GNSS-constrained InSAR correction for land subsidence mapping in Tianjin, China

Interferometric synthetic aperture radar (InSAR) is a primary geodetic technique for monitoring surface deformation, achieving high spatial resolution and extensive coverage. However, long-wavelength errors – such as orbital errors, atmospheric delays, and tidal loading – mask subtle deformation signals. Furthermore, reliance on a single reference station neglects plate motion gradients, causing time series displacement biases. These factors limit millimeter-level monitoring accuracy. Our study integrates InSAR and global navigation satellite system (GNSS) data, leveraging GNSS measurements to correct interferograms for long-wavelength errors and thereby enhance InSAR deformation accuracy. This integration also anchors InSAR-derived deformation to GNSS motions and helps establish a reference frame. Using Tianjin in northern China as our case study, we corrected long-wavelength errors in both ascending and descending Sentinel-1 interferograms with continuous GNSS observations. To evaluate the effectiveness of our long-wavelength error correction method, we compared the corrected interferograms against results derived from both the general atmospheric correction online service (GACOS) atmospheric correction and trend surface modeling. We conducted a systematic evaluation of correction accuracy and its impacts on four key results: phase changes, velocity estimates, cumulative displacement, and seasonal signals. Results show that the correction by using GNSS data improved accuracy in 84% of the 862 processed interferograms, while the performance of the correction depended on the baseline distance of the GNSS stations. The root mean square error (RMSE) of the time series displacement for the GNSS validation stations was maintained within the millimeter-level, improving the displacement accuracy by at least 80%. Regarding reference stability, the GNSS correction method proved to be more reliable than the single-reference approach. These results highlight the benefits of multiple GNSS stations in InSAR correction. Finally, our analysis of the fused vertical velocities, derived from both ascending and descending tracks, reveals a notable reduction in land subsidence across Tianjin since 2016, with subsidence rates and areas decreasing annually.