Standard Displacement Products

The product is packaged as a Netcdf4 file guided by file structure as is planned for the NISAR mission. The top-level group of the product is called “science”, which has a “grids” group containing the geo-coded datasets and a “radarMetaData” group containing specific information related to the SAR data. Within the “grids” group a distinction is made between a “data”, “imagingGeometry”, and “corrections” sub-group. The “data” group contains 2D datasets at 3’’ (~90 m) posting such as the “unwrappedPhase”, the “imagingGeometry” group contains 3D datasets posted laterally at 0.1° (~11km) such as the “incidenceAngle”, while the “corrections” group contains placeholders for potential future layers, such as for ionospheric, tropospheric, and solid earth tide corrections and where the posting is likely to vary depending on the correction. All 2D and 3D datasets are with respect to WGS84 (EPSG 4326). Detailed information on the projection information e.g. wkt is specified under the “crs*” datasets provided under the “data” and “imagingGeometry” groups. At the top-level of the product we also define a “productBoundingBox” as a GDAL compatible wkt polygon. Global attributes provide further information on the product version and citation references.

A description of the different datasets and their sampling is provided in the below, while a file structure for one of the sample products is shown in the figure at the end of this section.

*Included as placeholder for potential future layers

File structure overview of a GUNW product

The L2 GUNW product is generated by ARIA using the ISCE TopsApp processor. The below table provides and overview of the processing parameters.

Overview of the processing parameters.
*Other processing parameters correspond to ISCE topsApp defaults.

Spatial and temporal connectivity

The spatial extend of an individual GUNW product is that of a Sentinel-1 frame. During the interferogram formation one of the two acquisition dates (either the “reference” or “secondary”) will consist out of a single ESA sentinel-1 product, which is referred to as “spatial reference”, while the other acquisition date will use as many ESA sentinel-1 products it needs to cover the “spatial reference” over the ARIA AOI completely.

We explicitly avoid SLC stitching of ESA Sentinel-1 products originating from a different Instrument Processing Facility (IPF). The acquisition date with multiple IPF versions is to be used as “spatial reference”. Note that the derived GUNW products can still be stitched along-track and thus no impact is expected for the spatial connectivity. In case both acquisition dates have multiple IPF versions, the interferogram pairing combination is skipped, and a next available interferogram combination is attempted. This procedure is repeated until the temporal minimal match criteria is met.

Example of interferogram pairing of a “Reference” with a “Secondary” acquisition with a minimal match criterion set to 2. Following the spatial coverage and IPF pairing rules only the acquisitions made on 20150408 and 20150327 are considered valid for “Secondary”. As the IPF is changing for the “Secondary” acquisition on 20150408, while that of the “Reference” is consistent, the frame definition of the “Secondary” will be used as “spatial reference” for the 20150502-20150408 interferogram, leading to 4 standard products for this pairing. As the IPF is consistent for both the “Reference” and “Secondary” acquisition made on 20150327, the frame definition of the “Reference” will be used as “spatial reference” for the 20150502-20150327 interferogram, leading to 3 standard products for this pairing.

Sensor-DatasetName-A/D-L/R-tracknumber-mode-YYYYMMDD_YYYYMMDD-HHMMSS-XXYYYN/S_XXYYYN/S-PP/RP/PR/RR-systemTag-versionTag.nc

Example:
S1-GUNW-A-R-035-tops-20170206_20170107-020845-42995N_41129N-PP-a63b-v2_0_0.nc

The product version tag captures if changes have been made to the standard product processing workflow. Changes are summarized below. Note that the product version tag is different from the global attribute “version” variable in the NETCDF file which captures changes in the standard product data structure.

• QGIS, ArcGIS
• Panoply
• GDAL

Unwrapping Errors

Unwrapping is not trivial. GUNW products are provided as is, with no warranties that the products are correctly unwrapped. It is the user responsibility to apply masking and correct for unwrapping errors that might be present.


Both the “coherence” and “connectedComponent” layers included within the GUNW products act as quality markers for the unwrapping. The “connectedComponent” layer is an output from the unwrapping method which provides more information on the confidence of the unwrapping. Most often unwrapping errors observed within the products occur in regions with low coherence, i.e. connected component 0, or between disconnected regions, i.e. different connected components.

Best practice is to mask out water bodies as well as connected component zero. In addition, it is recommended to inspect if there are multiple connected components left and correct the unwrapped phase where needed.