ASF RTC Data Package (S1TBX)
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This folder contains Radiometric Terrain Corrected (RTC) products and their associated files. They were generated by the Alaska Satellite Facility (ASF) HYP3 RTC-S1TBX software version [HYP3_VER], running Sentinel-1 Toolbox (S1TBX) release [S1TBX_VER]. They are projected to [PCS], and the pixel spacing is [SPACING] m.

Processing Date/Time: [DATE] [TIME] UTC

The folder and each of its contents all share the same base name, using the following convention:  

S1x_yy_RTzz_aaaaaaaaTbbbbbb_c_def  
x:        Sentinel-1 Mission (A or B)  
yy:       Beam Mode  
zz:       Terrain Correction Resolution  
aaaaaaaa: Start Date of Acquisition (YYYYMMDD)  
bbbbbb:   Start Time of Acquisition (HHMMSS)  
c:        Processor (Gamma or S1TBX)  
d:        gamma-0 (g) or sigma-0 (s) output  
e:        amplitude (a) or power (p) output  
f:        not filtered (n) or filtered (f)

The source granule used to generate the products contained in this folder is:  
[GRAN_NAME]

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The side-looking geometry of SAR imagery leads to geometric and radiometric distortions, causing foreshortening, layover, shadowing, and radiometric variations due to terrain slope. Radiometric terrain correction converts unprocessed SAR data into geocoded tiff images with values directly relating to physical properties, alleviating the inherent SAR distortions. The process improves backscatter estimates and provides geolocation information, so images can be used as input for applications such as the monitoring of deforestation, land-cover classification, and delineation of wet snow-covered areas.

Refer to the ASF Sentinel-1 RTC User Guide for additional guidance on the use of this dataset:
https://asf.alaska.edu/wp-content/uploads/2019/02/Sentinel_RTC_Users_Guide.pdf

Visit the ESA sentinel online website for further information on the Sentinel-1 Toolbox (S1TBX):
https://sentinel.esa.int/web/sentinel/toolboxes/sentinel-1

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### To cite the data: ###
ASF DAAC [YEARPROCESSED] using S1TBX software. Contains modified Copernicus Sentinel data [YEARACQUIRED], processed by ESA.

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*Consider opening this document in a Markdown editor/viewer for easier reading.*  

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# Product Contents #  

The files generated in this process include:

1. Radiometric Terrain Corrected GeoTIFF data files for each polarization available
2. Browse images (PNG and KMZ format) in grayscale and color (when dual-pol is available)
3. A copy of the DEM used to correct the data (included in standard products; you can choose to omit this layer when custom ordering imagery)  
4. An incidence angle map  
5. A layover-shadow mask (included in standard products; you can choose to omit this layer when custom ordering imagery)  
6. An ArcGIS xml metadata file for each raster layer, displayed in the Item Description (ArcGIS Desktop) or Metadata (ArcGIS Pro)
7. Log file

See below for detailed descriptions of each of the products.

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## 1. Radiometric Terrain Corrected data files

GeoTIFF files are generated for each polarization available in the source granule. Each filename will include the polarization: VV or HH for primary polarization, and VH or HV for cross-polarization. To learn more about polarimetry, refer to https://sentinel.esa.int/web/sentinel/user-guides/sentinel-1-sar/product-overview/polarimetry.

These files have been processed to output [POWERTYPE]-0 [FORMAT].

[FILT] speckle filter has been applied to the RTC images. The default is to not apply a speckle filter, but the user can choose to apply a filter when ordering the RTC imagery. When the filtering option is selected, the S1TBX default Speckle Filter is applied during RTC processing to remove speckle while preserving edges. When applied, it is a Lee Sigma filter set to one look with a window size of 7x7, a sigma of 0.9 and a target window size of 3x3.

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## 2. Browse images in grayscale and color

PNG files are generated in two different resolutions for quick visualization of the backscatter data. Each png browse image is accompanied by an aux file containing the projection and geocoding information for the file.

All products will include a grayscale png browse image in both resolutions. It is a rendering of the primary polarization data, scaled to an ASF standard to display nicely in grayscale. The low-resolution image is designated by a simple .png extension, while the tag _large.png indicates the medium-resolution image. 

For dual-pol products, a false-color png browse image is generated in both resolutions. It is a rendering of the primary and cross-polarization data, scaled to an ASF standard to display nicely in color. These files are additionally tagged with _rgb, but otherwise have the same tags/extensions as the grayscale browse images.

KMZ files are generated in the higher resolution for use in Google Earth and other compatible applications. All products will include a grayscale kmz image, and dual-pol products will also include a color browse kmz image.

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## 3. DEM used to correct the data

The Digital Elevation Model (DEM) layer is included with standard products, but is optional when placing a custom order for imagery. This layer is tagged with _dem.tif

The best DEM publicly available for each granule is used in the RTC process, so different granules may be processed using different source DEM layers. The resolution of the source DEM varies depending on the location of the granule. The DEM is clipped from the source layer to the size needed for full granule coverage, or to the extent of the available DEM source data if full coverage is not available. It is then resampled from the native DEM resolution to [SPACING] m for use in RTC processing.

The DEM sources include the National Elevation Dataset (NED), the Shuttle Radar Topography Mission (SRTM), the Copernicus Land Monitoring Service EU-DEM (EUDEM), the Greenland Ice sheet Mapping Project DEM (GIMP), and the Reference Elevation Model of Antarctica DSM (REMA).

The source of the DEM for this particular product is [DEM], which has a native resolution of [RESA] arc seconds (about [RESM] meters).  

*Refer to the _dem.tif.xml file for additional information about the specific DEM included with this product, including use and citation requirements.*

__Summary of the DEMs used by ASF for RTC Processing__

The *NED* provides the best available public domain raster elevation data of the conterminous United States, Alaska, Hawaii, and territorial islands in a seamless format. The NED is derived from diverse source data, processed to a common coordinate system and unit of vertical measure. For more information, refer to https://pubs.er.usgs.gov/publication/70201572. To download the data, visit https://viewer.nationalmap.gov/basic and expand the Elevation Products (3DEP) section.

The *SRTM* was flown aboard the space shuttle Endeavour February 11-22, 2000. The National Aeronautics and Space Administration (NASA) and the National Geospatial-Intelligence Agency (NGA) participated in an international project to acquire radar data which were used to create the first near-global set of land elevations. For more information and to access the full SRTM dataset, refer to https://www.usgs.gov/centers/eros/science/usgs-eros-archive-digital-elevation-shuttle-radar-topography-mission-srtm-1-arc?qt-science_center_objects=0#qt-science_center_objects.

The *EUDEM* combines data from a variety of sources, including the ASTER DEM, the SRTM, and Russian topographic maps. For more information and to access the full EU-DEM dataset, refer to https://land.copernicus.eu/imagery-in-situ/eu-dem/eu-dem-v1.1.

The *GIMP* DEM was constructed by combining ASTER and SPOT 5 DEMs over the ice sheet periphery and margin with AVHRR photoclinometry for the interior and far north, and calibrating the data to approximate mean ICESat/GLAS elevations from 2003 to 2009. For more information and to access the full GIMP dataset, refer to https://nsidc.org/data/nsidc-0645.

The *REMA* DSM was constructed from hundreds of thousands of individual stereoscopic Digital Elevation Models extracted from pairs of submeter-resolution DigitalGlobe satellite imagery acquired between 2009 and 2017, and vertically registered to altimetry measurements from Cryosat-2 and ICESat. For more information and to access the full REMA dataset at the original 8-meter resolution, refer to https://www.pgc.umn.edu/data/rema.

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## 4. Incidence angle map

The incidence angle map is included with standard products, but is optional when placing a custom order for imagery. This layer is tagged with _inc_map.tif

This map records the local incidence angle for each pixel in the RTC image. The incidence angle is the angle between the incident radar beam and the direction perpendicular to the ground surface (based on the DEM), expressed in degrees.

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## 5. Layover-shadow mask

The layover/shadow mask indicates which pixels in the RTC image have been affected by layover and shadow. This layer is tagged with _ls_map.tif.  The pixel values are 1 for layover/shadow areas and 0 for areas that are not affected.

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## 6. ArcGIS-compatible xml metadata files

Each raster in this folder has an associated xml file. It is named with the same filename as the raster, but also includes a .xml extension. When any of the rasters are viewed in ArcGIS, the associated xml file is recognized by the software, and the contents will display in the Item Description (ArcGIS Desktop) or Metadata (ArcGIS Pro) for that raster. Once the file is viewed in ArcGIS, the software will update the xml file to include metadata inherent to the raster (geographic extent, raster format, etc.) along with the descriptive metadata included in the original xml file.

ArcGIS users should take care not to edit the xml files directly, or to change filenames outside of the ArcGIS environment, as it may render the metadata files unreadable by ArcGIS. 

Users who do not use ArcGIS to interact with the data may still find the information included in the individual xml files very useful, although the xml tag system makes it look cluttered in a text editor or browser window.

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## 7. Log file

A textfile is generated during processing, which includes the parameters used and step-by-step processing history for the product. It has a .log extension.

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### RTC Processing ###

The basic steps in the radiometric terrain correction process are as follows:  

1.  Get the DEM file covering this scene from the ASF DEM heap
2.  Download and apply precision orbit information
3.  Optional: Remove thermal noise
4.  Apply calibration parameters
5.  If SLC inputs are used, deburst the SLC
6.  Optional: Subset dataset to desired AOI or shapefile
7.  Optional: Apply speckle filtering
8.  If needed, Multi-look dataset: for SLC 30-meter product, 12 x 3 looks are used; for GRD 30-meter product, 3 x 3 looks are used
9.  Apply radiometric terrain flattening
10. Apply terrain correction
11. Optional: Create layover/shadow mask
12. Post processing creates GeoTIFF, PNG and KMZ files, along with associated metadata

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### The Sentinel-1 mission ###

The Sentinel-1A satellite was launched April 3, 2014, and the Sentinel-1B satellite was launched April 25, 2016. The satellites each have a 12-day repeat cycle. 

More information about the mission is available at:  
https://earth.esa.int/web/guest/missions/esa-operational-eo-missions/sentinel-1

Additional information about Sentinel-1 data, imagery, tools and applications is available at:  
 https://asf.alaska.edu/data-sets/sar-data-sets/sentinel-1/ 

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For assistance, contact the Alaska Satellite Facility:  
uso@asf.alaska.edu  
907-474-5041

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Revised 2020-07-30