Assessing Co‐Located NISAR and Sentinel‐1 Observations for Atmospheric Characterization

Tropospheric delay remains one of the largest sources of uncertainty in repeat‐pass Interferometric Synthetic Aperture Radar (InSAR) measurements, while accurate three‐dimensional deformation retrieval continues to be limited by the geometry of current spaceborne SAR systems. The recent availability of near‐simultaneous observations from the NASA‐ISRO Synthetic Aperture Radar (NISAR) mission and Sentinel‐1 provides a unique opportunity to investigate atmospheric signals using independent sensors observing nearly the same scene within minutes of each other. We analyze colocated NISAR L‐band and Sentinel‐1 C‐band interferometric observations over four regions spanning flat and mountainous terrain with varying vegetation cover. After applying the ionospheric correction provided in NISAR GUNW products, the residual NISAR interferometric phase exhibits strong agreement with corresponding Sentinel‐1 observations, with Pearson correlations exceeding 0.9 in several cases. The similarity between the two data sets suggests that both systems are observing a common atmospheric signal, which is interpreted primarily as tropospheric delay for the short (12‐day) interferometric intervals considered here. In contrast, tropospheric correction layers provided in the NISAR products often fail to reproduce the dominant spatial variability observed in either data set, particularly over flat terrain. These results demonstrate the potential of colocated NISAR and Sentinel‐1 observations for future evaluation of atmospheric correction products and motivate the development of multi‐frequency, multi‐geometry approaches for improved atmospheric characterization and three‐dimensional deformation estimation.