Temporal analysis of C-band InSAR decorrelation for canopy height mapping over dry forests and tropical savannas

Dry forests and savannas have highly heterogeneous horizontal and vertical structure of woody vegetation and high temporal variability in moisture and phenology. This presents distinct challenges to SAR-based approaches to mapping canopy height compared to tropical and temperate forests. Dense time-series of C-band (Sentinel-1A/1B) and in future L-band (NASA/ISRO NISAR) provide a pathway to reduce the impact of signal noise and environmental conditions and extract additional information more directly related to height, using repeat-pass Interferometric SAR (InSAR) techniques. This work explores the potential information in Sentinel-1(S1) C-band InSAR temporal decorrelation and Global Ecosystem Dynamics Investigation (GEDI)-derived relative height metrics for canopy height estimation. The study comprises two major parts 1) temporal decorrelation analysis and 2) canopy height estimation. The temporal dynamics of the 12-day InSAR correlation (𝛾) were analyzed over a wide range of canopy heights and woody ecosystems, with a focus on dry forests and savannas in Australia, India and South Africa as part of a NASA Carbon Monitoring System (CMS) investigation. We observe a strong seasonal pattern in InSAR correlations over low to medium-height vegetation. In contrast, the 𝛾 for taller canopies is unchanged throughout the observation period (1 year). We also observe differences in 𝛾 derived from VV and VH polarizations with expected higher 𝛾 values in VV for a majority of the scenarios. Further, for inverted canopy heights, we obtain a Pearson correlation r>0.5 with an RMSE of 3-6m for different forest types at a scale of 1 hectare. However, the inversion results are limited to low to moderate canopy heights as we observe a steady low 𝛾 for taller canopies. In summary, this study provides new insights into the applicability and limitations of using C-band InSAR data for canopy height estimation in dry forest and savanna studies. We also discuss the implementation of the proposed canopy height estimation algorithm on the NASA-Multi-Mission Algorithm and Analysis Platform (MAAP) for global scalability with current and future InSAR time series. The limitations with C-band could be overcome partly, if not completely, with longer wavelengths like L-band, such as the one proposed for the upcoming NISAR and ROSE-L missions.