Annual Ice Velocity of the Greenland Ice Sheet (1991-2000)

We derive surface ice velocity using data from 16 satellite sensors
deployed by 6 different space agencies. The list of sensors is given in
the Table S1. The SAR data are processed from raw to single look complex
using the GAMMA processor (www.gamma-rs.ch). All measurements rely on
consecutive images where the ice displacement is estimated from tracking
or interferometry (Joughin et al. 1998, Michel and Rignot 1999, Mouginot
et al. 2012). Surface ice motion is detected using a speckle tracking
algorithm for SAR instruments and feature tracking for Landsat. The
cross-correlation program for both SAR and optical images is ampcor from
the JPL/Caltech repeat orbit interferometry package (ROI_PAC). We assemble
a composite ice velocity mosaic at 150 m posting using our entire speed
database as described in Mouginot et al. 2017 (Fig. 1A). The ice velocity
maps are also mosaicked in annual maps at 150 m posting, covering July,
1st to June, 30th of the following year, i.e. centered on January, 1st
(12) because a majority of historic data were acquired in winter season,
hence spanning two calendar years. We use Landsat-1&2/MSS images
between 1972 and 1976 and combine image pairs up to 1 years apart to
measure the displacement of surface features between images as described
in Dehecq et al., 2015 or Mouginot et al. 2017. We use the 1978 2-m
orthorectified aerial images to correct the geolocation of Landsat-1 and
-2 images (Korsgaard et al., 2016). Between 1984 and 1991, we process
Landsat-4&5/TM image pairs acquired up to 1-year apart. Only few
Landsat-4 and -5 images (~3%) needed geocoding refinement using the same
1978 reference as used previously. Between 1991 and 1998, we process radar
images from the European ERS-1/2, with a repeat cycle varying from 3 to 36
days depending on the mission phase. Between 1999 and 2013, we used
Landsat-7, ASTER, RADARSAT-1/2, ALOS/PALSAR, ENVISAT/ASAR to determine
surface velocity (Joughin et al., 2010; Howat, I. 2017; Rignot and
Mouginot, 2012). After 2013, we use Landsat-8, Sentinel-1a/b and
RADARSAT-2 (Mouginot et al., 2017). All synthetic aperture radar (SAR)
datasets are processed assuming surface parallel flow using the digital
elevation model (DEM) from the Greenland Mapping Project (GIMP; Howat et
al., 2014) and calibrated as described in Mouginot et al., 2012, 2017.
Data were provided by the European Space Agency (ESA), the EU Copernicus
program (through ESA), the Canadian Space Agency (CSA), the Japan
Aerospace Exploration Agency (JAXA), the Agenzia Spaziale Italiana (ASI),
the Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR) and the National
Aeronautics and Space Administration (NASA). SAR data acquisitions were
coordinated by the Polar Space Task Group (PSTG). Errors are estimated
based on sensor resolution and time lapse between consecutive images as
described in Mouginot et al. 2017.  References Dehecq, A,
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from a complete satellite archive: Application to the
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