In forest areas, when using interferometric synthetic aperture radar (InSAR) technique to extract subcanopy topography, a key problem is how to reduce the impact of the forest-scattering process on InSAR height measurements. In this paper, we propose a new methodology for subcanopy topography extraction by making full use of single-baseline TanDEM-X InSAR data. The interferometric phase is used to estimate a high-resolution and high-precision digital elevation model (DEM), and the coherence magnitude is used to extract the scattering phase center (SPC) height associated with canopy scattering. Allowing for the difference in the sensitivity of TanDEM-X bistatic interferometry information for forests with different densities, two scenarios are involved: (1) For pixels characterized by dense forest, the X-band SAR signal cannot penetrate through the forest canopy layer. In this case, an existing scattering model is used to estimate the InSAR penetration depth from the coherence magnitude. Note that when the penetration depth is small compared with the height of ambiguity (HoA), the former can be recognized as the difference between the InSAR DEM and the actual surface height. Some sparse forest heights (or SPC heights) measured by the Ice, Cloud, and land Elevation Satellite-2 (ICESAT-2) system are then used to help determine the linear relationship between penetration depth and sparse forest height (or SPC height). Finally the SPC can be estimated and removed from the TanDEM-X InSAR DEM. (2) For pixels characterized by sparse vegetation, the X-band likely penetrates the vegetation volume layer to ground level. For this scenario, a coherence threshold is set to identify these pixels, and the corresponding SPC heights can be considered zero where the subcanopy topography is approximately equal to the InSAR-generated DEM. The proposed method was validated by combining TanDEM-X coregistered single look slant range complex (CoSSC) data from two test sites with different forest types with ICESAT-2 points. The results are as follows. For the Krycklan test site, compared with the validation data, the RMSE varied from 7.76 m for the InSAR DEM to 3.22 m for the reconstructed subcanopy topography, which is an improvement of 58.5%. For the Oyan test site, subcanopy topography achieved an RMSE of 7.26 m compared with the RMSE of 27.12 m before SPC height correction, which represents an improvement of 80.1%. In addition, we investigated the impact of the topographic slope and the necessity of using the coherence threshold. For inconsistencies between TanDEM-X bistatic InSAR data and ICESAT-2 measured forest height over time, the fitting between penetration depth and SPC height calculated by ICESAT-2 subcanopy topographic elevation and InSAR-generated DEM can be used. These investigations confirm the feasibility and reliability of the proposed method.

在森林地区，利用干涉合成孔径雷达（InSAR）技术提取冠层地形时，一个关键问题是如何减少森林散射过程对InSAR高度测量的影响。在本文中，我们充分利用单基线TanDEM-X InSAR数据，提出了一种新的冠层地形提取方法。干涉相位用于估计高分辨率和高精度数字高程模型（DEM），相干幅度用于提取与冠层散射相关的散射相位中心（SPC）高度。考虑到不同密度森林的TanDEM-X双基地干涉测量信息灵敏度的差异，涉及两种情况：（1）对于以密集森林为特征的像素，X 波段 SAR 信号无法穿透森林冠层。在这种情况下，现有的散射模型用于从相干幅度估计 InSAR 穿透深度。请注意，当穿透深度与模糊高度 (HoA) 相比较小时，前者可以被识别为 InSAR DEM 与实际表面高度之间的差异。然后使用冰、云和陆地高程卫星 2 (ICESAT-2) 系统测量的一些稀疏森林高度（或 SPC 高度）来帮助确定渗透深度和稀疏森林高度（或 SPC 高度）之间的线性关系。最后，可以从 TanDEM-X InSAR DEM 中估计并去除 SPC。(2) 对于以稀疏植被为特征的像素，X 波段可能穿透植被体积层到地面。对于这个场景，一个相干阈值被设置来识别这些像素，并且相应的 SPC 高度可以被认为是零，其中子冠层地形近似等于 InSAR 生成的 DEM。通过将来自具有不同森林类型的两个测试站点的 TanDEM-X 配准单视斜距复合体 (CoSSC) 数据与 ICESAT-2 点相结合，对所提出的方法进行了验证。结果如下。对于 Krycklan 试验场，与验证数据相比，RMSE 从 InSAR DEM 的 7.76 m 到重建的冠层地形的 3.22 m，提高了 58.5%。对于 Oyan 试验场，与 SPC 高度校正前的 RMSE 27.12 m 相比，次冠层地形实现了 7.26 m 的 RMSE，提高了 80.1%。此外，我们调查了地形坡度的影响以及使用相干阈值的必要性。对于 TanDEM-X 双基地 InSAR 数据与 ICESAT-2 随时间测量的森林高度之间的不一致，可以使用穿透深度与由 ICESAT-2 副冠层地形高程和 InSAR 生成的 DEM 计算的 SPC 高度之间的拟合。这些调查证实了所提出方法的可行性和可靠性。