Tomographic synthetic aperture radar (TomoSAR) is a promising technique for the estimation of forest above-ground biomass (AGB), but knowledge gaps still remain concerning the effects of forest type and ground topography. This article presents new results at P- and L-bands based on data acquired during the TomoSense campaign. The study area is a temperate forest, predominantly beech and spruce, with ground slopes ranging up to 40°. Analysis of vertical reflectivity profiles shows distinct differences for spruce and beech. Three AGB retrieval methods are analyzed, i.e., total vertical backscatter $I_{\text {tot}}$ , canopy backscatter from a height layer $I_{c}$ , and the ratio $I_{\text {cr}} = I_{c}/I_{\text {tot}}$ . All three methods show sensitivity to AGB for spruce, whereas for beech, this is only true for the two latter methods. For the P-band, a significant ground slope effect is observed, while less so for the L-band. The highest $R^{2}$ is obtained for spruce with HV polarization, $I_{c}$ and ground slopes less than 10°, i.e., $R^{2} = 0.86$ and RMSE =15.6% for P-band and $R^{2} = 0.75$ and RMSE =12.5% for L-band. Corresponding results by including all forest types are $R^{2} = 0.77$ and RMSE =11.4% for P-band and $R^{2} = 0.54$ and RMSE =12.0% for the L-band. Moreover, the performance of using $I_{\text {cr}}$ is similar to that of $I_{c}$ . The ratio $I_{\text {cr}}$ can be determined without absolute radiometric calibration which relaxes system requirements. This article reinforces the potential of TomoSAR for forest AGB estimation and draws attention to important effects of tree species and ground slope.

中文翻译：

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P 和 L 波段 SAR 层析成像对温带丘陵森林地上生物量的敏感性


层析合成孔径雷达（TomoSAR）是一种很有前景的森林地上生物量（AGB）估算技术，但关于森林类型和地面地形的影响仍然存在知识空白。本文根据 TomoSense 活动期间获取的数据介绍了 P 和 L 波段的新结果。研究区域是温带森林，主要是山毛榉和云杉，地面坡度高达 40°。垂直反射率剖面分析显示云杉和山毛榉存在明显差异。分析了三种 AGB 反演方法，即总垂直反向散射 $I_{\text {tot}}$ 、高度层冠层反向散射 $I_{c}$ 以及比率 $I_{\text {cr}} = I_ {c}/I_{\text {tot}}$ 。所有三种方法都显示云杉对 AGB 敏感，而对于山毛榉，这只适用于后两种方法。对于 P 波段，观察到显着的地面坡度效应，而对于 L 波段则不太明显。具有高压极化、$I_{c}$ 和地面坡度小于 10° 的云杉获得最高的 $R^{2}$，即 $R^{2} = 0.86$ 且 RMSE = 15.6%（P-） L 波段的 $R^{2} = 0.75$ 和 RMSE =12.5%。包含所有森林类型的相应结果为：P 波段的 $R^{2} = 0.77$ 和 RMSE =11.4%，L 波段的 $R^{2} = 0.54$ 和 RMSE =12.0%。此外，使用 $I_{\text {cr}}$ 的性能与 $I_{c}$ 类似。比率 $I_{\text {cr}}$ 无需绝对辐射校准即可确定，从而放宽了系统要求。本文强调了 TomoSAR 在森林 AGB 估算方面的潜力，并提请人们注意树种和地面坡度的重要影响。