Abstract
Assessments of forest cover, forest carbon stocks and carbon emissions from deforestation and degradation are increasingly important components of sustainable resource management, for combating biodiversity loss and in climate mitigation policies. Satellite remote sensing provides the only means for mapping global forest cover regularly. However, forest classification with optical data is limited by its insensitivity to three-dimensional canopy structure and cloud cover obscuring many forest regions. Synthetic Aperture Radar (SAR) sensors are increasingly being used to mitigate these problems, mainly in the L-, C- and X-band domains of the electromagnetic spectrum. S-band has not been systematically studied for this purpose. In anticipation of the British built NovaSAR-S satellite mission, this study evaluates the benefits of polarimetric S-band SAR for forest characterisation. The Michigan Microwave Canopy Scattering (MIMICS-I) radiative transfer model is utilised to understand the scattering mechanisms in forest canopies at S-band. The MIMICS-I model reveals strong S-band backscatter sensitivity to the forest canopy in comparison to soil characteristics across all polarisations and incidence angles. Airborne S-band SAR imagery over the temperate mixed forest of Savernake Forest in southern England is analysed for its information content. Based on the modelling results, S-band HH- and VV-polarisation radar backscatter and the Radar Forest Degradation Index (RFDI) are used in a forest/non-forest Maximum Likelihood classification at a spatial resolution of 6 m (70% overall accuracy, κ = 0.41) and 20 m (63% overall accuracy, κ = 0.27). The conclusion is that S-band SAR such as from NovaSAR-S is likely to be suitable for monitoring forest cover and its changes.
Highlights
A cross-comparison with the Advanced Land Observing Satellite (ALOS) PALSAR-based global forest cover map provided by the Japan the Japan Aerospace Exploration Agency (JAXA) at 25 m scales was carried out [52] (Figure 6a,b)
S-band radar total backscatter returns in deciduous stands slightlysuggest weakerstrong scattering from coniferous canopies returns 6)
Total backscatter at HH-polarisation is high due to ground/trunk interactions from the deciduous canopy, higher than for a coniferous canopy. This trend is evident for VV-polarisation
Summary
Forests play a pivotal role in biogeophysical feedbacks of the terrestrial biosphere with the climate system [1] and constitute one of the main components of the global carbon cycle [2] in the form of woody aboveground biomass (AGB) [3]. For information related to forest attributes at the stand level such as cleared-felled or canopy degradation, S-band radar data at 6 m resolution from NovaSAR-S could potentially provide a more useful data product than is currently available from Sentinel-1, ALOS-2 and other SAR sensors that provide global coverage. Simulations from microwave canopy radiative transfer models provide insight into the complex interactions of the radar wave with the different components of the canopy as a function of wavelength, incidence angle and polarisations [26,27,28,29]; for instance, increasing level of penetration with increasing wavelength Such models have shown that the low frequencies (P- and L-band), in particular co-polarized (horizontal transmit and horizontal receive—HH, vertical transmit and vertical receive—VV) channels produce stronger scattering from larger scattering elements (branches, trunks) with high frequencies (X- and C-band) from leaves and needles. The S-band (3.1–3.3 GHz) lies between the longer L-band (1–2 GHz) and the shorter C-band (5–6 GHz)
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