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Sentinel-3 measures three broad things: reflected light (ocean and land colour), surface temperature, and surface height or topography. Its instruments turn those observations into products for water quality, ecosystems, vegetation, fires, sea level, waves, sea ice, ice sheets, rivers and lakes. It does not use one camera or produce one type of image; each instrument senses a different physical property, and processing converts the measurements into usable environmental data.
What does Sentinel-3 measure?
Copernicus Sentinel-3 is a multi-instrument Earth-observation mission focused on systematic measurements of oceans, land, ice and related atmospheric effects. Its central measurement families are:
- Colour and reflectance: reflected sunlight measured over ocean and land.
- Surface temperature: sea-surface and land-surface temperature, plus thermal observations of active fires.
- Surface topography: sea-surface height, waves, wind-related measurements and water or ice elevation.
These are satellite observations. Terms such as ocean productivity, crop condition, land-cover change or river monitoring describe products and applications derived from the observations, not additional quantities measured by a separate “product sensor.”
Sentinel-3 also acquires data systematically rather than selecting occasional scenes. That repeat coverage supports environmental monitoring, climate analysis and ocean forecasting. The mission is operated jointly by the European Space Agency (ESA) and EUMETSAT, with ESA handling land products and EUMETSAT handling marine products as described in the mission material.
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Sentinel-3 instruments at a glance
| Instrument | Physical quantity | Technical characteristics | Typical products and uses |
|---|---|---|---|
| OLCI (Ocean and Land Colour Instrument) | Reflected light and spectral reflectance | 21 bands from 400–1020 nm; 300 m spatial resolution; 1270 km swath | Ocean colour, water quality, ecosystems, vegetation, crops, land cover, aerosols and clouds |
| SLSTR (Sea and Land Surface Temperature Radiometer) | Sea- and land-surface temperature; thermal fire signals | 500 m visible/near-infrared and short-wave-infrared resolution; 1 km thermal-infrared resolution; accuracy better than 0.3 K | Daily global surface temperature, active-fire detection and fire radiative power |
| SRAL (Synthetic Aperture Radar Altimeter) | Surface height, waves and related radar returns | Dual-frequency Ku- and C-band radar; approximately 300 m along-track resolution after SAR processing | Sea level, wave conditions, sea ice, ice-sheet, river and lake height |
| MWR (Microwave Radiometer) | Atmospheric water vapour for correction | Supports SRAL rather than replacing it as a height sensor | Atmospheric correction that improves altimetry performance |
How OLCI measures ocean and land colour
It measures reflected light, not an ordinary photograph
OLCI records reflected sunlight in 21 distinct spectral bands covering 400–1020 nanometres. Different materials reflect and absorb light differently across that range. Algorithms use those spectral patterns to produce ocean-colour, land-reflectance and vegetation products.
Consequently, “colour” in Sentinel-3 documentation means multispectral information interpreted into geophysical measurements. It is not limited to a true-colour picture that looks like a photograph to the human eye. Atmospheric correction and other processing are important because the signal reaching the satellite has passed through the atmosphere and may be affected by clouds or aerosols.
Ocean applications
OLCI observations support estimates and maps related to water quality and ocean ecosystems. Spectral changes can indicate suspended material, biological activity and other properties used in monitoring coastal and open-ocean conditions. The instrument’s broad swath helps build regular coverage for regional and global marine products.
Land applications
Over land, OLCI reflectance feeds vegetation indicators, crop-management information and land-cover mapping. The same spectral measurements can also contribute to estimates of atmospheric aerosols and cloud characteristics. These outputs should be understood as processed products: OLCI measures radiance or reflectance, while an index such as a vegetation indicator is calculated from those measurements.
How SLSTR measures sea and land surface temperature
Surface temperature is not air temperature
SLSTR measures the temperature of the observed surface—the skin of the sea or land—not the temperature of the air above it. That distinction matters when comparing satellite products with weather-station readings, which generally describe air temperature at a specified height.
ESA reports that SLSTR measures global sea- and land-surface temperatures every day, with accuracy better than 0.3 kelvin under the stated instrument specifications. A kelvin difference has the same size as a Celsius-degree difference, although the absolute temperature scales have different zero points.
Visible, infrared and dual-view observations
The instrument combines visible, near-infrared, short-wave-infrared and thermal-infrared channels. Visible, near-infrared and short-wave-infrared channels have 500 m spatial resolution; thermal-infrared channels have 1 km resolution. SLSTR uses a dual-view, along-track scanning design, observing the same area from two viewing angles to improve retrieval of surface properties and temperature.
Fire detection and fire radiative power
Two dedicated thermal-infrared channels are optimized for active-fire detection and fire-radiative-power measurements. Those channels allow products that identify burning areas and estimate the energy being emitted. A fire product is therefore a thermal interpretation of SLSTR data, not a separate optical photograph of the flames.
How SRAL and MWR measure surface topography
Radar altimetry over ocean and inland water
SRAL is a synthetic-aperture radar altimeter operating at Ku and C bands. It sends radar signals toward the surface and derives height-related information from the returned signal and the satellite’s precisely known orbit. ESA gives an approximate 300 m along-track resolution after SAR processing.
Over the oceans, the topography package measures sea-surface height, wave conditions and surface-wind-related information. The same approach extends to measurements over sea ice, ice sheets, rivers and lakes. These observations underpin products for sea-level studies, wave monitoring, sea-ice analysis and inland-water height.
Why the microwave radiometer is needed
Water vapour in the atmosphere changes the radar signal’s travel time. MWR measures atmospheric-column water vapour so the processing chain can apply an atmospheric correction to SRAL observations. MWR therefore provides essential support for the altimeter; it is not an optical imager and does not generate the same kind of colour product as OLCI.
What Sentinel-3 provides over land and ice
“Land data” is a combination of outputs from different instruments rather than one single land channel. The main categories are:
- Reflectance and vegetation: OLCI spectral observations produce vegetation indicators, crop information and land-cover products.
- Surface heat: SLSTR supplies land-surface temperature at its thermal resolution.
- Wildfires: SLSTR thermal channels detect active fires and estimate fire radiative power.
- Inland water: SRAL and its processing chain provide height observations for rivers and lakes.
- Ice: the mission contributes sea-ice extent and thickness information, as well as topography over sea ice and ice sheets.
ESA describes Sentinel-3 as providing a broader picture over land, complementing the higher-resolution land measurements of Sentinel-2. That is a role distinction, not a claim that every Sentinel-3 and Sentinel-2 product has a directly interchangeable resolution or revisit time.
Resolution, swath and revisit: what the numbers mean
| Specification | Value and qualification |
|---|---|
| OLCI spectral coverage | 21 spectral bands spanning 400–1020 nm |
| OLCI spatial resolution | 300 m |
| OLCI swath | 1270 km |
| SLSTR visible, near-infrared and short-wave-infrared resolution | 500 m |
| SLSTR thermal-infrared resolution | 1 km |
| SLSTR surface-temperature accuracy | Better than 0.3 K, according to ESA’s instrument specification |
| SRAL along-track resolution | Approximately 300 m after synthetic-aperture-radar processing |
| Average orbit altitude | 814.5 km |
| Ocean revisit context | About one day for SLSTR and about two days for OLCI in the two-satellite constellation; these are mission-level figures, not a guarantee for every location, product or download |
| Core-product latency | Typically within three hours of sensing, according to ESA’s data-products information |
Spatial resolution describes the scale represented by a measurement; it does not mean that every feature at that size can be identified reliably under all conditions. Clouds, viewing geometry, surface conditions and the product algorithm can affect usability.
How a Sentinel-3 measurement becomes a usable product
- Acquisition: an instrument records reflected light, emitted thermal radiation or returned radar energy as the satellite passes overhead.
- Calibration and geolocation: processing converts instrument counts into calibrated measurements and assigns them to locations on Earth.
- Atmospheric and geometric correction: algorithms account for effects such as atmospheric scattering, water vapour, viewing angle and the satellite’s motion.
- Geophysical retrieval: measurements are transformed into products such as surface temperature, reflectance, chlorophyll-related ocean information, fire radiative power or water height.
- Application analysis: scientists, agencies and software systems combine products over time to study trends, forecasts, ecosystem conditions, land use or hazards.
This chain explains why a mission page may list both a direct measurement and an application. “Sea-surface temperature” is a retrieved geophysical product from SLSTR observations; “ocean ecosystem monitoring” is a use of products derived from OLCI and other contextual data.
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What is the difference between Sentinel-2 and Sentinel-3?
Both missions observe Earth’s land surface, but they serve different monitoring roles. Sentinel-3 is designed for systematic, broad coverage of oceans and land, combining colour, temperature and topography instruments. Sentinel-2 supplies higher-resolution land measurements. In practical terms:
| Question | Sentinel-3 | Sentinel-2 |
|---|---|---|
| Primary emphasis | Oceans, broad land monitoring, surface temperature and topography | High-resolution land observation |
| Measurement families highlighted here | Colour/reflectance, surface temperature, radar altimetry | Land-imaging measurements |
| Best-known Sentinel-3 strength | Large-area, repeat monitoring and marine products | More detailed land measurements |
| Direct resolution comparison | Not a single uniform value because the instruments measure different quantities | Not stated in the cited mission material as a single comparison value |
The missions are complementary. A project might use Sentinel-3 for frequent, wide-area temperature or water observations and Sentinel-2 for more detailed land mapping, rather than choosing one satellite for every task.
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Current constellation
ESA’s current mission information lists Sentinel-3A launched on 16 February 2016, Sentinel-3B on 25 April 2018 and Sentinel-3C on 15 September 2026. The stated average orbit altitude is 814.5 km. Revisit figures depend on the constellation and on the instrument, latitude, product and acquisition geometry.
Who can use the data?
ESA describes Sentinel-3 data as free of charge and open to users worldwide. Its mission facts point users to Copernicus Data Space for access. Operational interfaces and product availability can change, so users should check the current catalogue and documentation when planning a download.
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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsCore products are typically available within three hours of sensing. “Typically” is important: latency can vary by product, processing status, ground-segment operations and delivery system. Marine and land products may also appear through different operational channels because ESA and EUMETSAT divide processing responsibilities.
How to choose the right Sentinel-3 measurement
- Choose OLCI when the question concerns spectral water or land properties, vegetation, crop condition, water quality or land cover.
- Choose SLSTR when the question concerns sea- or land-surface temperature, heat patterns, active fires or fire radiative power.
- Choose SRAL when the question concerns sea level, waves, sea-ice or ice-sheet topography, or the height of rivers and lakes.
- Use MWR-supported altimetry when atmospheric water vapour correction is relevant to a height measurement.
Before interpreting a product, check its processing level, spatial resolution, observation geometry, cloud or quality flags and time of acquisition. A 300 m OLCI pixel, a 1 km SLSTR thermal pixel and an approximately 300 m SRAL along-track result are not interchangeable units: they represent different sensing methods and physical quantities.
Why Sentinel-3 matters
Sentinel-3’s value comes from combining complementary measurements on a repeatable schedule. OLCI describes spectral reflectance, SLSTR describes surface heat and fire signals, and SRAL with MWR describes height and atmospheric effects on radar ranging. Together they support a broad view of ocean state, land condition, ice and inland water.
“All of Sentinel-3’s instruments acquire data systematically.”
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That systematic design is what turns individual satellite passes into time series used for environmental monitoring, climate work and operational forecasting. The mission’s measurements are most useful when their physical meaning, resolution and processing limits are kept explicit.
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