Sentinel-6B Extends Global Ocean Height Record

Introduction

On November 16, 2025, the Sentinel-6B satellite was launched from Vandenberg Space Force Base (VSFB) in California. The mission is a partnership between NASA, the National Oceanic and Atmospheric Administration (NOAA) and several European partners – the European Space Agency (ESA), the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT), the French National Center for Space Studies (CNES) and the European Commission. Its objective is to continue collecting data to extend the ocean height record, started in 1992 with the American-French satellite mission TOPEX/Poseidon. For the past three decades, NASA and its partners have operated a satellite in the same orbit, precisely tracking the height of oceans around the world, once every 10 days.

Sentinel-6B took flight almost five years to the day after its twin, Sentinel-6A, launched on November 20, 2020, also from VSFB, and was renamed Sentinel-6 Michael Freilich, in homage to the former head of NASA’s Earth Sciences Division – see Editor’s Corner [March–April 2020, 32:1, 1–2]. Together, the two missions constitute the international Sentinel-6/Jason – Continuity of Service (CS) mission, which will ensure continuity of past missions from TOPEX/Poseidon to Jason-3. Sentinel-6B will continue to measure sea level down to about 2.5 cm, extend the record of atmospheric temperatures, and continue sea level observations into the late 2020s.

The following article briefly presents the payload of Sentinel-6B (which is the same as that of Sentinel-6 Michael Freilich). It then describes the scientific applications planned for the mission, followed by a brief conclusion.

Sentinel-6B payload

The Sentinel-6B satellite carries several instruments to support the mission’s science objectives – see Figure 1. A radar altimeter bounces signals off the ocean surface to determine the distance to the ocean. An advanced microwave radiometer (AMR) retrieves the amount of water vapor between the satellite and the ocean, which affects the speed at which the radar pulses travel, thus providing a critical correction to the distance measured by the radar. Other onboard instruments are used to accurately determine the satellite’s position [e.g., Doppler Orbitography by Radiopositioning Integrated on Satellite (DORIS) and Laser Retroreflector Array]. The height of the ocean surface can be calculated by combining the satellite position with the distance to the ocean. Additionally, S- and X-band antennas perform data downlinks and a solar panel provides power.

Beyond these instruments, Sentinel-6B contains a Global Navigation Satellite System Radio Occultation (GNSS-RO) instrument that will aid weather forecasting. Observations made between the spacecraft instrument and other GNSS satellites as they disappear above the Earth’s limb, or horizon, will provide detailed information about variations in the layers of the atmosphere. This information will contribute to computer models that predict weather and improve forecasting capabilities.

Sentinel-6B Science

The following subsections provide a brief overview of Sentinel-6B’s science capabilities, which are identical to those of Sentinel-6 Michael Freilich and similar – although enhanced – to the capabilities of previous satellite altimetry missions.

Measuring the height of the ocean

Ocean height is an essential measurement because it provides a wealth of information about the movement of surface currents, the transfer of energy around the planet and an early warning system for large-scale climate phenomena, such as El Niño – Southern Oscillation (ENSO) – see more detail on ENSO below. Satellites obtain this data using altimeterswhich send a radar pulse to the ocean surface every second and measure the time it takes to return. Combining this data with the precise location of the satellite makes it possible to measure the height of the ocean water with an accuracy of a few centimeters.

But the simplicity of the measurement belies the amount of information that can be gleaned from high above the oceans. When water moves from one place to another, it tilts the ocean surface, and by measuring this tilt, sea level satellites allow scientists to calculate ocean currents – see Figure 2.

Tracking the expansion and contraction of water in the ocean

Ocean height data also provides information on ocean water temperature. Since water expands as it warms, a warm area of ​​ocean is several inches larger than a cold area – see Figure 3. Ocean height measurements can thus be used to reveal how the ocean stores and redistributes heat and energy, which are key drivers of Earth’s climate.

By observing ocean heights, Sentinel-6B will help improve the ability of forecasters to predict storm intensity and scientists to track long-term trends in heat storage. Ocean height information also describes ocean currents, eddies and tides, which helps scientists understand how heat, nutrients, carbon and energy are transported around the Earth. These observations are essential for understanding the Earth’s energy budget, ocean circulation and the role of the ocean in changing weather and climate conditions.

Using ocean height measurements to track ENSO

The movement of heat in the ocean is linked to weather and climate patterns across the world. For reasons that are not fully understood, the waters of the Pacific Ocean experience a periodic fluctuation between warm and cold in the eastern tropical Pacific; this cycle is called ENSO. During an El Niño event in the Pacific Ocean, unusually warm water (which is visible in satellite data as being higher than normal sea level) builds up along the equator to the east. The warm water pool changes precipitation patterns in the United States and Canada. This change extends across the globe, altering normal weather conditions. Conversely, La Niña events develop when colder waters accumulate along the eastern Pacific (and therefore at below-normal levels). In this way, satellite observations of sea level help scientists and forecasters better understand how the ocean is changing and what kind of weather conditions can be expected in the coming months – see Figure 4.

Higher sea levels generally mean warmer waters, not only at the surface, but at various depths. This means that high sea levels can also herald rapid intensification of storms. Meteorologists can use this information to track tropical storms that draw energy from warm areas of ocean water and intensify into hurricanes – often quickly.

Monitoring ocean changes

Sentinel-6B can also monitor changes in sea levels. More than 90% of the heat trapped by the Earth is stored in the oceans. This heat warms the water, which takes up more space and is responsible for about a third of the observed global sea level rise. The rest is due to melting glaciers and ice sheets, which also add water to the oceans. The result is a long-term sea level rise of more than 10 cm (4 in) since the early 1990s, when TOPEX/Poseidon was launched.

A record of changes in mean sea level over the past three decades reveals an annual oscillation that reflects the natural movement of water between the ocean and land, much like the beating of the planet’s heart – see Figure 5. The growth rate is not stable. Sea level change in the 1990s was less than half the rate of increase in the most recent decade.

Conclusion

This uninterrupted record of sea level change is the crowning achievement of the precision, stability and consistency of a series of satellite missions spanning more than three decades. This approach remains one of the most successful international collaborations to study our ever-changing Earth from space, and the launch of Sentinel-6B will extend this record to almost 40 years. With a vibrant international community of several hundred scientists and expert users, the discoveries made and the value created by these observations will undoubtedly extend to 2030 and beyond. Although Sentinel-6B is nearly identical to its predecessor, a broad community of scientists, forecasters, operational users, and policy makers eagerly await its observations, and the discoveries and utility they will provide for the remainder of this decade.

Joshua Willis
NASA/Jet Propulsion Laboratory
joshua.k.willis@jpl.nasa.gov

Séverine Fournier
NASA/Jet Propulsion Laboratory
severine.fournier@jpl.nasa.gov