Towards expressing InSAR in a terrestrial reference frame: Preliminary tests and results of tidal loading

We present initial tests and results of the application of tidal loading corrections to synthetic aperture radar interferograms for the ultimate purpose of aligning InSAR-derived displacements and velocities in a consistent terrestrial reference frame. InSAR has enjoyed supreme success as a geodetic observation over the last 20 years. However, these observations produce relative, rather than absolute, deformation measurements, thus requiring many long-term observations to be interpreted at local rather than global scales. For nearly 30 years, GNSS has been one of the core space geodetic techniques, used alongside Satellite Laser Ranging (SLR) and Very Long Baseline Interferometry (VLBI) to define terrestrial reference frames. Here we test applying fundamental corrections that are nowadays standard within GNSS processing and used as part of the reference frame alignment processes, but have previously been overlooked with InSAR data, to correct InSAR time series and line-of-sight velocity solutions. These corrections address many current limitations of InSAR, as it moves from the realm of large, local signals, such as earthquakes and localized subsidence, to providing data capable of constraining smaller, long-wavelength signals, such as interseismic fault motions and uplift on the scale of mountain belts and post-glacial adjustment. Additionally, this ultimate will allow us to place local-scale vertical signals identified by InSAR, such as coastal subsidence, in the context of global phenomena such as eustatic sea level change. We find that large tidal loading signals, such as solid Earth tides and ocean tides near coastlines, can produce significant apparent long-wavelength deformations of a couple of fringes, which may have previously been attributed to orbital errors or ionospheric perturbations. While these may still be a significant source of error, we conclude that well understood phenomena such as tidal loads should be considered when aligning and comparing deformation InSAR time series and velocities with those from other geodetic techniques such as GNSS.