Known Caveats and Limitations

Pixel offset between scenes in same swath

We process Sentinel-1 “Ground Range Detected” (GRD) “scenes” individually, despite the radar acquisition coming from a continuous swath. Because these scenes within the same swath are not overlapping, there can be a ~1 pixel gap between scenes due to resampling during terrain correction. This issue has also been observed for Sentinel-1 RTC quality data on the Google Earth Engine because they also process each scene individually for similar concerns around memory and file sizes when procesing many scenes at once.

Extremely low, but non-zero, backscatter at tile edges

Prior to March, 2018, Sentinel-1 “Ground Range Detected” (GRD) preprocessed data exhibit an artifact commonly known as “GRD border noise” which affects image borders in both the range and azimuth directions. This was corrected by the Sentinel-1 “Instrument Processing Facilities” (IPF) in versions 2.90 and newer, but this artifact remains in imagery processed before this version.

The character of this noise makes it difficult to remove through simple procedures like thresholding because many dark targets, like water, may be within the same range of backscatter.

During preprocessing steps, we run the built-in noise removal step from SNAP, “Remove-GRD-Border-Noise”. While this algorithm helps the issue, it is unfortunately far from perfect and leaves many artifacts in the image. To help address this issue, we perform a very conservative masking step after RTC scene generation that cleans up some of these missed artifacts.

Still, artifacts from this original processing issue remain in the dataset and may impact your analysis if you do not consider this issue. Depending on your application, you may find that a simple threshold is suitable for masking this noise or use outlier detection or other methodologies robust to noise when doing things like creating image composites or temporal metrics.

For more information on this issue in GRD products, see the document, Masking “No-value” Pixels on GRD Products generated by the Sentinel-1 ESA IPF.