stealth AUVs rest on the seafloor listening for military targets. New, silicone-based soft robots—so-called bionic AUVs—mimic animals to blend in and observe organisms less intrusively. Uncrewed surface vessels (USVs) allow scientists (and the military) to make observations in remote locations (Braginsky et al. 2020). A catamaran-style USV can even tow a plankton net for sampling microplastics, plankton, and other floating particles (Bazilchuk 2022). For young ocean scientists and engineers, AUVs represent an emerging opportunity to push the boundaries of ocean exploration, a chance to go where no oceanographer has gone before (e.g., Katzschmann et al. 2018; Li et al. 2021; Mazzolai et al. 2022).

4.6 Hybrid Platforms

Hybrid platforms combine the best features of ROVs and AUVs. Formally known as hybrid remotely operated vehicles (HROVs), these platforms permit researchers to operate the same vehicle with or without a tether (e.g., Bowen et al. 2009; Yoerger et al. 2021). As outlined by Xiang et al. (2015), an HROV can launch underwater in ROV mode and be guided to a research site, then operate as an AUV and return to the surface when its mission is complete. 

Hybrid platforms allow oceanographers to reach the deepest depths in the ocean. The Japan Agency for Marine-Earth Science and Technology, or JAMSTEC, developed the UROV11K—with 12.4 miles (20 km) of fiber-optic cable. In 2017 the vehicle reached a maximum depth of 35,758 feet (10,899 m) in the Mariana Trench (Nakajoh et al. 2018). Unfortunately, the UROV11K has since sunk (Wang et al. 2020). HROVs remain a specialized robotic platform, yet their ability to receive and transmit data and move more flexibly and at greater distances than ROVs will likely propel their further development as a tool for undersea exploration and research.

4.7 Benthic Landers

Not all robotic craft operate in the water column. A group of platforms known as benthic landers, nicknamed ocean elevators, sink to the seafloor. Once landed, they carry out any number of observations, such as imaging, remote sensing of ocean properties, sample collection, or experimentation. Landers have proved particularly useful for studying submarine canyons, where episodic events such as underwater mudslides occur. They’re also perfect for observations in deep oceanic trenches that are largely inaccessible to other craft. The deepest-diving landers, the hadal landers, deploy baited cameras and traps to attract, film, and even capture deep-sea organisms. In 2014 oceanographers used landers to catch a snailfish, the deepest known vertebrate, collected at 26,135 feet (7,966 m) in the Mariana Trench (Gerringer et al. 2017). In 2017 a JAMSTEC lander filmed a snailfish at 26,831 feet (8,178 m), the deepest observation of any fish to date (JAMSTEC 2017). Hadal landers have also been used to study sediments and oxygen consumption in oceanic trenches (Lumberg et al. 2018; Glud et al. 2021). Though less sexy than propelled vehicles, landers offer one of the most cost-effective means for making observations in the deep ocean (e.g., Giddens et al. 2021; Du et al. 2023).

4.8 Mini-Autonomous Underwater Explorers 

Sometimes smaller is better. Enter the mini-autonomous underwater explorer (M-AUE). The M-AUE system operates as a swarm of instruments, a dense collection of subsurface drifters and GPS-enabled surface floats that track upper ocean motions in a kind of virtual electronic net. Roughly the size of stout 1.5-liter sports bottles, the subsurface drifters can provide a three-dimensional view of the upper ocean over distances of hundreds of feet to a few miles. Because the drifters can be programmed to automatically maintain their position in the water column, they can mimic the drift patterns of plankton, harmful algae, or oil spills (e.g., Jaffe et al. 2017). The fine-scale information provided by M-AUEs has already revealed insights into how marine larvae find their way home (e.g., Garwood et al. 2020). M-AUEs offer a preview of an emerging field of study known as underwater swarm robotics, the design and development of multi-robot systems that interact and coordinate their behavior to solve problems and carry out complex tasks (e.g., Connor et al. 2021). Such systems offer opportunities to better understand the connections between physical and biological processes in the ocean (e.g., Dickey 1991). 

4.9 Ocean Moorings

Ocean moorings provide a fixed platform to which various oceanographic instruments can be attached to acquire measurements of ocean properties for long periods of time. They excel in sampling temporal scales at an unprecedented resolution—minutes to weeks—and they can be left unattended for months to years at a time.

The foundation of an ocean mooring involves a piece of machinery all too familiar to us. Next time you’re stuck at a railroad crossing, check out the wheels on the train. At several hundred pounds each, they make great anchors. And indeed, that’s what oceanographers use them for. (And just to clear this up, there’s absolutely no truth to the rumor that oceanographers steal wheels from parked trains at night.) Train wheels provide the foundation for a steel cable to which floats (often hollow glass balls) and various oceanographic instruments can be attached. The cable may stretch vertically all the way from the seafloor to the surface, or it may rise from the seafloor to a particular depth of interest. Meteorological instruments attached to the surface floats collect weather data. Below the surface, oceanographic instruments may be attached at various intervals to collect depth-specific information. 

While several types of instruments may operate simultaneously at fixed depths, profiler moorings allow instruments to travel up and down a cable and sample the water column in a manner similar to an instrument on a cable on a ship. In either case—fixed or profiling—when an oceanographer wants to retrieve the instruments, they send a secretly coded acoustic signal to the mooring that activates a release mechanism, unhooking the cable from the railroad wheel and permitting the instruments to float to the surface. The wheel remains on the seafloor, forever a part of Davy Jones’s locker.

Moorings have provided some of the longest continuous observations of the ocean on record. Off the coast of Bermuda in the Sargasso Sea (or the Bermuda Triangle, as some people call it), oceanographers with the Ocean Flux Program (OFP) have maintained a mooring since 1978. The OFP mooring hosts sediment traps, devices that collect and store sinking particles. Its data—more than 40 years’ worth—offers invaluable insights on the ocean’s recent past. These data have been especially useful for assessing the effects of climate change in the ocean (e.g., Conte and Weber 2014; Cael et al. 2021).

Because ocean moorings can provide critical meteorological information, the US established in 1967 the National Data Buoy Center (NDBC). The NDBC oversees the operations and data quality control of hundreds of moored buoys deployed in US coastal and oceanic waters across the Atlantic and Pacific. It also provides quality control and dissemination of data from hundreds of partner stations. Its website provides near-real-time data from stations in the Atlantic, Pacific, and Indian Oceans. The NDBC serves a wide range of scientific, military, governmental, private, and civilian needs. Fishers, sailors, and boaters, among others, go there for up-to-date information about the waters in which they travel.

Despite their many advantages, the harsh conditions to which moorings are subjected can cause breakdowns, corrosion, and, on occasion, total loss. Moorings attract fish that bite and fray the cables. Fish attract fishers, who accidentally snag moorings when trawling or intentionally latch onto moorings and steal their parts. A cable snaps or the anchor accidentally releases, and the mooring drifts away, never to be seen again. On some occasions, the release fails to respond to its acoustic signal, in which case the crew must drag an anchor through the water hoping to catch the subsurface cable and bring the mooring to the surface. Rescue missions to find moorings take up costly ship time and fray the nerves of the oceanographers responsible for maintaining them. Nonetheless, the benefits they serve, such as El Niño prediction, tsunami warning, storm warning, and weather forecasting, more than outweigh their costs. We would be blind to what’s happening beneath the ocean’s surface without them.

4.10 Uncrewed Aerial Platforms

Increasingly, scientists are turning to uncrewed aerial platforms—drones—remotely controlled flight-capable platforms outfitted with video cameras and various sensors. This category of platforms also includes remotely piloted aerial systems and multirotor copters (e.g., Klemas 2015; Nguyen et al. 2022). The use of drones and drone-based sensors in oceanographic research represents one of the most recent examples of how new technological developments can revolutionize a field (e.g., Gray et al. 2022). 

One of the earliest uses of drones for oceanographic research was simply to follow whales and dolphins (Fiori et al. 2017). Images and video from drones have enabled scientists to map intertidal reefs (e.g., Murfitt et al. 2017), track discharge plumes of groundwater (Lee et al. 2016), survey the size and shape of beaches (Casella et al. 2020), and measure snow thickness on sea ice (Molar-Candanosa 2022). 

A new generation of sensors—deployable on drones and aircraft—now permit oceanographers to measure a wide variety of ocean properties. Multispectral imagers detect several different wide bands of electromagnetic radiation (colors). Hyperspectral imagers detect hundreds of narrow bands of electromagnetic radiation. Both routinely fly aboard airborne platforms to monitor the environment (Stuart et al. 2019). Using a drone-based hyperspectral sensor, researchers were able to document the extent and concentration of a harmful algal bloom in two bays in southern China (Li et al. 2021).

Not all modern oceanographic measurements require cutting-edge platforms. Kites, outfitted with state-of-the-art cameras or sensors, serve many coastal applications. Kite aerial photography has been used to map the distribution of organisms in the rocky intertidal and near-coastal zone and to acquire high-resolution images of entire islands or beaches (Bryson et al. 2013).

 Balloons play a role in oceanographic research too. During the 2018 Microbiology-Ocean-Cloud-Coupling in the High Arctic (MOCCHA) campaign, oceanographers deployed a tethered balloon to sample biologically generated aerosols and the properties of low clouds (Zinke et al. 2021). In the near future, scientists hope to deploy steerable stratospheric balloons, such as Stratollites. As big as a blue whale, these craft can maintain a position above Earth’s surface for several days (Good et al. 2018). Stratollites may be used to monitor storms, assist natural disaster efforts, and even remotely sense the ocean. By 2024 you’ll be able to ride 19 miles high above Earth in a Stratollite (World View 2023).

And while technically not an aerial platform, the newest tool for carrying oceanographic instruments over large swaths of the ocean involves a hybrid device called a saildrone. Part windsurfer, part oceanographic buoy, this uncrewed wind- and solar-powered device can stay at sea for months at a time. On a 60-day voyage from San Francisco to Guadalupe Island, a saildrone carrying 16 ocean and atmospheric sensors provided unprecedented data on key oceanographic and atmospheric processes (Gentemann et al. 2020). NOAA soon hopes to deploy fleets of saildrones to replace some of its aging buoys at a fraction of the cost (Voosen 2018). Of course, on windless days, saildrones might also make a nice resting spot for migrating seabirds or a peaceful place for a textbook-weary student to get away from it all.

4.11 Satellite Oceanography

Space-based observations of the ocean have revolutionized oceanography. Though physical oceanographers had long suspected the chaotic nature of ocean circulation, the first satellite images of ocean color in 1978 drew a collective “oh, my.”  The surface of the ocean and the phytoplankton suspended within it resembled the sky in Vincent van Gogh’s painting Starry Night (1889): variable, complex, and organized in loose, rotating structures. The ocean’s mesoscale structure—the highly variable eddies that characterize nearly all of the ocean’s flows—suddenly became visible. Seemingly overnight, satellite oceanography, the study of the world ocean using sensors aboard space-based platforms, changed the way we thought about and studied the ocean. Barber and Hilting (2000) likened it to “seeing the ocean for the first time.”

Prior to this grand awakening, weather and military satellites had routinely observed the Earth, ice caps, and ocean. But on June 26, 1978, the National Aeronautics and Space Administration (NASA) launched the very first satellite dedicated solely to oceanography, the Seafaring Satellite, or Seasat. Four months later, NASA launched the Nimbus 7, which carried the Coastal Zone Color Scanner (CZCS), the first sensor designed to measure ocean color. Its success  drew greater attention to the optical properties below the ocean’s surface and gave birth to the field of bio-optical oceanography, the study of the submarine light field and its variations due to biological processes (Smith and Baker 1978). This extraordinary technology continues to make significant contributions to our understanding of the world ocean in the 21st century.

4.11.1 Sea Surface Temperature 

Covering 71 percent of Earth’s surface, the world ocean dominates temperature regulation on our planet. As NOAA puts it, “the ocean is the largest solar energy collector on Earth” (Lindsey and Dahlman 2020). Through heat exchange across the air–sea interface—the boundary between the ocean and the atmosphere—the world ocean adds or removes heat from the atmosphere and vice versa. These exchanges of heat drive the motions of the atmosphere and the ocean and prove important for a whole heckuva lot of other things we’ll learn about in the chapters ahead.

Satellite oceanography owes much of its early development to sensors used in aerial oceanography. One group of sensors, collectively known as radiometers, detects different kinds of electromagnetic radiation, the energy of light. Because the ocean and the stuff it contains remove, reflect, and scatter different wavelengths of electromagnetic radiation, we can learn about the ocean by measuring the wavelengths of light emitted by its surface. Sensors that detect different types of surface-emitted radiation are known as passive sensors. 

On April 1, 1960, NASA launched the world’s first weather satellite, the TIROS-1 (Television Infrared Observation Satellite), one of a series of  satellites carrying television cameras into space. Images of cloud and weather patterns ushered in an era of space-based weather forecasting. TIROS set the stage for the Earth-observing satellite systems that now deliver weather information every hour of every day across the globe. 

Scientists soon developed additional tools for space-based observations of Earth. Measurements of the temperature of the sea surface—a property known as sea surface temperature (SST)—began with the Nimbus satellites launched between 1964 and 1978. These satellites carried high-resolution infrared radiometers designed to measure the temperatures of cloud tops and the sea surface at night (when sunlight wouldn’t interfere with the measurement).  In 1978 NASA launched TIROS-N (N for NOAA), which carried the first of a series of Advanced Very-High-Resolution Radiometers (AVHRRs), multichannel, infrared-sensing instruments for measuring SST. The data from AVHRRs flown on TIROS-N and subsequent satellites represent the longest near-continuous measurements of SST from space, a period of more than 40 years (e.g., Casey et al. 2010; O’Carroll et al. 2019; Minnett et al. 2019). 

One limitation of AVHRR sensors is their inability to “see” through clouds, which block surface-emitted infrared radiation. To overcome this limitation, NASA and the Japanese Aerospace Exploration Agency launched satellites that carried passive microwave sensors. These sensors detect microwave radiation which passes through clouds and from which estimates of SST and other properties can be derived. Unfortunately, microwave sensors lack the resolution of infrared sensors and tend to be less accurate (Castro et al. 2008). So oceanographers take the best of both worlds and blend data from infrared and microwave sensors into a combined product, the Multi-scale Ultra-high-Resolution SSTs (MUR-SSTs). Global and regional maps of MUR-SSTs adhere to standards developed by the Group for High Resolution Sea Surface Temperature, which promotes best practices for monitoring, processing, and reporting satellite-derived SST (Minnett et al. 2019).

SST measurements serve operational oceanography, climate change research, fisheries, marine heatwave tracking, and other interests (e.g., Beggs 2010; O’Carroll et al. 2019). NOAA’s Coral Reef Watch uses satellite SST data to inform resource managers, scientists, and the public about the potential for coral bleaching, the phenomenon whereby coral animals expel their algal “helpers” in response to prolonged elevated seawater temperatures. In recent decades, coral bleaching has devastated corals around the world (e.g., Hughes et al. 2018). Satellite-based forecasts of coral bleaching allow scientists to “rescue” rare coral species and house them in a laboratory. When the warm waters subside, the corals can be returned to their natural environment. It’s not the ideal solution, but coral rescues play an important role in coral conservation and management (e.g., Vardi et al. 2021).

4.11.2 Sea Ice Extent

Soon after the Titanic sank following its collision with an iceberg in 1912, the governments of the US and Europe formed the International Ice Patrol. Tasked with monitoring the whereabouts of icebergs, seaborne chunks of fractured glaciers, the Ice Patrol was the first to regularly monitor ice conditions at polar latitudes. Interest in polar ocean ecosystems and climate change led to efforts in the 1960s to develop tools for observing icebergs and sea ice, the frozen surface of polar oceans (Zwally et al. 1983). Fortunately, passive microwave sensors proved ideal for the job.

Observations of sea ice using passive microwave sensors aboard aircraft began in the Arctic in 1967 (Gloersen et al. 1992). The first passive microwave instruments were flown on Russian satellites in 1968 (Tikhonov et al. 2016). But the Electrically Scanning Microwave Radiometer flown aboard the Nimbus 5 satellite in 1972 began the era of “all-weather, all-seasons” sea ice observations that has continued to this day (e.g., Parkinson et al. 1987). A 40-year record from passive microwave sensors has revealed striking and alarming retreats of sea ice in the Arctic and Antarctic Oceans (Parkinson 2019). Multiyear ice, sea ice that remains present for longer than one year—thought to be critical for maintaining sea ice volume—has shown similar declines (Haibo et al. 2020).

A desire for higher-resolution imagery led to the launch of active microwave sensors, instruments that produce their own electromagnetic signal and detect the reflected signal (like the radar gun used to catch speeders). By bouncing a beam of energy off the surface of the ocean, oceanographers can measure a number of ocean properties with very high resolution (e.g., Gens 2008). An active microwave system known as synthetic aperture radar (SAR) flew aboard Seasat and performed so well it inspired a generation of SAR-carrying satellites in the 1980s and 1990s (Tsatsoulis and Kwok 1998). SAR works by electronically mimicking a large antenna, enabling it to produce detailed images of sea ice and other properties. SAR remains an integral part of Earth-observing satellite systems in the 21st century. 

4.11.3 Sea Surface Height

By bouncing a beam of energy off the surface of the ocean, oceanographers can measure the shape of the sea surface–the sea surface topography, the bumps and depressions of the ocean surface. The best-known devices, the satellite altimeters, measure the time it takes for a microwave radar signal to bounce off the ocean surface and return to the satellite. Based on the return time and precise knowledge of the height and path of the satellite, oceanographers can calculate the distance between the sea surface and the satellite.

Satellite altimeter measurements allow mapping of sea surface height, the height of the sea surface relative to a baseline, such as Earth’s ellipsoid or geoid (i.e., its equi-gravitational surface; see Chapter 20). Sea surface height measurements permit oceanographers to learn about ocean circulation and ocean tides. It also provides knowledge of sea level changes in response to a warming ocean and melting ice caps. As you might expect, these observations and the computations required to generate useful satellite products—the results provided by satellite measurements—involve very accurate measurements and lots of computing power.  

Arguably the most successful oceanographic satellite altimeter mission to date is TOPEX/Poseidon, the Ocean Topography Experiment/Poseidon mission. Launched in 1992 in a collaboration between NASA and the French space agency, Centre National d’Etudes Spatiales, TOPEX/Poseidon allowed oceanographers to observe changes in world ocean circulation over seasonal and annual cycles, advanced our understanding of tides and sea level rise, and enabled significant refinement of general circulation models. It provided unprecedented observations of the 1997–1998 El Niño, among the strongest in 100 years. After more than 13 years and nearly 62,000 orbits of Earth, TOPEX/Poseidon ended operations in October 2005. Legendary oceanographer Walter Munk (1917–2019) called it “the most successful ocean experiment of all times” (Munk 2002).

The Jason satellite altimetry missions extended the data record begun by TOPEX/Poseidon. Jason-3 (launched in 2016) currently operates in tandem with the altimeter-carrying Sentinel-6 Michael Freilich  satellite. This satellite was named after the former director of NASA’s Earth Science Division, Michael Freilich (1954–2020)—a “passionate advocate” of measurements of Earth from space (Cook 2020). This satellite can track sea level rise with an error of less than a tenth of an inch (<1 mm) per year (Donlon et al. 2021).

4.11.4 Ocean Color

If you’ve ever spent time near an ocean in middle latitudes, you’ve undoubtedly noticed that the color of the water changes from season to season. Here in Southern California, the winter ocean appears blue, turns green in spring, blues up in summer, and greens again slightly in fall. These changes in ocean color originate from changes in the abundance of phytoplankton and their primary light-absorbing pigment, chlorophyll. Think green trees in summer and fallen leaves in winter. It’s a similar phenomenon in the ocean. As the concentrations of phytoplankton vary, so does the color of the water: more phytoplankton, greener water. Viewed from space, these organisms give the ocean a palette of greenish hues in what might otherwise be a blue desert.

Oceanographers define ocean color as the “relative amounts of water-leaving radiance in the various portions of the visible spectrum” (Austin 1993). In other words, ocean color is the color of the ocean. Pure ocean water absorbs mostly every color except blue. That’s why the open ocean appears blue. It’s the only color remaining after all others are absorbed. The CZCS (mentioned above) provided biological oceanographers their first global views of ocean color, whose hues varied with the concentration of phytoplankton in the water. Images from the CZCS revealed an intimate connection between ocean physics (i.e., seasonal stratification, upwelling, eddies) and phytoplankton. 

The CZCS greatly exceeded expectations and outlived its life expectancy, but it ceased operations in 1986. A decade later, the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) lifted off from Vandenberg Air Force Base (August 1, 1997). By this time, oceanographers had developed in-water optical sensors and improved biochemical analyses of seawater. A new generation of oceanographers, the bio-optical oceanographers, stepped in. Their mission was to advance our understanding of biological oceanography through a study of ocean optics. Of them, Barale writes: “The space oceanographers . . . were like those pioneers who opened new trade routes, as what they discovered . . . surpassed many times what was expected in the beginning” (Barale 2010).

The design and execution of SeaWiFS built on the lessons learned from the CZCS, including the need for continuous ground truthing, the practice of verifying satellite observations with measurements obtained from within the water column. Though traditionally carried out on ships, ground truthing with AUVs is now under development (e.g., Bailey and Werdell 2006; Bennion et al. 2019). Buoys are also used, such as the Marine Optical Buoy system moored off the coast of Hawaii (e.g., Brown et al. 2007). 

More than a dozen missions have been launched since SeaWiFS. The Moderate Resolution Imaging Spectroradiometer (MODIS) launched aboard the Terra satellite (December 1999) and the Aqua satellite (May 2002) brought improvements to sensor design and performance. The Visible and Infrared Imager/Radiometer Suite (VIIRS), the successor to MODIS, aboard the Suomi National Polar-orbiting Partnership, and a host of international sensors and satellites devoted to ocean color measurements ensure that global measurements of the ocean biosphere continue to this day.

4.12 Ocean Observatories

If robots had underwater cities, they might look like ocean observatories, an integrated array of ocean-observing sensors and platforms on and above the seafloor designed to address questions of scientific and societal importance (e.g., Crise et al. 2018). Ocean observatories were conceived in the 1980s by American geological oceanographers as the oceanographer’s version of an astronomical or volcano observatory (Delaney et al. 1987). Like these observatories, ocean observatories carry out observations over broad spatial and temporal scales. Uniquely, however, ocean observatories span hundreds of miles in waters thousands of feet deep. They involve fixed and mobile platforms connected either physically via cables or virtually via acoustic or optical signals. At heart, ocean observatories provide long-term and sustained observations of the ocean, in contrast to the brief encounters offered by shipboard oceanography.

To get an idea of what they look like, imagine an oceanic neighborhood with strings of instrumented moorings and AUV docking stations spaced at regular intervals like gas stations. Bottom-attached instruments of all shapes and sizes cover the seafloor like tiny houses. Attached cables act as their power lines. Dish-shaped antennae transmit and receive acoustic and optical data. Drifters and gliders and AUVs fly through the surrounding water like a scene from The Jetsons (Hanna-Barbera 1962–1963). Bottom-roaming benthic explorers excavate sediments and retrieve and deploy instrument packages like All-Terrain Walkers on an undersea Tatooine (e.g., Lucas 1977–2019). Of course, the occasional human visitor interrupts the robotic seascape—arriving via ship or submersible or as a deep-sea diver—acting as a necessary overlord to keep the city operating smoothly.

Ocean observatories generally fall into one of three categories, regional cabled arrays, coastal arrays, and global arrays. Each serves different goals and purposes.

Regional cabled arrays represent what geologists envisioned when they proposed ocean observatories in the 1980s. These arrays use optical-electrical telecommunication cables to provide power and two-way communication capabilities to suites of instruments on the seafloor. The regional cabled array off the coasts of Washington and Oregon serves as the best example. This array, touted as “the first US ocean observatory to span a tectonic plate,” consists of seven primary nodes, each with its own set of instruments and name (such as International District Vent Field 1). One array, the Axial Caldera Cabled Array, captured the eruption of an active undersea volcano on the Juan de Fuca Ridge on April 24, 2015 (Delaney et al. 2016).

Coastal arrays focus on oceanographic processes that occur on or above the continental shelf (e.g., Trowbridge et al. 2019). A coastal array off the Oregon Shelf, the Coastal Endurance Array, employs moorings (fixed and profiler) in concert with gliders to provide high-resolution, top-to-bottom observations of ocean and atmospheric processes. The Coastal Endurance Array has observed hypoxia events (i.e., low oxygen), ocean heatwaves, and the response of marine organisms to a solar eclipse (Barth et al. 2018).

Global arrays refer to ocean observatories in select deep water locations that shed light on major science themes, such as air–sea exchange, climate variability, ocean circulation, and ocean ecosystems. Global arrays rely on deep ocean moorings and gliders to sample vertical and horizontal variability in the upper ocean. The Global Irminger Sea Array off Greenland has already provided tantalizing new observations that may change how we view circulation in this part of the world ocean (de Jong et al. 2018).

Ocean observatories represent an emerging conceptual, technological, and strategic shift in how oceanographers carry out their work. But there are concerns that the enormous financial outlay needed to build and sustain ocean observatories will drain resources from traditional approaches (Witze 2013; Kintisch 2015). As stated by the National Research Council (2015), a “healthy balance” should be sought between “infrastructure to provide access to the ocean and advance the science . . .  funds for scientists and trainees to conduct research and provide value for the infrastructure investment.” How oceanographers and funding agencies achieve this balance may determine the long-term vitality of future oceanographic research.

As a final note, I want to emphasize that ocean observatories will prove a massive undertaking. They represent, perhaps, the culmination of nearly 150 years of oceanographic research that began with the Challenger expedition. Heightened awareness of the rapid changes in Earth’s climate and the growing knowledge of multiple stressors on the marine environment makes the mission of ocean observatories ever more urgent (e.g., Speich et al. 2019). With the UN Decade of Ocean Science for Sustainable Development upon us (2021–2030), the opportunities for ocean scientists look ever more promising. As Visbeck (2018) put it:

The increased awareness of the importance of the ocean to the future of humanity gives grounds for cautious optimism and motivation for ambitious multilateral cooperation. The scientific community has been given a stage on which to shine during the Decade of Ocean Science for Sustainable Development. Let us come together, respect our disciplines and agendas, but also be ready to embark on an exciting and transformative journey to realize the ocean we need for the future we want. 

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