The Eyes of the Space Whales: A History of Satellite Oceanography

About 71% of Earth’s surface is covered by water, most of which consists of oceans. This vast and ever-changing expanse remains one of the least explored regions on our planet. The oceans are not just immeasurable masses of water, but global ecosystems that regulate Earth’s climate, support biodiversity, and are vital for all living things.

Studying in detail the complex processes behind the formation and evolution of oceans, humans not only dive into its depths, but also try to make sense of it from above. This is why monitoring satellites—and even entire satellite constellations – have been placed in orbit, becoming indispensable tools for oceanographic research today.

Getting Started: The Skylab Research Toolkit

The early 1970s marked the starting point for orbital oceanic monitoring programs. Before then, only a few satellites performed this role, and then only indirectly. For example, the Soviet Kosmos-243 (launched in 1968) and Kosmos-384 (1970) satellites were equipped with microwave radiometers that allowed them to measure sea surface temperature and wind speed from orbit. However, this was insufficient for full-scale observation of the oceans.

Consequently, oceanography at the time was limited to data obtained from ships and research aircraft. Oceanographic research conducted from ships, however, had major limitations, since they moved very slowly and could cover only relatively small sections of the ocean’s surface during a single voyage.

Aircraft were more practical tools, and had been used for research almost since the dawn of aviation in the early 20th century. Reconnaissance aircraft provided aerial photographs of coastal areas and carried out the first remote sensing using infrared and microwave radiometers. On the one hand, oceanographic aviation could capture much more detailed images of the ocean surface, since it operated at relatively low altitudes compared to satellites. On the other hand, this very advantage also limited oceanographic aviation, since it could not provide global, planetary-scale coverage. Additionally, its use was always dependent on weather conditions.

To gain a comprehensive and truly global understanding of the oceans as a single dynamic system, NASA proposed a different approach: observing the ocean surface from orbit. The first orbital platform equipped with instruments capable of conducting oceanographic research was the first American space station, Skylab, which was launched on May 14, 1973.

Although Skylab was not specifically designed for satellite oceanography, it nevertheless played a significant role in studying large water bodies. Astronauts aboard the station conducted visual observations and photographed the Earth, including its oceans. The crew’s equipment included both handheld cameras, such as Hasselblads, and professional research instruments that were part of the specialized Earth Resources Experiment Package (EREP).

EREP was an advanced suite of measurement instruments for its time, comprising six main devices designed for comprehensive observation of the Earth’s surface. In particular, it included:

1. S190A Multispectral Camera — a system of six identical cameras that simultaneously captured images of the surface, each using its own narrowband filter. This approach allowed S190A to image in six different spectral ranges, from visible to near-infrared light.
2. S190B High-Resolution Camera — equipped with a long-focus lens, this camera provided highly detailed images of the Earth’s surface. For oceanography, it enabled close study of coastal zones, ice drift dynamics, nearshore eddies, and similar phenomena.
3. S191 Infrared Spectrometer — this device measured the intensity of radiation emitted from the Earth in two key ranges: shortwave and thermal infrared. Its main task was to accurately measure surface temperatures (both land and water) and to analyze the composition of the Earth’s atmosphere.
4. S192 Multispectral Scanner — this device operated across 13 narrow spectral channels, from visible to infrared. S192 was unique for its time because it collected data digitally, allowing for computer-based analysis, automated classification, and study of the Earth’s topography and resources.
5. S193 Microwave Radiometer/Scatterometer/Altimeter — for the purposes of oceanography, this instrument was used to measure sea surface height, map currents and waves, monitor water surface temperature, and determine wind speed and direction over the ocean.
6. S194 L-Band Radiometer — a passive microwave instrument operating at relatively long wavelengths (L-band). It was designed for experimental studies of remote sensing capabilities, such as soil moisture on land and water salinity.

During Skylab’s 171 days of operation from 1973 to 1974, the station was visited by three different crews. Using EREP, they captured approximately 350,000 images of the Earth’s surface, including the oceans. The photographs, taken from an altitude of about 435 km, provided the first documented evidence of the global dynamic processes occurring in the oceans. During this period, astronauts recorded major ocean currents and eddies, the distribution of large phytoplankton blooms, the movement of icebergs and sea ice drift in polar regions, and identified the main mechanisms of interaction between the Earth’s atmosphere and ocean waters. However, Skylab was only a precursor to fully dedicated oceanographic satellites, which would be launched into orbit by the late 1970s.

Seasat-1: the world’s first SAR satellite and a pioneer of space oceanography

On June 27, 1978, NASA launched Seasat-1, the first satellite entirely dedicated to oceanographic research. Although the Seasat-1 mission lasted only 99 days and was prematurely ended due to a technical failure, it permanently transformed the perception of what satellite oceanography could achieve. Seasat-1 demonstrated that satellites could provide high-quality data essential for weather forecasting, climate studies, shipping monitoring, and, most importantly, measuring global ocean levels.

The satellite was developed by NASA’s Jet Propulsion Laboratory. The project cost approximately $85 million, which was an ambitious budget for the 1970s. The original mission plan envisioned the spacecraft operating for 3–5 years. Seasat-1 carried five primary scientific instruments designed specifically for oceanographic research:

1. Radar Altimeter — This instrument measured the time it took for a radio pulse sent from the satellite to reach the ocean surface and return. This approach allowed for extremely precise measurements of sea surface height. As oceanographic satellites evolved, radar altimeters remained a reliable and accurate tool for measuring ocean levels.
2. Scanning Multichannel Microwave Radiometer (SMMR) — This passive instrument collected microwave radiation emitted from the ocean surface. It allowed scientists to accurately measure sea surface temperature, wind speed, atmospheric water vapor content, and even the amount of liquid water in clouds.
3. Microwave Scatterometer — This instrument measured the reflection of microwave pulses from the water surface. The roughness of the water, caused by wind, affected the reflected signal, allowing scientists to calculate wind speed and direction over the ocean, which is crucial for storm prediction.
4. Synthetic Aperture Radar (SAR) — The most innovative instrument aboard Seasat-1 and the first of its kind ever placed on a satellite. SAR provided high-resolution images of the ocean surface, regardless of weather conditions or sunlight. It was extremely effective for observing sea ice, waves, and detecting ships.
5. Laser Retroreflector — This instrument was used for precise tracking of the satellite’s orbit from ground stations, ensuring the high accuracy of all other measurements.

Once Seasat-1 reached its operational orbit, it immediately began conducting research. By analyzing data from the radar altimeter, scientists were able to quickly identify and study major ocean currents such as the Gulf Stream and perform topographic mapping of the seafloor, including underwater mountains and ridges. The satellite conducted one of the first mappings of Earth’s gravitational field (the geoid), paving the way for some of the most advanced maps of Earth’s gravitational distribution over oceanic areas. The geoid represented an idealized shape of the Earth, helping scientists understand how mass and gravitational anomalies affect the planet’s surface.

During the mission, the effectiveness of the Synthetic Aperture Radar (SAR) was also demonstrated. SAR proved indispensable for operations in polar regions, providing detailed images of sea ice and showing exceptional performance in cloudy conditions and at night. Seasat-1’s scatterometer produced the first global maps of ocean wind speed and direction, greatly aiding weather forecasting and the study of storm dynamics.

Despite Seasat-1’s remarkable capabilities, there was nevertheless a significant flaw in its electronics that came to light on October 10, the 99th day of the mission. A short circuit in the satellite’s electrical system instantly disabled the satellite, leaving the ground team with no chance to restore communication. Even so, despite its extremely short duration, the Seasat-1 mission delivered a vast amount of data and tested cutting-edge technological solutions for its time. Today, SAR, which first appeared on Seasat-1 in 1978, is still widely used across nearly all areas of satellite monitoring, from precision agriculture to the military sector.

Seasat-1 proved that satellites could be reliable tools for global ocean monitoring and laid the foundation for all subsequent oceanographic space programs that emerged in the United States and Europe during the 1980s and 1990s.

The next generation: Nimbus-7, GEOSAT and ERS-1

On October 24, 1978, two weeks after the loss of Seasat-1, NASA launched another satellite with oceanographic capabilities: Nimbus-7. As the name suggests, Nimbus-7 was the seventh satellite in its series, which was primarily intended for meteorological, rather than oceanographic, purposes. Its main objective was to study three domains: the atmosphere, the oceans, and the cryosphere (glaciers and ice).

Nimbus-7 deserves mention here because it was the first satellite to measure ocean color using the CZCS (Coastal Zone Color Scanner), which allowed scientists to track chlorophyll (plankton) concentrations and the condition of coastal waters. The satellite was also equipped with a Scanning Multichannel Microwave Radiometer (SMMR), which provided long-term, almost daily data on sea surface temperature, wind speed, and, most importantly, sea ice concentration in polar regions. Nimbus-7 operated far beyond its planned lifespan, collecting data until 1987 and only fully ceasing operations in 1994.

The United States also soon began work on a successor to Seasat-1. This spacecraft was to be named the GEOdetic SATellite (or GEOSAT for short). From the very start, this satellite was designed for military purposes, and its primary operator was not NASA, but the U.S. Navy.

With GEOSAT, the U.S. military aimed to significantly improve the navigation of submarine-launched ballistic missiles, which were a key component of the American nuclear triad. Ballistic missiles rely on inertial navigation systems to determine their position during flight. The accuracy of these systems directly depends on knowledge of the local gravitational field of the area into which they are launched. Since the Earth’s gravitational field is not uniform, as clearly demonstrated by Seasat-1 during geoid mapping, the Navy wanted a satellite equipped with a radar altimeter to create a more accurate geoid map. This is why GEOSAT carried only a single instrument on board: a radar altimeter.

The satellite’s technical simplicity and the presence of only a single instrument in its equipment allowed for significant cost savings in its production. The main developer and manufacturer for GEOSAT was the Applied Physics Laboratory at Johns Hopkins University. The design of the altimeter itself was much improved as compared to Seasat-1, and the instrument was equipped with more reliable electronics. Additionally, GEOSAT processed the returned radio signal (echo) directly on board, thanks to the integration of an onboard microcomputer.

GEOSAT was launched into orbit on March 12, 1985, aboard an Atlas E rocket and operated for five years, through 1990. Its mission was divided into two main phases. The military geodetic mission (GM mode) lasted 18 months (from April 1985 to September 1986), during which GEOSAT produced the most detailed map yet of the Earth’s gravitational field. Once the first phase of the geodetic mission was completed and the Navy’s needs were met, GEOSAT began its second phase: conducting a civil ocean observation mission that lasted more than three years (exact repeat mission, or ERM mode). The collected data were later made public and became available to the global scientific community. Operating in ERM mode, GEOSAT became the second satellite after Seasat-1 to provide detailed data for monitoring sea level changes, a key indicator in climate change research.

GEOSAT also inspired the European Space Agency (ESA), leading to the development of the European Remote Sensing Satellite 1 (ERS-1) oceanographic satellite. Unlike GEOSAT, ERS-1 was designed as a multifunctional platform, focusing on the use of active microwave sensors (SAR, scatterometer), allowing it to collect data independently of cloud cover or day-night cycles.

ERS-1 was equipped with five primary instruments for observing the oceans and polar ice caps:

1. AMI (Active Microwave Instrument) — an active microwave instrument operating in two primary modes: synthetic aperture radar (SAR) and wind scatterometer.
2. RA (Radar Altimeter) — a radar altimeter similar to the one installed on GEOSAT.
3. ATSR (Along-Track Scanning Radiometer) — a radiometer scanning along the satellite’s path, designed to measure surface water temperatures.
4. MWR (Microwave Radiometer) — a passive microwave radiometer used to measure water vapor and liquid water content in the atmosphere. These data were critically important for atmospheric correction of altimeter measurements.
5. PRARE (Precise Range And Range-rate Equipment) — a precise orbit determination system needed to achieve maximum accuracy of the satellite altimeter data.

During the first four years of its mission, ERS-1 focused on studying the Earth’s ocean surfaces, recording changes in ocean currents, and monitoring global sea level changes (at ten different time intervals). The SAR installed on the satellite also became an indispensable tool for systematic monitoring of sea ice thickness and ice sheets in Greenland and Antarctica.

Later, ESA decided to enhance ERS-1’s capabilities by launching its twin, ERS-2, on April 21, 1995. Shortly after the second satellite reached orbit, a short-lived tandem mission between the two spacecraft began, applying radar interferometry (InSAR) methods. This new paired methodology made it possible to create highly precise digital elevation models (DEMs) and detect minute ground movements, such as those occurring after earthquakes.

The ERS-1/ERS-2 tandem mission thus went far beyond oceanographic observations. The paired studies continued until March 2000, when ERS-1 was unable to enter emergency acquisition mode due to a failure in its attitude control system. The satellite could therefore not perform a maneuver meant to orient its solar panels toward the Sun and recharge its batteries. Nevertheless, ERS-1 nearly tripled its planned three-year lifespan, operating for a total of eight and a half years. For its part, its twin, ERS-2, continued its monitoring mission until September 2011.

Cooperation at the turn of the millennium: American-French Topex/Poseidon

By 1992, both the United States and Europe had accumulated their own experience in launching and operating oceanographic satellites, and both were interested in launching their first full-scale collaboration in satellite oceanography. This mission, named TOPEX/Poseidon, involved specialists from NASA, as well as experts from the French space agency CNES (Centre national d’études spatiales).

The seeds of this initiative were planted in the late 1970s and early 1980s, shortly after the loss of Seasat-1. The driving force behind the mission was the U.S. TOPEX program (Ocean Topography Experiment), which was a direct continuation of the scientific developments begun with Seasat-1. Both projects involved key scientists from the Jet Propulsion Lab (JPL). French experts from CNES became a crucial international partner by providing their own, more compact and innovative radar altimeter, Poseidon.

As a result, the TOPEX/Poseidon spacecraft was equipped with two separate radar altimeters:

• TOPEX — a dual-frequency radar operating simultaneously in two frequency bands (C and Ku). This allowed effective correction of signal delays caused by the ionosphere, which was key to achieving far greater accuracy compared to previous oceanographic satellites.
• Poseidon-1 — a single-frequency, more compact, and energy-efficient radar produced by CNES, primarily used to validate the reliability of new, miniaturized technologies, while also providing supplementary data to complement TOPEX measurements.

For the mission’s success, it was crucial to place the satellite into a high orbit with a precise repeat cycle. TOPEX/Poseidon was therefore positioned at an altitude of 1,336 km above Earth, with a repeat cycle every 10 days. For the scientific team managing the project, precise orbital repetition was essential for distinguishing the Earth’s stable geoid shape from variable topographic signals caused by ocean currents, tides, and other phenomena.

The TOPEX/Poseidon satellite mission was launched on August 10, 1992. Almost immediately after the satellite began transmitting its first observation data to Earth, it became clear on both sides of the Atlantic that a new “gold standard” in satellite oceanography had been established. Sea level measurement accuracy reached 4.2 cm, a phenomenal result for satellite sensors at the time.

The satellite regularly monitored the smallest changes in major currents such as the Gulf Stream and Kuroshio, as well as their smaller, energetic eddies, improving ocean modeling and weather forecasting. Its high accuracy and continuous global coverage enabled the creation of some of the world’s most precise models of ocean tides. TOPEX/Poseidon was the first to measure the average global rate of sea level rise, and by the end of 1997/1998, it successfully predicted a major climate event: the El Niño and La Niña hurricane cycles.

As it turned out, the planned mission duration of 3–5 years was exceeded many times over. Launched in 1992, the TOPEX/Poseidon satellite officially ended its mission only on January 18, 2006, although it had effectively been deactivated since October 2005. Moreover, the mission only ended due to an anomaly in the satellite’s orientation system, which prevented TOPEX/Poseidon from maintaining its precise orbit. The oceanographic satellite became a striking example of scientific and technological synergy between American precision altimetry technology (TOPEX) and French innovations in miniaturizing radar altimeters (Poseidon).

Since then, the 21st century has seen a whole series of satellite missions launched to monitor ocean waters and polar ice sheets. These missions have helped open the scientific community’s eyes to the issues of climate change and global warming. Check out our next article to learn about the JASON and Sentinel-6 satellites, as well as modern methods of satellite oceanography.