Biomass Retrieval in Topographic Boreal Forest using Polarimetric and Tomographic SAR in P- and L-band

Abstract: Forests and their associated habitats are an integral part of Earth's biosphere and provide essential environmental services. There is an ever present need for accurate and economic monitoring of forest parameters such as biomass - of great interest both locally, for forest management and resource utilization, and globally as it represents stored atmospheric CO2 and one of the largest uncertainties in climate modelling.
Forest monitoring using orbital assets has the benefit of consistent, large scale coverage with short revisit times. Radars have an advantage over optical sensors in that they are active sensors independent of solar illumination and are much less sensitive to weather, with synthetic aperture radar (SAR) providing images comparable to optical data in resolution. The longer wavelengths used by radars interact differently with the forest canopy, with the longest wavelengths able to penetrate down to the ground even in dense forest. Radar backscatter therefore includes information about the forest structure, but may also contain significant contributions from the underlying ground surface.
Electromagnetic modelling of these interactions, the focus of the first paper, is important in order to support the retrieval of relevant parameters from the returned signal. It is demonstrated that much of the variations in intensity and polarization seen in P-band SAR images can be attributed to the direct and indirect reflections of vertical stems on sloping ground. The sensitivity of forest backscatter polarization to the geometry and structure of the scene forms the basis of biomass retrieval models such as the one evaluated in the second paper, which includes all linear polarimetric components as well as a ground slope term. This model, previously evaluated for two Swedish boreal forest sites, is here revisited with a new data set and the results are shown to be consistent with previous findings, although a systematic increase in backscatter attributed to varying moisture conditions result in higher biomass estimates.
The third and final paper includes the first evaluation of biomass retrieval using tomographic SAR (TomoSAR) data representative of a proposed L-band mission with two formation flying satellites. Tomography can resolve the vertical forest profile and offers a new dimension for forest structure investigation. A linear model using the HH-polarized backscatter is found to retrieve forest biomass with an RMSE of 26-30%, performing better than higher resolution data from an airborne SAR despite only utilizing a single polarization.