Sentinel-3 is an Earth observation heavy satellite

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developed by the

European Space Agency The European Space Agency (ESA) is a 23-member International organization, international organization devoted to space exploration. With its headquarters in Paris and a staff of around 2,547 people globally as of 2023, ESA was founded in 1975 ...

as part of the Copernicus Programme. As of 2024, it consists of 2 satellites: Sentinel-3A and Sentinel-3B. After initial commissioning, each satellite was handed over to EUMETSAT for the routine operations phase of the mission. Two recurrent satellites, Sentinel-3C and Sentinel-3D, will follow in approximately 2025 and 2028 respectively to ensure continuity of the Sentinel-3 mission. Each Sentinel-3 satellite is designed to operate for seven years in a sun-synchronous low-Earth orbit. The satellites use multiple sensors to measure topography, temperature, marine ecosystems, water quality, pollution, and other features for ocean forecasting and environmental monitoring.

Overview

The Sentinel-3 satellites travel in a sun-synchronous orbit at an altitude of approximately , with an inclination of 98.6° and orbit cycle of approximately 100 minutes. The local time of

descending node An orbital node is either of the two points where an orbit intersects a plane of reference to which it is inclined. A non-inclined orbit, which is contained in the reference plane, has no nodes. Planes of reference Common planes of referenc ...

is 10:00 a.m. and nominal duration is 7.5 years. A pair of Sentinel-3 satellites enables a short revisit time, allowing the earth to be imaged at least once every two days by the OLCI instrument, and at least once per day by the SLSTR instrument at the equator. This is achieved using both Sentinel-3A and Sentinel-3B satellites in conjunction. The satellite orbit provides a 27-day repeat for the topography package, with a 4-day sub-cycle.

Pre-launch

On 14 April 2008, the European Space Agency and

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signed a contract to build the first GMES Sentinel-3 in its Cannes Mandelieu Space Center. Bruno Berruti led the team that was responsible for delivering the Copernicus Sentinel-3 satellites from the drawing board into orbit. The satellite platform was delivered to France for final integration in 2013. The communications systems were completed by Thales Alenia Space España in early 2014.

Launch

Sentinel-3A was subsequently launched on 16 February 2016 on a Rokot vehicle from the Plesetsk Cosmodrome, located near Arkhangelsk, Russia. This first launch was followed by the launch of Sentinel-3B on 25 April 2018, also aboard a Rokot. Each satellite is designed to operate for 7 years.

Objectives

The Sentinel-3 mission's main objectives are to measure sea-surface topography, land- and sea-surface temperature, land- and ocean-surface colour with accuracy in support of ocean forecasting systems, and for environmental and climate monitoring. Sentinel-3 builds directly on the heritage pioneered by ERS-2 and Envisat satellites. Near-real time data will be provided for ocean forecasting, sea-ice charting, and

maritime safety Maritime safety as part of and overlapping with water safety is concerned with the protection of life ( search and rescue) and property Property is a system of rights that gives people legal control of valuable things, and also refers to th ...

services on the state of the ocean surface, including surface temperature,

marine ecosystem Marine ecosystems are the largest of Earth's aquatic ecosystems and exist in Saline water, waters that have a high salt content. These systems contrast with freshwater ecosystems, which have a lower salt content. Marine waters cover more than 7 ...

s, water quality, and pollution monitoring. Further objectives of the mission include: *Measure sea-surface topography, sea-surface height, and significant wave height *Measure

ocean The ocean is the body of salt water that covers approximately 70.8% of Earth. The ocean is conventionally divided into large bodies of water, which are also referred to as ''oceans'' (the Pacific, Atlantic, Indian Ocean, Indian, Southern Ocean ...

and land-surface temperature *Measure ocean and land-surface colour *Monitor sea and land ice

topography Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the landforms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sci ...

*Sea-water quality and

pollution Pollution is the introduction of contaminants into the natural environment that cause harm. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). Pollutants, the component ...

monitoring *Inland water monitoring, including rivers and lakes *Aiding

marine weather forecasting Marine weather forecasting is the process by which mariners and meteorological organizations attempt to forecast future weather conditions over the Earth's oceans. Mariners have had rules of thumb regarding the navigation around tropical cyclone ...

with acquired data *

Climate Climate is the long-term weather pattern in a region, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteoro ...

monitoring and modelling * Land-use change monitoring *Forest cover mapping *Fire detection *

Weather forecasting Weather forecasting or weather prediction is the application of science and technology forecasting, to predict the conditions of the Earth's atmosphere, atmosphere for a given location and time. People have attempted to predict the weather info ...

*Measuring Earth's thermal radiation for atmospheric applications

Instruments

Sentinel-3 makes use of multiple sensing instruments:

SLSTR

SLSTR (Sea and Land Surface Temperature Radiometer) determines global sea-surface temperatures to an accuracy of better than . It measures in nine spectral channels and two additional bands optimised for fire monitoring. The first six spectral bands cover the visible and near-infrared (VNIR) spectrum as well as the short-wave infrared (SWIR) spectrum; VNIR for bands 1 to 3, and SWIR for bands 4 to 6. These 6 bands have a spatial resolution of , while bands 7 to 9 as well as the two additional bands have a spatial resolution of . For the SLSTR instrument on Sentinel-3, calibration on-board is one of the most important features for the thermal and infrared channels. This instrument has two black body targets used for calibration, one at similar temperature to the optics (about ), and one at a higher temperature (), such that the temperature range corresponds to ocean surface temperatures measured by the instrument.

OLCI

OLCI (Ocean and Land Colour Instrument) is a medium-resolution imaging

spectrometer A spectrometer () is a scientific instrument used to separate and measure Spectrum, spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomeno ...

that uses five cameras to provide a wide field of view. The OLCI is an along-track or "push broom" scanner, meaning that the

sensor array A sensor array is a group of sensors, usually deployed in a certain geometry pattern, used for collecting and processing electromagnetic or acoustic signals. The advantage of using a sensor array over using a single sensor lies in the fact that an ...

is arranged perpendicular to the path of flight. This method essentially eliminates the scale distortion near the edge of an image that is common with across-track or "whisk broom" scanners. OLCI has 21 spectral bands with wavelengths ranging from the optical to the near-infrared. Bands vary in width from 400 nm to 1020 nm, and serve a variety of different purposes, including measuring

water vapour Water vapor, water vapour, or aqueous vapor is the gaseous phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Water vapor ...

absorption,

aerosol An aerosol is a suspension (chemistry), suspension of fine solid particles or liquid Drop (liquid), droplets in air or another gas. Aerosols can be generated from natural or Human impact on the environment, human causes. The term ''aerosol'' co ...

levels, and

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absorption. SLSTR and OLCI are optical instruments with an overlap of their swath path, allowing for new combined applications. Due to climate changing factors, inland coastal regions have become an increased area of concern, and from 2002 to 2012 the Medium Resolution Imaging Spectrometer ( MERIS) provided quality observations for analysis. The OLCI improves upon the MERIS in that it was built with six additional spectral bands, higher-end

signal-to-noise ratio Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to noise power, often expressed in deci ...

(SNR), reduced solar glar, a maximum of 300 m spatial resolution, and increased ground coverage allowing it to sense

cyanobacteria Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...

levels within inland coastal ecosystems. OLCI is currently the only sensor in space able to detect cyanobacteria.

SRAL

Sea-level variations from Sentinel-3A ESA359994

SRAL ( Synthetic Aperture Radar Altimeter) is the main topographic instrument to provide accurate topography measurements over sea ice, ice sheets, rivers, and lakes. It uses dual-frequency K_u and C band and is supported by a

microwave radiometer A microwave radiometer (MWR) is a radiometer that measures energy emitted at one millimeter-to-metre wavelengths (frequencies of 0.3–300 GHz) known as microwaves. Microwave radiometers are very sensitive receivers designed to measure thermally ...

(MWR) for atmospheric correction and a DORIS receiver for orbit positioning. This allows the instrument, which is based on legacy missions such as CryoSat and the

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missions, to provide a 300-meter resolution and a total range error of 3 cm. The instrument operates its pulse repetition frequency at 1.9 kHz (low-resolution mode - LRM, real aperture radar) and 17.8 kHz (synthetic aperture radar - SAR).

DORIS

DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) is a receiver for orbit positioning.

MWR

MWR (MicroWave Radiometer) measures water vapour, cloud water content, and the thermal radiation emitted by the Earth. The MWR sensor has a radiometric accuracy of .

LRR

LRR ( Laser retroreflector) accurately locates the satellite in orbit using a laser ranging system. When used in combination with SRAL, DORIS, MWR, they will acquire detailed topographic measurements of the ocean and in-land water.

GNSS

GNSS (

Global Navigation Satellite System A satellite navigation or satnav system is a system that uses satellites to provide autonomous geopositioning. A satellite navigation system with global coverage is termed global navigation satellite system (GNSS). , four global systems are op ...

) provides precise orbit determination and can track multiple satellites simultaneously.

Satellite operation and data flow

Sentinel-3 is operated by the European Space Operations Centre (ESA) during the Commissioning Phase (E1) and EUMETSAT from the routine operations phase onwards. The in-orbit operations for Sentinel-3 are managed by EUMETSAT in Darmstadt, Germany. This includes monitoring the health of the satellite and the instruments, and coordinates housekeeping

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and commands at the main flight control center in Darmstadt, Germany. EUMETSAT maintains a Back-Up Control Centre in Torrejon, Spain. In addition, the ESA operates an x-band core station in

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, Norway. This station is responsible for receiving the data collected by Sentinel-3. The data is then analysed by the Sentinel Collaborative Ground Segment and compiled into the Copernicus Space Component (CSC). The CSC is an earth observation program run by the ESA with the objective of providing high quality continuous monitoring of the earth. EUMETSAT generates and disseminates the marine and near-real-time atmospheric data from its own facilities in Darmstadt.

Applications

Extreme temperatures in Eureka ESA22181121

The applications of Sentinel-3 are diverse. Using the collection of sensors on-board Sentinel-3 is able to detect ocean and land temperature and colour change. The Ocean and Land Color Instrument (OLCI) has a resolution with 21 distinct bands allowing global coverage in less than four days. This sensor can then be used to by researches to do water quality and land-monitoring research. The satellite also has the ability to monitor the temperature of the sea, land and ice through the Sea and Land Surface Temperature Radiometer (SLSTR). Sentinel-3 also has the ability to detect changes in sea-surface height and sea-ice using the

synthetic aperture radar Synthetic-aperture radar (SAR) is a form of radar that is used to create two-dimensional images or 3D reconstruction, three-dimensional reconstructions of objects, such as landscapes. SAR uses the motion of the radar antenna over a target regi ...

altimeter and the

, two of the most complex sensors on the satellite. The observations acquired by the mission will be used to in conjunction with other ocean-observing missions to contribute to the Global Ocean Observing System (GOOS) which aims to create a permanent system of ocean observation. *Ocean colour and land reflectance data *Sea, land, and ice surface temperature *Active fire and burnt area monitoring *Sea surface topography data

Gallery

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and the

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in January, 2021