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History of modifications

List of datasets covered by this document

Related documents

Acronyms

General definitions

Interferometric Wide (IW): The Interferometric Wide (IW) swath mode is the main acquisition mode of Sentinel-1 over land, including ice sheets. It acquires data with a 250 km swath at 5 m by 20 m spatial resolution. IW mode captures three sub-swaths using Terrain Observation with Progressive Scans SAR (TOPS). (Adapted from: https://sentinels.copernicus.eu/)

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Single Look Complex (SLC): Level-1 Single Look Complex (SLC) products are Synthetic Aperture Radar (SAR) images in the slant range by azimuth imaging plane, in the image plane of satellite data acquisition. Each image pixel is represented by a complex magnitude value and therefore contains both amplitude and phase information. The imagery is geo-referenced using orbit and attitude data from the satellite. SLC images are produced in a zero Doppler geometry. (Adapted from: https://sentinels.copernicus.eu/)

Scope of the document

This document is the Product User Guide and Specification for Ice Velocity (IV) as part of the Copernicus Ice Sheets and Ice Shelves service. It describes the datasets produced, and their specifications.

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• Ice velocity is given for Greenland and Antarctica in product WP2-FDDP-IV-CDR
• Gravimetric mass balance is given for Greenland and Antarctica in product WP2-FDDP-GMB-CDR5
• Surface elevation change is given for Greenland in product WP2-FDDP-SEC-CDR-GrIS

Executive summary

The dataset contains annual ice velocity maps of the Greenland Ice Sheet derived from Sentinel-1 data. The data represent the current state-of-the-art in Europe for production of satellite-based ice velocity data records. It follows on from the ESA Greenland Ice Sheet Climate Change Initiative and is guided by the GCOS (Global Climate Observing System) targets for the Ice Sheets Essential Climate Variable.

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This dataset is produced on behalf of the Copernicus Climate Change Service (C3S) and available through the Climate Data Store.

Product Change Log

Greenland ice sheet velocity

Product description

The product contains annually averaged ice velocity (IV) for Greenland derived from Sentinel-1 (S1) SAR data (Figure 1). The surface velocity is derived by applying advanced iterative offset tracking techniques (Nagler et al., 2015). The primary processor for IV generation is the ENVEO software package (ESP v2.1), see the related ATBD [D1]. The system’s core module performs coherent and incoherent offset tracking utilizing long stripes of S1 single look complex (SLC) data acquired in interferometric wide (IW) swath mode. The ESP v2.1 is a state-of-the-art IV retrieval algorithm designed for SAR sensors (e.g. Sentinel-1, TerraSAR-X, European Remote-sensing Satellite (ERS)-1, ERS-2, Advanced Land Observation Satellite (ALOS) Phased Array type L-band SAR (PALSAR), Cosmo-SkyMed), and has been tested rigorously through intercomparisons with other packages and extensive validation efforts.

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Target requirements

Table 1 shows the C3S IV product achievement versus the 2022 GCOS requirements for measurement uncertainty and spatial/temporal resolution of ice sheet velocity. The C3S product fulfills all threshold requirements for spatial and temporal resolution and falls well within the goal accuracy range. Further details on the the target requirements for each product within the service are provided in the Target Requirements and Gap Analysis Document (TRGAD, [D3])

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Data usage information

Product Data Format and Content

The product is provided as a NetCDF file with separate layers for the velocity components: v_x, v_y, v_z and v_v (magnitude of the horizontal components), and maps showing the valid pixel count and uncertainty (std) (Table 2). The ice velocity map is annually averaged and provided at 250m grid spacing in North Polar Stereographic projection (EPSG: 3413). The horizontal velocity is provided in true meters per day, towards easting (v_x) and northing (v_y) direction of the grid, and the vertical displacement (v_z), is derived from a digital elevation model (TanDEM-X 90m Digital Elevation Model (DEM); Rizzoli et al., 2017). For all maps a nodata value of 3.4028235e+38 is used.

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Product Known Limitations

The following lists some known product limitations:
1) The IV products contain separate layers for the horizontal (Easting, Northing) and the vertical components of velocity. This is, however, not the true 3D velocity, which requires both ascending and descending image pairs acquired close in time. The vertical component is derived from the difference in height of start and end position of the displacement vector taken from a DEM.
2) The IV products do not have a time stamp for a single date, but give the average velocity over the time-period covered (1 year).
3) For various reasons, the tracking software sometimes fails to find matching features leading to gaps in the velocity fields. This can be caused by a lack of surface features or when features, for example crevasses, rapidly change due to shearing leading to low correlation. Other reasons for gaps in the IV maps can be areas affected by radar shadow or anomalous pixels that are filtered out. A simple distance-weighted averaging filter is applied to get rid of outliers and to fill small gaps in the data (<5 pixels), further filtering/gap filling is left to the user if required. 
4) Due to different acquisition modes, sensor type, resolution and processing strategy there can be differences between S1 IV products and IV products derived from other sensors that complicate a direct comparison between the data sets. Because of differences in resolution, the image patches used for feature tracking have different dimensions impacting the type of features that can be resolved. S1 can for instance not capture the high velocity gradients that may be found in shear zones with the same detail as for example TerraSAR-X (TSX). On the other hand, due to the regular repeat acquisition the temporal sequence of S-1 is much higher than that of TSX and the covered area of the IV maps is much larger.
5) In-situ Global Positioning System (GPS) data for validation of ice velocity are only sparsely available. Velocity products are therefore also compared with ice velocity maps retrieved from other sensors (e.g. S1 vs TSX) to estimate product performance and uncertainty. As an additional quality test, velocity results on stable terrain (rock outcrops), where no movement is expected, are analysed.

Product Nomenclature

The Filename of the IV Products consists of:

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Example: C3S_GrIS_IV_250m_S1_20201001_20210930_v1_4.nc, Explanation: Annually averaged Greenland Ice Sheet surface velocity derived from Sentinel-1 for Oct 1^st 2020 – Sept 30^th 2021 provided at 250 m grid spacing, processing version 1_4 for C3S.

Structure of NetCDF files

The product metadata is included in the NetCDF file and is shown in Table 4. The NetCDF file contains 7 sub-datasets. Table 5 shows as an example the metadata for the "land_ice_surface_easting-_velocity" sub-dataset.

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Driver: netCDF/Network Common Data Format
Files: C3S_GrIS_IV_250m_S1_20201001_20210930_v1_4.nc
       C3S_GrIS_IV_250m_S1_20201001_20210930_v1_4.nc.aux.xml
Size is 5984, 10801
Coordinate System is:
PROJCRS["WGS 84 / NSIDC Sea Ice Polar Stereographic North",
    BASEGEOGCRS["WGS 84",
        DATUM["World Geodetic System 1984",
            ELLIPSOID["WGS 84",6378137,298.257223563,
                LENGTHUNIT["metre",1]]],
        PRIMEM["Greenwich",0,
            ANGLEUNIT["degree",0.0174532925199433]],
        ID["EPSG",4326]],
    CONVERSION["unnamed",
        METHOD["Polar Stereographic (variant B)",
            ID["EPSG",9829]],
        PARAMETER["Latitude of standard parallel",70,
            ANGLEUNIT["degree",0.0174532925199433],
            ID["EPSG",8832]],
        PARAMETER["Longitude of origin",-45,
            ANGLEUNIT["degree",0.0174532925199433],
            ID["EPSG",8833]],
        PARAMETER["False easting",0,
            LENGTHUNIT["metre",1],
            ID["EPSG",8806]],
        PARAMETER["False northing",0,
            LENGTHUNIT["metre",1],
            ID["EPSG",8807]]],
    CS[Cartesian,2],
        AXIS["easting",south,
            ORDER[1],
            LENGTHUNIT["metre",1]],
        AXIS["northing",south,
            ORDER[2],
            LENGTHUNIT["metre",1]],
    ID["EPSG",3413]]
Data axis to CRS axis mapping: 1,2
Origin = (-640000.000000000000000,-655500.000000000000000)
Pixel Size = (250.000000000000000,-250.000000000000000)
Metadata:
  crs#false_easting=0
  crs#false_northing=0
  crs#grid_mapping_name=polar_stereographic
  crs#latitude_of_projection_origin=90
  crs#spatial_ref=PROJCS["WGS 84 / NSIDC Sea Ice Polar Stereographic North",
    GEOGCS["WGS 84",
        DATUM["WGS_1984",
            SPHEROID["WGS 84",6378137,298.257223563,
                AUTHORITY["EPSG","7030"]],
            AUTHORITY["EPSG","6326"]],
        PRIMEM["Greenwich",0,
            AUTHORITY["EPSG","8901"]],
        UNIT["degree",0.0174532925199433,
            AUTHORITY["EPSG","9122"]],
        AUTHORITY["EPSG","4326"]],
    PROJECTION["Polar_Stereographic"],
    PARAMETER["latitude_of_origin",70],
    PARAMETER["central_meridian",-45],
    PARAMETER["false_easting",0],
    PARAMETER["false_northing",0],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]],
    AXIS["Easting",SOUTH],
    AXIS["Northing",SOUTH],
    AUTHORITY["EPSG","3413"]]
  crs#standard_parallel=70
  crs#straight_vertical_longitude_from_pole=-45
  crs#unit=metre
  land_ice_surface_velocity_magnitude#coordinates=y x
  land_ice_surface_velocity_magnitude#description=magnitude of horizontal ice velocity
  land_ice_surface_velocity_magnitude#grid_mapping=crs
  land_ice_surface_velocity_magnitude#units=m/day
  land_ice_surface_velocity_magnitude#_FillValue=3.4028235e+38
  NC_GLOBAL#comment=Ice velocity map of Greenland derived from Sentinel-1 SAR data acquired from 2020-10-01 to 2021-09-30. The surface velocity is derived applying feature tracking techniques. The ice velocity map is provided at 250m grid spacing in North Polar Stereographic projection (EPSG: 3413). The horizontal velocity is provided in true meters per day, towards EASTING(vx) and NORTHING(vy) direction of the grid, and the vertical displacement (vz), is derived from a digital elevation model. Provided is a NetCDF file with the velocity components: vx, vy, vz and vv (magnitude of the horizontal components), along with maps showing valid pixel count and uncertainty (stdx, stdy). The product was generated by ENVEO.
  NC_GLOBAL#contact=copernicus-support@ecmwf.int
  NC_GLOBAL#Conventions=CF-1.7
  NC_GLOBAL#creation_date=2022-12-12
  NC_GLOBAL#history=product version 1.4
  NC_GLOBAL#institution=Copernicus Climate Change Service
  NC_GLOBAL#keywords=EARTH SCIENCE CLIMATE INDICATORS CRYOSPHERIC INDICATORS GLACIAL MEASUREMENTS ICE SHEET VELOCITY CRYOSPHERE GLACIERS/ICE SHEETS
  NC_GLOBAL#license=C3S general license
  NC_GLOBAL#project=C3S_312b_Lot4_ice_sheets_and_shelves
  NC_GLOBAL#reference=Main: Nagler, T.; Rott, H.; Hetzenecker, M.; Wuite, J.; Potin, P. The Sentinel-1 Mission: New Opportunities for Ice Sheet Observations. Remote Sens. 2015, 7, 9371-9389. https://doi.org/10.3390/rs70709371
  NC_GLOBAL#source=Copernicus Sentinel-1A and Sentinel-1B
  NC_GLOBAL#summary=Ice velocity derived for Greenland Ice Sheet gridded at 250m averaged from 2020-10-01 to 2021-09-30.
  NC_GLOBAL#title=Ice Velocity of the Greenland Ice Sheet
  x#axis=X
  x#long_name=x coordinate of projection
  x#standard_name=projection_x_coordinate
  x#units=m
  y#axis=Y
  y#long_name=y coordinate of projection
  y#standard_name=projection_y_coordinate
  y#units=m
Geolocation:
  LINE_OFFSET=0
  LINE_STEP=1
  PIXEL_OFFSET=0
  PIXEL_STEP=1
  SRS=GEOGCS["WGS 84",DATUM["WGS_1984",SPHEROID["WGS 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],AUTHORITY["EPSG","6326"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],AXIS["Latitude",NORTH],AXIS["Longitude",EAST],AUTHORITY["EPSG","4326"]]
  X_BAND=1
  X_DATASET=NETCDF:"C3S_GrIS_IV_250m_S1_20201001_20210930_v1_4.nc":x
  Y_BAND=1
  Y_DATASET=NETCDF:"C3S_GrIS_IV_250m_S1_20201001_20210930_v1_4.nc":y
Corner Coordinates:
Upper Left  ( -640000.000, -655500.000) ( 89d18'52.26"W, 81d33'27.74"N)
Lower Left  ( -640000.000,-3355750.000) ( 55d47'51.49"W, 59d11'51.92"N)
Upper Right (  856000.000, -655500.000) (  7d33'22.15"E, 80d 4'16.22"N)
Lower Right (  856000.000,-3355750.000) ( 30d41'23.64"W, 58d47'38.46"N)
Center      (  108000.000,-2005625.000) ( 41d55' 3.66"W, 71d36'42.42"N)
Band 1 Block=748x1351 Type=Float32, ColorInterp=Undefined
  Min=0.000 Max=23.788 
  Minimum=0.000, Maximum=23.788, Mean=0.083, StdDev=0.396
  NoData Value=3.4028234663852886e+38
  Unit Type: m/day
  Metadata:
    coordinates=y x
    description=magnitude of horizontal ice velocity
    grid_mapping=crs
    NETCDF_VARNAME=land_ice_surface_velocity_magnitude
    STATISTICS_MAXIMUM=23.78809928894
    STATISTICS_MEAN=0.082814344301973
    STATISTICS_MINIMUM=3.1557854072162e-06
    STATISTICS_STDDEV=0.39557537806555
    units=m/day
    _FillValue=3.4028235e+38  

Data access information

All products

Data will be made available through the Copernicus Climate Data Store (CDS), which is the sole data distributor. Registration (free) is required to access the CDS and its toolbox software suite. Data can be downloaded from the website and used under the License to Use Copernicus Products (included on download page). Data may also be viewed online. All requests for information or further data should be channelled through the CDS Knowledge Base.

References

GCOS, 2022. The 2022 GCOS Implementation Plan. Geneva: World Meteorological Organization, 85. https://gcos.wmo.int/en/publications/gcos-implementation-plan2022

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