Rising seas are no longer some far-off concern, but rather a measurable, accelerating reality. On November 17, 2025, Sentinel-6B started its watch from 830 miles above the Earth, continuing a record of sea level measurements that is more than three decades long and vital to our understanding of ocean dynamics and climate change.
The Sentinel-6B launched at exactly 9:21:42 pm PST from Space Launch Complex 4E at Vandenberg Space Force Base atop a SpaceX Falcon 9. It marked the 500th flight of a previously flown Falcon 9 booster, with first stage B1097 concluding its third mission. After staging, the booster conducted a boost-back burn, entry burn, and final landing burn to land at LZ-4, only 1,400 feet away from where it took off—a demonstration of the engineering that goes into making SpaceX rockets reusable.
The spacecraft, built by Airbus Defense and Space in Germany, separated from the Falcon 9’s second stage 57 minutes after liftoff following two upper-stage burns. It is 5.82 meters long and weighs 1,190 kilograms. Sentinel-6B carries the Poseidon-4 radar altimeter and NASA’s Advanced Microwave Radiometer for Climate (AMR-C). The Poseidon-4 transmits Ku-band and C-band microwave pulses toward the ocean surface and times their return to determine sea surface height with accuracy better than 3 centimeters. The dual-frequency system corrects for ionospheric interference. AMR-C measures atmospheric water vapor to further refine the satellite’s altimetry data.
Similarly, highly accurate orbit determination is required: the position of the Sentinel-6B satellite itself is fixed to within about 1 centimeter by the three independent systems comprising the DORIS receiver, the GNSS-POD package, and Laser Retroreflector Array. This positional accuracy, in combination with radar range measurements, enables the determination of global mean sea level on millimeter scales over the 10-day repeat cycle of the satellite.
This $1-billion mission is an international collaboration shared among NASA and its European partners: ESA, EUMETSAT, NOAA, the European Commission, and CNES. Sentinel-6B follows on from Sentinel-6 Michael Freilich, launched in 2020; it will first fly in tandem with its twin but 30 seconds later, to cross-calibrate measurements, before eventually taking over as the reference altimetry mission. This continuity preserves a dataset which began in 1992 with TOPEX/Poseidon and continued through the Jason series, chronicling an average global sea level rise of about 3 millimeters per year—now closer to 4 millimeters per year. On its 66-degree-inclined orbit, Sentinel-6B will map up to 95 percent of Earth’s ice-free oceans every 10 days.
The cloud-penetrating capability of the radar altimeter will enable measurements only a few kilometers away from coastlines, supporting operational oceanography, coastal hazard assessment, and weather forecasting. The data products will be ingested into NOAA’s Ocean Prediction Center, hurricane intensity models, and STAR’s Ocean Heat Content algorithms, improving forecasts of tropical storm development. Beyond ocean height, the Sentinel-6B GNSS Radio Occultation payload will profile atmospheric temperature, pressure, and humidity by tracking the signals from navigation satellites as they bend, or refract, through the atmosphere.
These profiles sharpen climate models and improve the accuracy of weather forecasts. Open data from the mission means that scientists, as well as maritime and coastal operators around the world, can draw upon its measurements for uses that range from ensuring navigation safety to managing flood risk. “Sentinel-6B will ensure that we continue to collect the high-precision data needed to understand our changing climate, safeguard our oceans and support decisions that protect coastal communities around the world,” said ESA Director of Earth Observation Programs Simonetta Cheli.
EUMETSAT Director-General Phil Evans added that in a generation, sea-level rise has almost doubled and now puts 900 million people living in coastal regions at risk. By bringing together the latest in radar altimetry, ultra-accurate orbital tracking, and global cooperation, Sentinel-6B takes this fundamentally essential Earth observation legacy further into the future. Its measurements will track not just the pace of rising seas but also deliver actionable intelligence to adapt infrastructure, protect ecosystems, and maintain maritime safety in an era of rapid environmental change.
