Introduction<\/p>\n
On November 16, 2025, the Sentinel-6B satellite 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 \u2013 the European Space Agency (ESA), the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT), the French Centre National d\u2019\u00c9tudes Spatiales (CNES), and the European Commission. Its objective is to continue collecting data to extend the ocean height record, which was started in 1992 with the U.S.\/French TOPEX\/Poseidon<\/a> satellite mission. During the past three decades, NASA and its partners have operated a satellite in the same orbit, precisely tracking the height of the oceans across the globe, once every 10 days.<\/p>\n Sentinel-6B took to the skies almost five years to the day after its twin, Sentinel-6A, which launched November 20, 2020, also from VSFB, and was renamed Sentinel\u20136 Michael Freilich, honoring the former head of NASA\u2019s Earth Science Division \u2013 see The Editor\u2019s Corner<\/a> [March\u2013April 2020, 32:1, 1\u20132]. Together, the two missions comprise the international Sentinel-6\/Jason – Continuity of Service (CS) mission, which will provide continuity with past missions from TOPEX\/Poseidon through Jason-3. Sentinel-6B will continue to measure sea level to about one inch (2.5 cm), extend the record of atmospheric temperatures, and continue sea level observations through the end of the 2020s.<\/p>\n The article that follows briefly introduces Sentinel-6B\u2019s payload (which is the same as Sentinel\u20136 Michael Freilich).\u00a0It then describes the planned science applications of the mission, followed by a brief conclusion.<\/p>\n Sentinel-6B Payload<\/p>\n The Sentinel-6B satellite carries several instruments to support the mission\u2019s science goals \u2013 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 ocean, which affects the travel speed of radar pulses, providing a critical correction to the distance measured by the radar. Other onboard instruments are used to precisely determine the satellite\u2019s position [e.g., Doppler Orbitography by Radiopositioning Integrated on Satellite<\/a> (DORIS) and Laser Retroreflector Array<\/a>]. The height of the ocean surface can be calculated by combining the satellite\u2019s position with the distance to the ocean.\u00a0In addition, S- and X-band antennas perform data downlinks, and a solar array supplies power.<\/p>\n Beyond these instruments, Sentinel-6B contains Global Navigation Satellite System Radio Occultation<\/a> (GNSS-RO) instrument that will aid with weather prediction. Observations made between the spacecraft instrument and other GNSS satellites as they disappear over Earth\u2019s limb, or horizon, will provide detailed information about variations in the layers of the atmosphere. This information will contribute to computer models that predict the weather and enhance forecasting capabilities.<\/p>\n Sentinel-6B Science<\/p>\n The subsections that follow give a short preview of Sentinel-6B\u2019s science capabilities, which are identical to those of Sentinel-6 Michael Freilich and similar \u2013 albeit enhanced \u2013 to the capabilities of previous satellite altimetry missions.<\/p>\n Measuring Ocean Height<\/p>\n Ocean height is a critical measurement because it provides a host of information about the movement of surface currents, transfer of energy around the planet, and an early warning system for large-scale climate phenomena, like El Ni\u00f1o\u2013Southern Oscillation (ENSO) \u2013 see further discussion of ENSO below. Satellites obtain this data using altimeters, which send a radar pulse to the ocean surface every second and measure the time it takes to return. Pairing these data with the satellite\u2019s precise location provides a measure of the height of the ocean water with an accuracy of within a few centimeters.<\/p>\n But the simplicity of the measurement belies the volumes of information that can be gleaned from the height of the oceans. As water moves from one place to another, it tilts the surface of the ocean, and by measuring this tilt the sea level satellites allow scientists to calculate ocean currents \u2013 see Figure 2.<\/p>\n Tracking the Expansion and Contraction of Water in the Ocean<\/p>\n Ocean height data also provide information about ocean water temperature. Since water expands as it warms, a warm patch of ocean measures several inches taller than a cold patch \u2013 see Figure 3. Ocean height measurements thus can be used to reveal how the ocean stores and redistributes heat and energy, which are key drivers of Earth\u2019s climate.<\/p>\n By observing ocean heights, Sentinel-6B will help improve forecasters\u2019 ability to predict storm intensity and scientists\u2019 ability to track long-term trends in heat storage. Information on ocean height also outlines ocean currents, eddies, and tides, which helps scientists understand how heat, nutrients, carbon, and energy are transported around Earth. These observations are essential for understanding Earth\u2019s energy balance, ocean circulation, and the role of the ocean in shaping weather and climate patterns.<\/p>\n Using Ocean Height Measurements to Track ENSO<\/p>\n The movement of heat within the ocean is linked to weather and climate conditions across the globe. For reasons not completely understood, the waters of the Pacific Ocean experience a periodic fluctuation between warm and cool in the eastern tropical Pacific; this cycle is called ENSO.\u00a0During an El Ni\u00f1o event in the Pacific Ocean, unusually warm water (which is visible in the satellite data as higher than normal sea levels) builds up along the equator in the east. The pool of warm water shifts rainfall patterns across the United States and Canada. This change is telescoped around the globe, altering normal weather patterns. Conversely, La Ni\u00f1a events develop when cooler waters accumulate along the eastern Pacific (and hence, lower than normal sea levels). In this way, the satellite observations of sea level help scientists and forecasters better see how the ocean is changing and the type of weather conditions to expect in the coming months \u2013 see Figure 4.<\/p>\n Higher sea levels usually mean warmer waters, not just at the surface, but over a range of depths. This means that high sea levels can also herald rapidly intensifying storms. Meteorologists can use this information when tracking tropical storms that gain energy from warm patches of ocean water and intensify into hurricanes – often rapidly.<\/p>\n Monitoring Ocean Changes<\/p>\n Sentinel-6B can also monitor changes in sea level. Over 90% of the heat trapped by the Earth is stored in the oceans. That heat warms the water, which takes up more space and accounts for about one-third of the observed global rise in sea level. The remainder is driven by melting glaciers and ice sheets, which add water to the oceans as well. The result is a long-term rise in sea level by more than 10 cm (4 in) since the early 1990s, when TOPEX\/Poseidon was launched.<\/p>\n A record of global mean sea level change for the past three decades reveals an annual oscillation that reflects the natural movement of water between the ocean and the land, much like the heartbeat of the planet \u2013 see Figure 5. The rate of rise is not steady.\u00a0The change in sea level in the 1990s was less than half the rate of rise in the most recent decade.<\/p>\n Conclusion<\/p>\n This unbroken record of sea level change stands as a crowning achievement to the accuracy, stability, and consistency of a series of satellite missions across 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 stretch the record to nearly 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 no doubt extend through 2030 and beyond. Although Sentinel-6B is nearly identical to its predecessor, a broad community of scientists, forecasters, operational users, and policymakers anxiously await its observations and the discoveries and utility they will bring through the remainder of this decade.<\/p>\n Joshua Willis
NASA\/Jet Propulsion Laboratory
joshua.k.willis@jpl.nasa.gov<\/a><\/p>\n