Capabilities of L-band TomoSAR for Forest Structure Characterization Towards Spaceborne Implementations

Kurzfassung

With decreasing frequency synthetic aperture radar (SAR) pulses penetrate more and more into and through vegetation layers, and interact with vegetation elements located at different heights within the forest volume and with the underlying ground. This, combined with the ability of SAR tomographic (TomoSAR) techniques to reconstruct the 3D radar reflectivity opens new perspectives on the design of space borne SAR configurations to explore and map forest structure parameters on large (global) scales with high spatial and temporal resolution. L-band (wavelength around 20 cm) appears more versatile for forest structure applications than other frequencies as it provides a balanced sensitivity to structurally relevant scattering components from the canopy top to the ground in a wide range of forest types and conditions. Moreover, the higher temporal stability of the scatterers leads to lower temporal decorrelations in repeat-pass configurations than at higher frequencies, and depending on the repeat-pass interval may even allow the implementation of quantitative structure parameters retrievals. Finally, considering bandwidth constraints and international regulations for the space borne realisation of SAR missions, L-band appears as the optimal compromise between possible spatial resolution and/or interferometric performance and penetration for forest structure applications. In this presentation, the potentials and challenges of L-band TomoSAR for the (quantitative) characterization of 3D forest structure are evaluated, discussed and summarized with respect to: 1. Information content – First, the ability of L-band to penetrate into and through the canopy down to the ground is assessed directly in terms of both the ground-to-volume power ratio and the performance to estimate the location of the underlying ground. Second, the capability in deriving physical forest structure descriptors from L-band TomoSAR profiles is discussed within a recently proposed framework in which stands can be projected in relation to their horizontal and vertical complexity. One important point that will be addressed is the information content and the role of the different polarization channels. 2. TomoSAR imaging geometry – Conditioned to the imaging algorithm, different TomoSAR geometries in terms of regularity, spacing in the direction orthogonal to the line of sight, and number of the acquisition tracks can result into different vertical resolutions and ambiguities in the TomoSAR profiles. The imaging geometry depends primarily on flight / orbital constraints, but it can change across a scene as a consequence of a change of incidence angle. The impact of such changes on the L-band range, sensitivity, and retrieved spatial gradients of the structure descriptors is evaluated. 3.Technical implementation – At present, there is no TomoSAR configuration in which all the images can be acquired simultaneously. As a consequence, any configuration is affected by (more or less negligible) changes of the reflectivity profile in time. Bistatic implementations rely on repeated single-pass interferometric acquisitions with different baselines. The advantage of such configuration with respect to conventional repeat-pass ones in mitigating the disturbance of the L-band temporal decorrelation on the TomoSAR structural characterization within typical space borne acquisition intervals is addressed. The analysis is supported by experimental TomoSAR results achieved in the framework of actual airborne SAR (DLR’s F-SAR platform) temperate and tropical forest campaigns.