Abstract
The paper investigates the penetration properties of an airborne Ku-band frequency modulated continuous waveform (FMCW) profiling radar named Tomoradar and a satellite near-infrared lidar into the boreal forest of Finland. We achieve the accumulative energy distributions based on the Tomoradar waveforms and the satellite lidar waveforms generated from the high-density airborne lidar data within Tomoradar footprints. By comparing two groups of the height percentiles and energy percentiles derived from the accumulative energy distributions, we evaluate the relationship of penetrations between the Ku-band microwave and near-infrared laser according to the coefficients of the determination (COD), and the root mean square errors (RMSE) of linear regression analyses. The quantitative analysis results demonstrate that the height and energy percentiles derived from Tomoradar waveforms correlate well with those from satellite lidar waveforms with the mean correlation coefficients of more than 0.78 and 0.85. The linear regression models for the height and energy percentile produce excellent fits with the mean CODs of 0.95 and 0.90 and the mean RMSEs of 1.25 m and 0.03, respectively. Less than 15% of height percentiles and 87.54% of the energy percentiles in the sixth stratum near the ground derived from Tomoradar waveforms surpass those from satellite lidar waveforms. Hence, the Ku-band microwave can penetrate deeper into the forest than the near-infrared laser at the same spatial scale. In addition, quadratic fitting models are established to describe the differences of the height percentile (DHP) and the energy percentile (DEP) to expound the canopy height and closure contributions numerically. The facts that the CODs of the DHP and DEP individually are more than 0.96 and 0.89 and the fitting residual histograms approximate to normal distributions reveal the reliabilities of the proposed fitting models. Thus, the penetration analyses are valid for the explorations on the FMCW radar applications and the data fusion of the Ku-band radar and near-infrared lidar in the forest investigations.
Highlights
We discover that the Tomoradar waveform shifts towards the ground relative to the satellite lidar waveform
More height and energy percentiles can represent the characteristics of EM penetration into the forest and bring about more complicated evaluation indicators
We suggest that two groups of height and energy percentiles with 12 metrics are favourable for the penetration analyses of satellite lidar and Tomoradar
Summary
Forests play prominent roles in determining carbon storage, climate and ecological functionalities [1,2]. Monitoring the forest structure’s spatial and temporal characteristics contributes to understanding carbon stock modelling and global environmental changes [3,4]. Remote sensing technologies have been proved to be highly effective for acquiring more accurate and spatially continuous forest structure properties on a large scale in a Remote Sens. Passive optical imaging systems can only collect the reflected sunlight from the exterior canopy surface and are incapable of obtaining the vertical forest structures inside the canopy [5,6]. Active remote sensing devices, including lidar and radar systems, can explore the understory vegetation and the ground to become the primary technologies in forest investigation
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