Sentinel-2

Understanding Sentinel-2 Satellite Data

This is the first entry in a series of posts about how to use data from Europe’s Sentinel-2 satellites. Stay tuned as the remaining posts are going to be published over the next couple of months. As always, feedback is very much welcome. Please let us know which topics you want us to include.

Today we start with an introduction to Copernicus and the Sentinel missions before diving into Sentinel-2 product levels and data concepts from granules to user products. How to download Sentinel-2 data and how to decode the SAFE file format is going to be described in the next posts.

Introduction to Copernicus

The Sentinel satellites are part of Europe’s ambitious Copernicus programme (formerly known as Global Monitoring for Environment and Security (GMES)). The overall mission is composed of five constellations of satellites, each targeting a different application domain.

The Sentinel missions implement two interesting and important concepts. First, they fly in constellations of multiple satellites which provide more frequent data. Second, the data is released under an Open Data policy allowing free access to images and therefore fostering enhanced knowledge generation and application developments.

  • Sentinel-1 (4 satellites) is a Synthetic Aperture Radar (C-band) mission designed to monitor sea ice zones, the Arctic environment, and the risk of land surface motion as well as generate forest, water, and soil mapping.
  • Sentinel-2 (2 satellites) is an optical mission with a multi-spectral instrument mainly for agricultural applications such as crop monitoring and management, vegetation and forest monitoring (e.g. leaf area index, chlorophyll concentration, carbon mass estimations), monitoring land cover change for environmental monitoring, observation of coastal zones (marine environmental monitoring, coastal zone mapping), inland water monitoring, glacier monitoring, ice extent and snow cover mapping.
  • Sentinel-3 (3 satellites) carries multiple instruments, mainly for sea-surface as well as sea and land ice topography, sea- and land-surface temperature, ocean- and land-surface colour, sea- and inland-water quality and pollution monitoring, land-use change monitoring, forest cover mapping, fire detection, marine and general weather forecasting, snow cover monitoring, measuring Earth’s thermal radiation for atmospheric applications, and last but not least for overall environmental and climate monitoring and modelling.
  • Sentinel-4 will be placed on geostationary Meteosat Third Generation satellites.
  • Sentinel-5 plus Sentinel-5 Precursor are carried on the polar-orbiting MetOp Second Generation satellite.
    Both, Sentinel-4 and Sentinel-5, are dedicated to monitor the composition of the atmosphere for air quality, stratospheric ozone and solar radiation as well as climate monitoring.

Beside the above mentioned domains all Sentinels will fully support natural disaster management (risk analysis, loss assessment, disaster management, etc.) and humanitarian aid in crisis situations.

Sentinel-2

After this short overview of the Sentinel missions we are going to focus on Sentinel-2 (S-2) and provide some more details about data from this recently started optical mission.

The Sentinel-2 constellation has currently one satellite in orbit, the second one is scheduled to start next year. The two satellites will fly at a height of 786 km and 180° degrees apart. The final constellation will provide a revisit time of five days at the Equator and proportionately more frequent in higher latitudes. It systematically covers all land and coastal areas between 84°N and 56°S. The data collection within this region includes islands greater than 100 km² in area, all islands in the European Union, all other islands within 20 km of a coastline, the whole Mediterranean Sea, all inland water bodies, and all closed seas.

Each satellite carries a single multi-spectral instrument (MSI), which provides 13 bands in the visible, near infra-red, and short wave infra-red part of the spectrum, at different ground resolution. The Table below shows a summary of the bands, their resolution and their application.

Band Band range (nm) /
Band Center (nm)
Spatial
Resolution (m)
Purpose in L2 processing context
B1 433-453 / 443 60 Atmospheric Correction
B2 458-523 / 490 10 Sensitive to Vegetation Aerosol Scattering
B3 543-578 / 560 10 Green peak, sensitive to total chlorophyll in vegetation
B4 650-680 / 665 10 Max Chlorophyll absorption
B5 698-713 / 705 20 Not used in L2A context
B6 734-748 / 740 20 Not used in L2A context
B7 765-785 / 783 20 Not used in L2A context
B8 785-900 / 842 10 Leaf Area Index (LAI)
B8a 855-875 / 865 20 Used for water vapour absorption reference
B9 930-950 / 945 60 Water vapour absorption atmospheric correction
B10 1365-1385 / 1375 60 Detection of thin cirrus for atmospheric correction
B11 1565-1655 /1610 20 Soils detection
B12 2100-2280 / 2190 20 AOT determination

Processing

Satellite data is provided in multiple levels typically ranging from 0 to 3, where 0 is unprocessed instrument and payload data at full resolution, and 3 means derived information, like a single percentage number.

Level-0 and Level-1A processing are performed by the ground segment and are in general not accessible to users. So we skip them here.

Level-1B and Level-1C processing is performed routinely by the S-2 ground segment and the results are available to users. We therefore provide a short overview on these products and go into more detail for Level-1C products later on.

Level-1B products represent Top-Of-Atmosphere calibrated radiances (TOA) in sensor geometry, and include radiometric corrections and the refinement of the physical geometrical viewing model (which is not applied at this level). Products are available as JPEG2000 lossless compressed images plus associated metadata, all capsuled within a SAFE file container (basically a zip-file, more details about SAFE in a later post). Level-1B is composed of granules (see also Figure below), one granule represents a sub-image (one for each band) of the satellite track, with a size of 25 km across-track and approx. 23 km along-track on ground. Each Level-1B granule has a data volume of approximately 27 MB. Level-1B products require expert knowledge of orthorectification techniques.

Level-1C products are radiometric and geometric corrected TOA products. The corrections include orthorectification and spatial registration on a global reference system (combined UTM projection and WGS84 ellipsoid) with sub-pixel accuracy. Level-1C images are delivered in tiles of 100×100 km (see also Figure below), each of which is approximately 500 MB. Products are available as JPEG2000 lossless compressed images plus associated metadata, all capsuled again within a SAFE file container.
Currently, this type of data is the main target for almost all users.

Level-2 products are Bottom-Of-Atmosphere (BOA) corrected orthophotos. In addition they contain an Aerosol Optical Thickness (AOT) map, a Water Vapour (WV) map, and a Scene Classification Map (SCM) together with Quality Indicators (QI) for cloud and snow probabilities at 60 m resolution. They are resampled and generated with an equal spatial resolution for all bands, based on the requested resolution (10 m, 20 m, or 60 m)

Unfortunately, Level-2 products are currently not routinely generated by the ground segment but have to be generated by the users themselves (e.g. by using the Sentinel toolbox, provided by ESA for free). However, the provisioning of Level-2 products, as a systematically generated core product of the ground segment, is currently assessed.

Sentinel-2 Data Structure

Datatakes & Datastrips

OK, lets start from the beginning i.e. a satellite flies over an area and scans the Earth surface. For the Sentinels this is named a “datatake” – the continuous acquisition of an image from one Sentinel-2 satellite in a given MSI imaging mode. The maximum length of an imaging datatake is 15,000 km (e.g. the continuous observation from northern Russia to southern Africa).

Since it is not possible for a single ground receiving station to cover such an area the term “datastrip” is introduced: within a given datatake, a portion of image downlinked during a pass to a given station is termed a “datastrip”. If a particular orbit is acquired by more than one station, a datatake is composed of one or more datastrips. The maximum length of a datastrip downlinked to a ground station is approximately 5,000 km.

Granules & Tiles

Now back to Level-1C products. In the whole concept of providing orthorectified, spatially co-registered (to a global reference system) data to users a confusion has been introduced. Specifically by allowing the usage of the same expression for different things. In order to avoid this confusion we will strictly use the term “granules” for the 25×23 km sub-images at Level-1B and the term “tiles” for 100×100 km sub-images a Level-1C and higher. The figure below (Courtesy of ESA) shall provide an impression of the meaning of these two terms. The underlying datastrip (the satellite image) forms the basis.

Sentinel-2 Granules & Tiles
Sentinel-2 Granules & Tiles

Since datatakes are usually composed of datastrips provided by multiple ground receiving stations (an exception would be if the data is transmitted via a geostationary satellite downlink) one question arises: “how are two adjacent ground stations handled?” The situation is illustrated in the figure below.

Tile Pair Consolidation (<a href="https://sentinels.copernicus.eu/documents/247904/685211/Sentinel-2+Products+Specification+Document+%28PSD%29/0f7bedeb-9fbb-4b60-91aa-809162de456c">Source</a>)
Tile Pair Consolidation (Source)

Another question, we have frequently discussed here at EOX, is the relationship between a datatake, the UTM reference tiling  schema, the user defined Area-Of-Interest (AOI), and the actual product delivered to the user – the so called “User Product”. The figure below provides a first impression of these relations, which will be explained in more detail together with the SAFE file format.

Relations between Sentinel-2 Data Concepts (<a href="https://sentinels.copernicus.eu/documents/247904/685211/Sentinel-2+Products+Specification+Document+%28PSD%29/0f7bedeb-9fbb-4b60-91aa-809162de456c">Source</a>)
Relations between Sentinel-2 Data Concepts (Source)

 

Thanks for reading this far but let’s stop for now. Come back to learn how to download Sentinel-2 User Products, how to decode the used SAFE file format, and how to further process the data.

6 thoughts on “Understanding Sentinel-2 Satellite Data

  1. This is very helpful, thanks! Please keep the series going.
    I’m interested accessibility and processing of S-2 data.

  2. Very useful and helpful the tutorial about Sentinel 2 data. The remote sensing user community in Brazil appreciate your blog and will wait the upcoming post with more details about the topic.
    Thank you in advance

    JY

  3. Very useful and helpful the tutorial about Sentinel 2 data. I appreciate your blog and will wait the upcoming post with more details about the topic.
    Thank you in advance

Comments are closed.