Sentinel – 6A, now named “Sentinel – 6 Michael Freilich” after NASA's Earth Sciences Division retired director, a former researcher at the NASA Jet Propulsion Laboratory, and professor and associate dean at Oregon State University's College of Earth, Oceanic, and Atmospheric Sciences. Dr. Michael Freilich extensively contributed not only to Sentinel -6 but to Earth science worldwide. 

Sentinel – 6 is part of the European Union's Earth Observation Program, Copernicus, and was possible due to international cooperation among ESA, the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT), NASA, and the National Oceanic and Atmospheric Administration (NOAA), with funding support from the European Commission and technical support from France's National Center for Space Studies. As typical in some of the Copernicus missions, Sentinel-6 is composed of two identical satellites, with Sentinel-6B following its twin five years after (2025) the first launch (scheduled for this November 21st). 

Sentinel-6 Science Goals

The Sentinel-6/Jason-CS (Continuity of Service) will provide data on Earth's changing oceans currently supplied by the French-US Jason series of ocean topography satellites reaching the end of their lives. Global sea-level measurements have also been done by Topex-Poseidon, ERS, Envisat, CryoSat-2, and Copernicus Sentinel-3.  

Sentinel-6 will map up to 95% of Earth's ice-free ocean every ten days, collecting accurate sea surface height measurements. Due to the extension of oceans covering 70% of Earth, global and systematic measurements are only possible using satellites. It will provide data on rising seas caused by climate change, ocean currents, wind speed, and wave height, vital for maritime safety and meteorology, enhancing weather and hurricane forecasts. The data will support operational oceanography, environmental protection, and ocean-resource, and coastal management. It will be especially helpful for planning for floods and policymaking, protecting the lives of those in vulnerable areas. 

Even though we might not see it when visiting the beach, sea-level rise and storm flood affect coastlines worldwide, threatening low-lying countries like the Netherlands, and coastal communities, including multiple megacities. 

The mission will also collect precise atmospheric temperature and humidity data. Climate change affects Earth's oceans and surface, impacting all atmosphere levels, from the troposphere to the stratosphere. Combined with existing data from similar instruments, Sentinel-6 will help better understand how Earth's climate changes over time and understand and mitigate the consequences of sea-level rise. 

Sea level raised on a global average by 3.2mm a year between 1993 and 2018. However, it has been accelerating over the past years. While the global average trend is towards rising levels, there are many regional differences, and in some places, it is decreasing. It is equally important to understand trends and averages as regional differences. Oceans and atmosphere are inextricably connected. The sea absorbs more than 90% of the heat from greenhouse gases, causing thermal expansion of seawater. This expansion is responsible for about one-third of today's sea-level rise, while ice loss from the continental glaciers and the polar ice sheets is responsible for about 45%; data obtained from satellites measuring the sea surface temperature and changing ice. The discharge from water bodies on land also contributes to sea-level rise, but how much is unknown. Also, there are many local differences, which can be caused by events such as El Niño, for example. 

Sentinel-6 Michael Freilich has about 1.5 tonnes, and it is about the size of a small pickup truck. It counts with six critical scientific instruments. Two of them will measure the distance from the satellite to the ocean's surface. Three will precisely determine the satellite's position in orbit, and a sixth will vertically measure the atmosphere's temperature and humidity. 

Sentinel-6 Instruments  

For providing millimeter precision and timely observations of the global ocean's topography, each Sentinel-6 satellite has the Poseidon-4 radar altimeter as the primary sensor. It will observe sea levels and ice thickness in the polar regions, measure surface wind speed, sea state, and geostrophic ocean currents. The radar works by measuring the time taken for radar pulses to travel to Earth's surface and back to the satellite. Additionally, the satellite carries several navigation instruments, and precise satellite location data combined with altimetry measurements provide water, land, and ice heights. 

Sentinel-6 is counts with an advanced microwave radiometer (AMR-C) to observe water vapor in the atmosphere, which impacts the speed of the altimeter's radar pulses. Together the instruments will enable researchers to see smaller and more complicated ocean features, especially near the coastlines. 

Sentinel-6 also has the Global Navigation Satellite System - Radio Occultation (GNSS-RO) instrument tracks radio signals from navigation satellites that orbit Earth, and it measures Earth's atmosphere's physical properties. When a satellite drops below (or rises above) the horizon, the radio signal passes through the atmosphere; during this process, the signal slows, its frequency changes, and its path bends, an effect called refraction. Scientists use this data to measure minute changes in atmospheric density, temperature, and moisture content. Sentinel-6 will detect these signals through the atmosphere's vertical extent, from the top and almost to the ground, through thick clouds, which is vital because weather phenomena develop in all atmosphere's layers, not only from near Earth's surface where we feel their effects. It will allow accurate to determine the temperature, pressure, and humidity through the atmosphere's layers, giving incredible insights into our planet's dynamic climate and weather. 

Although error margin exists in every scientific measurement, collecting data from satellites at the top atmosphere's layer, near a vacuum situation, and no signal refraction, avoid bias in the weather predictions. Currently, meteorologists gather information from various sources and need to compensate for biases in the data.  

Sentinel-6 International Cooperation

NASA JPL, a Caltech division, contributes with three science instruments: the AMR-C, the GNSS-RO, and the Laser Retroreflector Array. NASA also will contribute with launch services, ground systems for the operation of NASA's science instruments, and science data processors for two of these instruments. 

Sentinel-6 A will launch on a Space X Falcon 9 rocket from California, in the United States. ESA's Kiruna ground station will track the spacecraft's first days, and the North Pole Satellite Station in Alaska will receive the spacecraft's early signals after separation from the launcher. 

Once safely in orbit, ESA's ESOC Operations Centre in Darmstadt, Germany, will take over for the next three days. The mission control team will guide the Launch and Early Orbit Phase, the riskiest phase due to space hazards exposure. It is when the satellite unfolds its solar arrays, wakes up to test its core functioning. It is also when teams at ESOC will perform two-orbit maneuvers into the correct path. Sentinel-6 needs to fly in tandem with Jason-3, separated by 30 seconds, about 230 kilometers until the older satellite is moved to another orbit. That makes this operation more complicated, and timing here is critical. ESA's Space Debris Office will monitor and calculate the risk of collisions with space debris and advise how to keep the mission safe on earlier days.  

Once the initial phase is over, EUMETSAT will complete the orbit acquisition, taking over command, control, commissioning responsibility, routine operations, and distribution of the mission's data. 

To know more, visit NASA's and ESA's website.  And don’t forget to follow us on Linkedin, Twitter, or Facebook to get our updates. 

This article was written by Juliane Verissímo - Marketing Department of VisionSpace.