Conference Agenda

Phase triplets are an interferometric observable derived by summing the interferometric phases around closed loops formed by three SAR acquisitions [1]. In the absence of perturbing effects, the closure sum should be zero. Deviations from this condition, referred to as phase non-closure, are primarily attributed to multi-looking effects, which occur when two or more scattering populations with distinct phase behaviours coexist within a single resolution cell [2]. Previous studies have demonstrated the potential of phase triplets to monitor temporal surface and vegetation changes, including soil moisture variations [1][2], vegetation growth [3], and fluctuations in vegetation water content [2]. However, the physical mechanisms underlying phase non-closure, and in particular its dependence on wave polarization, remain insufficiently understood.

This study aims to advance our understanding of surface and vegetation mechanisms along time by investigating the polarimetric dependence of phase non-closure. To this end, early data from the ESA BIOMASS P-band SAR mission, acquired during the commissioning phase, are employed. With its fully polarimetric capabilities and long wavelength (70 cm), BIOMASS provides sensitivity to scattering processes throughout the entire canopy, from top to bottom, offering a unique opportunity to study the temporal behaviour of forest backscatter under dense vegetation conditions.

Such capabilities are explored over the Gabonese rainforest, a natural environment characterized by high above-ground biomass, complex canopy structure, and diverse tree species composition. Triplets of temporally proximate P-band acquisitions (3 days intervals) are analysed to assess how polarization influences both the magnitude and spatial distribution of phase non-closure. The analysis focuses on small-baseline interferometric pairs to minimize geometric decorrelation, thereby isolating the contributions of volumetric and polarimetric effects. In addition, complementary airborne LiDAR data are used to discriminate forested from non-forested areas and to relate phase non-closure patterns to canopy height and structural heterogeneity.

Results reveal a clear polarization-dependent behaviour: over open areas, HH and VV channels exhibit low and spatially stable closure deviations, whereas in forested regions, HV and cross-polarized combinations display larger and more variable phase non-closure values. These findings suggest that phase non-closure carries valuable information about canopy structure and dielectric heterogeneity. Understanding its polarimetric sensitivity can improve the interpretation of multi-temporal P-band interferometric data for biomass estimation, vegetation dynamics monitoring, and physical model validation in tropical forests.

[1] F. De Zan, A. Parizzi, P. Prats-Iraola and P. López-Dekker, "A SAR Interferometric Model for Soil Moisture," in IEEE Transactions on Geoscience and Remote Sensing, vol. 52, no. 1, pp. 418-425, Jan. 2014.

[2] F. De Zan, M. Zonno and P. López-Dekker, "Phase Inconsistencies and Multiple Scattering in SAR Interferometry," in IEEE Transactions on Geoscience and Remote Sensing, vol. 53, no. 12, pp. 6608-6616, Dec. 2015.

[3] Y. Yuan, M. Kleinherenbrink and P. López-Dekker, "On Crop Growth and InSAR Closure Phases," in IEEE Transactions on Geoscience and Remote Sensing, vol. 62, pp. 1-12, 2024.