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Everything You Should Know About Sentinel-6

UPDATE: Sentinel-6 successfully launched on Saturday, November 21st. To see our photo gallery and launch video, click here.

The latest in ocean and atmosphere observation technology is going to be taking to the skies in November to contribute to nearly 30 years of continuous sea-level data collected by the European Space Agency. Here is everything you should know about the 'Sentinel-6 Michael Freilich' satellite.

Artist's rendering of Sentinel 6 during fairing separation (ESA - P. Carril)


The ESA and NASA have been using radar beamed down to the ocean floor from satellites in orbit since the early 90's to measure sea level accurately to less than a millimeter. Beginning with the TOPEX-Poseidon satellite launched in August of 1992, we have been able to track sea-level rise and include it in our broader understanding of climate change. While its original mission was planned for only three years, its revolutionary capabilities went on to contribute over 10 years of data including year-to-year changes in ocean temperature, the first global tide map, and even improving our understanding of Earth's gravity field. The project was directed by both NASA and CNES, the French government space agency.

This legacy continued throughout the 2000's with the Copernicus satellite constellation. The "Jason" series of satellites provided continuous ocean-topography information beginning with 'Jason-1' in 2001. Now as 'Jason-3' nears the end of its lifespan it will be replaced by 'Sentinel-6 Michael Freilich', an international cooperative project between NASA and the European Space Agency, NOAA, EUMETSAT, AirBus, and SpaceX. The spacecraft is named after Dr. Michael Freilich, the former director of NASA's Earth Science Division, who passed on August 13th, 2020.

Sea level is one of the key measurements required for analyzing climate change, and most important is how these values change over time. Since we have been collecting continuous and precise sea level data over the course of several decades we can make accurate predictions for what we can expect in the decades to come.

These measurements indicate that not only is sea level definitively rising, but in recent years the rate of that rise seems to be increasing. This information is critical to communities along coastlines all around the world as they craft legislation and plan for the future. In July 2018 the UK National Oceanographic Centre published a study which suggests that flooding from rising sea levels could cost more than $14trillion worldwide annually by 2100 if current rising trends persist.

'Sentinel-6' is equipped with several different instruments developed by the NASA Jet Propulsion Laboratory to help us better understand our changing oceans and atmosphere. The satellite uses a radar transmitter to pulse the Earth, it's echo precisely measures the distance to the sea surface and helps to determine wave height and surface wind speed based on the roughness of the swells. Sentinel is carrying the Poseidon 4 Radar Altimeter as well as a new Advanced Microwave Radiometer that will help researchers see more complex ocean features than ever before, especially near coastlines. It will map 95% of the planet's oceans every 10 days at an orbital altitude of 830.2 miles.

But what makes 'Sentinel-6 Michael Freilich' truly unique from its predecessors is a little trick it does to help see into our atmosphere with impressive detail. The 'Global Navigation Satellite System - Radio Occultation (GNSS-RO) instrument tracks the radio signals of navigation satellites elsewhere in orbit, watching them dip below and rise above the horizon. As those radio signals pass through the atmosphere they change frequency and their paths bend, the same way refraction bends a straw in water. From this data, scientists are able to detect minute changes in atmospheric density, temperature, and moisture content. When this data is combined with information from other sources it gives researchers a better understanding of how the Earth's climate is changing over time and contributes to more accurate weather forecasts.


"Like the long-term measurements of sea level, we also need long-term measurements of our changing atmosphere to better understand the full impacts of climate change," said Chi Ao, the GNSS-RO instrument scientist at JPL. "Radio occultation is a wonderfully precise and accurate way to do that."


Sentinel-6 in the iABG cleanroom in 2020 (ESA–S. Corvaja)


The Sentinel-6 vehicle is developed by AirBus. You may know them from their popular line of commercial airliners, but the company has gained extensive experience in space since the launch of their first Earth observation satellite1986. None of their spacecraft have ever failed in orbit. Eumetsat is responsible for flight operations and interpreting data throughout the mission for European users, while the NOAA will process data for users in the US. On September 24th the vehicle was delivered to Vandenberg Air Force Base in Lompoc, California where it will undergo final checks before launch. 'Sentinel-6 Michael Freilich' also has a twin, known as 'Sentinel-6B', which is expected to follow suit in 2025.

SpaceX will be launching the mission from pad SLC-4 at Vandenberg. This will be the company's 16th launch from the West Coast facility, which caters exclusively to missions with polar orbits. They will also be attempting a 'Return to Launch Site' landing of the previously un-flown first stage. After the rocket places the second stage into position for an orbital burn, the first stage performs a "boostback" maneuver to aim itself back towards land. It will touch down on a landing pad just under half a mile from where it took off.

SpaceX will be using the 'Sentinel-6' launch to validate changes being made to the Falcon 9's engines before proceeding with NASA's upcoming Dragon Crew-1 mission to the International Space Station. Initially scheduled for Oct. 31st, Crew-1 has been delayed while NASA and SpaceX conduct tests to confirm the safety of Falcon 9 for human flight following an automatic abort on October 3rd only two seconds before the liftoff of a GPS-III navigation satellite.

SpaceX CEO Elon Musk responded on Twitter, saying that there was an "unexpected pressure rise in the turbomachinery gas generator." This component burns a small amount of fuel to operate a turbine, and this turbine primes the turbopumps injecting fuel into the combustion chamber. Since this abort they have successfully launched a few batches of Starlink satellites, and marking the company's 100th successful flight.

But the Falcon 9 that will fly 'Sentinel-6 Michael Freilich' will have one of its engines replaced, and will contribute to what has been described as a "tremendous amount of testing" by Tim Dunn, director of NASA's Launch Services Program at Kennedy Space Center. 'Sentinel-6' is currently scheduled for 12:17 p.m PST on November 21st.

Falcon 9 after liftoff. (SN–Ryan Bale)


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