Sentinel-3

The Sentinel-3 platform has different sensors onboard. Its mission is to measure sea surface topography, sea and land surface temperature, and ocean and land surface colour with high accuracy and reliability. By these features Sentinel-3 supports ocean forecasting systems, environmental monitoring and climate monitoring.

Sentinel-3 Data

Sentinel-3 has 3 different sensors

SLSTR: Sea and Land Surface Temperature Radiometer - it determines global sea surface temperatures at an accuracy of better than 0.3 K. SLSTR measures in nine spectral channels and two additional bands. A 1.3 micron band aimed at cirrus detection and a 2 micron band aimed at vegetation, mineral and atmospheric corrections for very turbid waters.
OLCI: Ocean and Land Colour Instrument - with 21 bands, a design optimised to minimise sun-glint and a resolution of 300 m over all surfaces. OLCI marks a new generation of measurements over the ocean and land. The swath of OLCI and nadir SLSTR fully overlap.
SRAL: Synthetic Aperture Radar Altimeter - provides measurements at a resolution of approximately 300 m in SAR mode along track. SRAL is supported by a MicroWave Radiometer (MWR) and a Precise Orbit Determination (POD) package, a GPS receiver (GNSS), a DORIS instrument and a Laser Retro-Reflector (LRR).
Note: This SRAL data is not supported by the Sentinel-3 Toolbox. To work with this data it is recommended to use the Sentinel Altimetry Toolbox .

Also Envisat data is provided in Sentinel-3 like format (4th reprocessing)

MERIS: The MEdium Resolution Imaging Spectrometer (MERIS) was a programmable spectrometer on the Envisat mission, operating in the solar reflective spectral range. Although primarily dedicated to ocean colour observations, MERIS broadened its scope of objectives to atmospheric and land surface related studies. The OLCI sensor can considered as the successor of MERIS.
AATSR: The Advanced Along-Track Scanning Radiometer (AATSR) was one of nine instruments on board the Envisat satellite. It was the third in a series of instruments designed primarily to measure Sea Surface Temperature (SST) remotely, following on from ATSR-1 and ATSR-2 on board ERS-1 and ERS-2, respectively. All those are the ancestors of the SLSTR sensor onb board of Sentinel-3.
IMPORTANT Note:
Currently (S3TBX v9.0) the AATSR data in Sentinel-3 like format do not provide geo-location information in SNAP. Applying the provided data (v2.0.5 of 4th reprocessing) does not give good results. Therefore, it is better to provide the data without geo-location information.

Import Data

The data is usually distributed in a zip archive. This needs to be extracted first. Afterwards the data can be imported via the respective entry in the application menu at 'File / Import / Optical Sensors / Sentinel-3'. For SLSTR tow special options exist. Sentinel-3 SLSTR L1B (1km) and Sentinel-3 SLSTR L1B (500m). The 500m option will only open the bands which are provided at this resolution. The 1km options will resample all data to the resolution. When using the generic option, then the bands will be imported unmodified at their native resolution.
To import the data the xfdumanifest.xml needs to be selected in the import dialog.

Of course it is also possible to simply use the open dialog from the menu 'File / Open Product...' or the toolbar. Here by default the generic reader is used.

Another option to open Sentinel-3 data is to drag'n'drop the *.SEN3 folder or the xfdumanifest.xml files onto the Product Explorer in SNAP.

Recommended Flags

SNAP generates for OLCI Level-2 data a set of recommended flags. Those flags are defined by an group of experts, named the Sentinel-33 Quality Working Group (S3-QWG). This group observes the data and suggests means to ESA/Eumetsat to maintain high quality. It is suggested to consider these flags when working with Sentinel-3 Level-2 data.
Detailed information: Sentinel-3 Recommended Flags

Data Access

Sentinel-3 data can be obtained from different sources:

ESA/Copernicus Open Access hub

The ESA/Copernicus service provides Sentinel-3 data at Level-1 and Level-2 land data. Also Level-2 synergy data is provided. Besides Sentinel-3 data, Sentinel-1 and Sentinel-2 is provided too.
https://scihub.copernicus.eu/

EUMETSAT/Copernicus Online Data Access

The EUMETSAT/Copernicus service provides Sentinel-3 data at Level-1 and Level-2 water data. https://www.eumetsat.int/coda

DIAS cloud environments

The Copernicus Data and Information Access Service (DIAS) cloud environments provide processing resources, tools and complimentary data sources at commercial conditions to further facilitate the access to Sentinel data.

NASA/OBPG Distributed Active Archive Center for satellite Ocean Biology

The service provided by the NASA's Ocean Biology Processing Group distributes Sentinel-3 data at Level-1 and Level-2. https://oceancolor.gsfc.nasa.gov/data/overview/

References

Sentinel Online: Sentinel Online provides various resources about the Sentinel platforms, the data and tools to work with it.
https://sentinel.esa.int/web/sentinel/home