Processing Steps
The Alaska Satellite Facility provided all source data as Level-1 burst Single-Look Complex (SLC) data, including digitized voltage values, instrument calibration constants, satellite timing, attitude, and position information. The SLC burst data were processed to full multi-burst SLC scenes using the GAMMA Modular SAR Processor (MSP) package.
The data have been mapped to grey scale values in the range of 0 - 255 (bytes) using a nonlinear stretch to allow visual discrimination of topographical features within the images. As a result, the pixel values for each year are not directly comparable; however, if no change has occurred, they should be relatively consistent from scene to scene.
The mosaics are produced from four descending and two ascending tracks. In some cases, the image tracks overlapped, but at any given pixel, only one image was used. If the overlapping images were ascending (Tracks 74 and 90) and descending (Tracks 26, 112, 141, 170), the descending image was used (see the shapefile attribute field to determine track direction). If two parallel images overlapped, the top polygon in the shapefile should correspond to the image that was included in the mosaic. In some locations, it is possible that one mosaic will have ascending geometry and another descending geometry because of missed acquisitions (i.e., a consistent geometry is used whenever sufficient data are acquired). In such areas, mountains and other topography features will appear to be illuminated from opposite sides. If the images are flickered between, the different viewing geometries can give the impression of large shifts, even though the data are generally well registered. For regions with like-viewing geometry, the co-registration is far better than the 50-m posting, except in cases where the topography is extreme.
In instances where no viable data were available, a blank tile (.tif
) was still generated. The JPEG file (.jpg
) that accompanies each four-tile mosaic provides a quicklook view of expected data output when all four tiles are combined.
Error Sources
With the exception of extreme terrain regions, geometric accuracy is better than a single 50 m pixel (Joughin et al., 2016).
Terrain Correction
The data were terrain-corrected with MEaSUREs Greenland Ice Mapping Project (GIMP) Digital Elevation Model, Version 1. Due to this correction, some slight differences in registration relative to MEaSUREs Greenland Ice Sheet Mosaics from SAR Data, Version 1, mosaics exist.