1.3.3.1. Considerations for Using and Interpreting CGM InSAR data

When interacting with and using the SCEC CGM InSAR product, we need to take several things into account that affect how we interpret this data: how we’re viewing the displacement at the surface, with what spatial reference are we interpreting this data, and how many dimensions of displacement estimates we are actually using at any given time.

Look Direction

When looking at an interferogram or InSAR product, the direction of measured displacement depends on how the satellite is looking at the surface, so to accurately interpret surface motions we need to know what direction the satellite is facing when it collects its data. This product is created from Sentinel-1 Synthetic Aperture Radar (SAR) satellite data and both SAR satellites in the Sentinel-1 system are right-looking satellites. One is on an ascending track and the other on a descending track [Figure 1.2], so in southern California, the ascending satellite is generally looking ENE, while the descending satellite is looking WNW.

One-dimensional Deformation Measurement

These InSAR measurements fundamentally represent only one dimension of displacement information, and this direction is the line-of-sight (LOS) from the surface to the Sentinel-1 SAR satellite. In the areas where we have both an ascending and descending coverage [Figure 1.3], we technically have two different LOS directions, which can be combined to produce a two dimensional product. We currently leave this combination to the user to complete, as well as any 3-dimensional decomposition, as decomposition is still an active area of research [e.g. Shen and Liu, 2020, Wright et al., 2004].

Relative Measurement: No Built-in Reference Frame

InSAR measurements are not absolute measurements of displacement. They are made relative to a satellite, relative to a reference pixel or region, and relative to a SAR image of a certain date; they are not placed in an absolute reference frame during processing. This fact is one major difference between InSAR and other geodetic methods, where other methods like Global Navigation Satellite System (GNSS) measurements are processed within a precisely defined global reference frame.

The SCEC InSAR CGM product currently uses two GNSS station locations to define its relative motions (two tracks share one station, the other two tracks share the other station). These GNSS station point location names are listed within the metadata of the HDF5 product package for each track.