Identifying Wet Troposphere Delay in L-Band InSAR Using Weather Radar Reflectivity

Synthetic Aperture Radar (SAR) pulses undergo variable propagation delays in the atmosphere due to changes in pressure, temperature, and humidity within the troposphere, causing large error in Interferometric SAR (InSAR) measurements of land surface displacement. Wet troposphere delay, resulting from condensed water and water vapor clouds, can introduce delays of tens of centimeters that significantly impact surface displacement estimates. This study provides unequivocal evidence of the wet troposphere's impact on InSAR phase measurements by examining spatial patterns in NOAA NEXRAD weather radar reflectivity and interferometric phase outliers. We utilize a feature-comparison approach with reflectivity data from NEXRAD radar stations to identify artifacts from wet tropospheric delays in InSAR phase measurements derived from rapid repeat-pass data acquired by UAVSAR L-band SAR. NEXRAD's 5-min scanning interval, compared to UAVSAR's 30-min revisit time, enabled detection of phase artifacts caused by fast-moving and developing clouds. We identify regions in InSAR interferograms with troposphere-induced phase artifacts by matching features common to InSAR phase outlier masks and NEXRAD high reflectivity masks. Matched results between InSAR phase noise and NEXRAD reflectivity show phase delays of up to 25 radians in L-band, corresponding to 48 cm of delay. Comparison with tropospheric delays calculated using the Generic Atmospheric Correction Online Service for InSAR (GACOS) showed global weather models lack sufficient spatial and temporal resolution to accurately estimate observed wet troposphere delays. While our study focused on UAVSAR, findings apply to other SAR missions, including L-band NISAR and ALOS2/4, aiding identification and interpretation of InSAR results affected by tropospheric delays.