Frequently Asked Questions - Sentinel 2

Sentinel 2 is part of the Copernicus earth observation program developed by the European Space Agency (ESA) to study the earth’s surface.  The Sentinel 2 portion of the program consists of a pair of satellites that are designed to acquire reflected sunlight in the optical wavelengths.  It is especially sensitive to variations in vegetation so is extremely useful for studying crops and forests.

Sentinel 2A was launched on 23 June 2015, the first image was acquired on 27 July 2015, and the program became operational on 15 October 2015.  The satellite orbits the earth at an altitude of 786 km and has a swatch width of 290 km.  This provides a return time of 10 days as compared with the 16-day return time of the Landsat program.  Sentinel 2B is planned to launch on 7 March 2017.  When this is properly positioned in its final orbit the pair of satellites will provide near-global coverage every 5 days, with quicker return times at mid latitudes.

Data are acquired at three spatial resolutions and should be handled as separate files. There are four bands with a 10m spatial resolution.  These are in the blue, green, red, and near infrared parts of the electromagnetic spectrum.  The second file has six bands with 20m spatial resolution and the third file has three bands with 60m resolution.  The wavelengths are described below.  Note that four of the 20m bands are in the red edge and near infrared wavelengths.

Sentinel 2 Bands

From https://earth.esa.int/web/sentinel/missions/sentinel-2/instrument-payload/resolution-and-swath

Learn more about this program at:
   the ESA Sentinel 2 website https://earth.esa.int/web/sentinel/missions/sentinel-2  
   and the USGS site:  https://eros.usgs.gov/sentinel-2

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You can obtain Sentinel 2 data from the ESA and from the USGS.  Users must register for free access to data at either site.  Currently ESA distributes Sentinel 2 data in entire swaths.  These files are very large, sometimes 7+ GB when compressed.  You can access these data directly from the ESA Copernicus Open Access hub at:  https://scihub.copernicus.eu/   Users should read the online User Guide prior to searching for data.

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The USGS distributes most of the Sentinel 2 data but has cut these data into much smaller tiles of 100 km X 100 km in the Level-1C top of atmosphere reflectance.  These data are much easier to work with and users are encouraged to look for data here first.  You can access these data at the USGS Earth Explorer site at:  https://earthexplorer.usgs.gov/

You should begin your data search by first defining a small region of interest.  You can expand this later as needed.  You can enter a start and stop date to further limit your search.  Next in the Data Sets tab scroll down and select Sentinel | Sentinel 2

Click on the Results tab to display data that meets your search criteria.  You can click on the Footprint icon to show the scene coverage.  To the right of this is a Browse icon.  This will display the image to help you decide if the scene meets your needs.  Users are encouraged to only download one or two images initially.  Once you gain experience working with these data you can come back and get more.

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ENVI version 5.4 opens Sentinel-2 data as 3 separate files based on their spatial resolution; 10m, 20m, and 60m.  After opening the data and exploring it, you should save one or more of these datasets in the standard ENVI format to make data access quicker in the future.  The YCEO Lab currently supports ENVI 5.4.1 and can be used to open all Sentinel 2 data.

First you will need to extract (unzip) these data using a tool such as 7-Zip.  Data for the file will be placed in a new folder.  From the ENVI version 5.4 main menu select File | Open.  Navigate to the new folder and select the file MTD_MSIL1C.xml.  ENVI will display the 10m resolution Color InfraRed view of the scene and the lower resolution (20m and 60m) files will be available in the Data Manager.

You can examine the data and if you wish to use these data in the future you are encouraged to save the file in ENVI format.  From the ENVI main menu select File | Save As | ENVI Standard.  Select the 10m file and save the output to a new folder structure for your project; it should not be placed in any of the original Sentinel folders.  Consider including 10m as part of the filename to indicate the resolution.

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If you are using ENVI version 5.4 or greater please see the much simpler instructions on importing Sentinel 2 data.  The YCEO Lab currently uses ENVI 5.4.1 so if you are working in the Center, please follow the separate ENVI 5.4 instructions. 

These instructions are for those using ENVI version 5.3.

Sentinel 2 data use very long folder and filenames that may cause problems in the Windows operating system.  You should extract (unzip) the data into a folder near the top of the file structure.  By this we mean U:\  rather than something like:  U:\Project\Rasters\Sentinel\MyNewData\ImageDate.  Once the data are extracted rename the new data folder to a shorter name.  For example, a typical Sentinel data folder name might be: 
S2A_OPER_PRD_MSIL1C_PDMC_20160613T000058_R084_V20160612T181447_20160612T181447.SAFE.  You should rename this to something like:  June10

In December 2016 the ESA revised their naming conventions and data packaging structures.  This impacts how we can access these data.  On the USGS Earth Explorer site data in the original format can be identified by the ENITY ID beginning with “S2A_OPER” and were acquired until 5 December 2016.  These data can be opened in ENVI 5.3.x. 

Data acquired from 6 December 2016 on have an ENTITY ID beginning with “L1C_T” and use the new format and naming convention.  If you do not have ENVI version 5.4 you can open these files using the European Space Agency program SNAP in the Sentinel-2 Toolbox, available here.  Instructions on how to import these data can be found below.

S2A_OPER format:

From the main ENVI menu select File | Open As | Optical Sensors | Sentinel 2.  Navigate into the new folder and select the XML file.  This will also be a very long filename such as:
S2A_OPER_MTD_SAFL1C_PDMC_20160613T000058_R084_V20160612T181447_20160612T181447.xml

ENVI should open the data into files based on spatial resolution.  You can examine the data and save the file to ENVI format if you wish to use these data in the future.  From the ENVI main menu select File | Save As | ENVI Standard.  Save this file to a new folder structure for your project; it should not be placed in any of the original Sentinel folders.  Consider including 10m or 20m as part of the filename to distinguish the resolution.

In some cases ENVI will generate an error message such as: 
       File: S2A_OPER_MSI…..  File does not appear to be a valid JPEG2000 file
This message is generated when the XML header file references data that are not part of the distribution package provided by the USGS.  You will need to edit the XML file to remove these references. 

First determine what files are provided in your package.  Using the Windows File Explorer open the Snetinel data folder.  Search for and double click on the folder Granule | S2A… (the long name again) | IMG_Data.  Expand the column width to show the full file name.  A typical filename is:
  S2A_OPER_MSI_L1C_TL_SGS__20160612T214651_A005086_T12TWM_B01.jp2

You want to keep any data references to the files ending with T12TWM and remove the rest.

First make a backup copy of the XML file used to open the data.  Now open the original XML file using the text editor Notepad++ found on the YCEO systems.  Scroll down to the  <Product_Organisation> section and examine the many <Granule_List> … </Granule_List> entries.  Remove all but the entry that refers to your specific data set.  In this example, this would be the <Granule_List> that has images with T12TWM as the end part of the filenames.  Save the XML file and return to ENVI to open this as you did earlier.  ENVI should create three new files based on the spatial resolution.

L1C_T format:

ENVI 5.3.1 cannot be used to open Sentinel 2 data in this new format.  You can open these data using the ESA Sentinel-2 Toolbox program SNAP.  This is installed on the YCEO Lab systems.  You can also download this software from the ESA site:
  https://sentinel.esa.int/web/sentinel/toolboxes/sentinel-2

The ESA website has links to documentation and YouTube videos to instruct users.  While you can view and manipulate these data quite easily in SNAP, it is a bit difficult to export these data into a format that can be used by ENVI.  In order to export these data, they must all have a common spatial resolution.  So if you want to use the four 10-meter bands you must resample the entire file to 10 meters. (this is a very large file!)  You can then open this in ENVI and spectrally subset this to extract just the four bands of interest.  You should then delete the large work files.  Again, if you have ENVI version 5.4 you do NOT need to do this.

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