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- Bennet, Patrik, 1995
(author)
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Characteristics of Vertical Forest Backscatter Profiles Measured using Radar Tomography
- 2023
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Licentiate thesis (other academic/artistic)abstract
- Forests are an integral part of the Earth’s climate system, having shaped the conditions of life throughout millions of years, but the coverage and conditions of forests are under rapid change. To monitor this change and predict the impact on climate, yearly global mapping of forest Above-Ground Biomass (AGB) is needed. Synthetic Aperture Radar (SAR) is a well-suited technique for this purpose, able to sense through dense canopies with high spatial resolution. Tomographic SAR (TomoSAR) resolves the vertical distribution of backscatter by constructing 3D tomograms of forested areas. Here, results obtained in the TomoSense project are presented, where sensitivity of P- and L-band TomoSAR to AGB of a temperate forest is analysed. For the first time, differences between the AGB dependence of spruce and beech vertical backscatter profiles are identified. Moreover, unique observations of ground slope influence on TomoSAR AGB retrieval are presented. The effect for P-band is significant, but not for L-band. For spruce, AGB was estimated for P-/L-band (ground slopes below 10◦), with R2 = 0.86/0.75 and RMSE = 15.6/12.5%. Without separating forest types, R2 = 0.77/0.54 and RMSE = 11.4/12.0%. Finally, unique L-band radar tomography observations using the BorealScat tower-based radar are presented. Time series of the vertical backscatter profile of a boreal forest during the summer of 2018 is analysed. Weekly diurnal cycles up to 1.3 dB are observed, showing clear differences depending on both canopy height layer and polarization. These differences are new results which are likely related to tree transpiration phenomena but needs further study.
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| 2. |
- Bennet, Patrik, 1995, et al.
(author)
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Diurnal Cycles of L-Band Tomographic SAR Backscatter in a Boreal Forest During Summer: Observations by the Borealscat Tower Radar
- 2023
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In: International Geoscience and Remote Sensing Symposium (IGARSS). ; 2023-July, s. 2195-2198
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Conference paper (peer-reviewed)abstract
- The BorealScat tower radar experiment, in Remningstorp, Sweden, acquired tomographic L-band backscatter time series over a boreal forest stand consisting of Norway spruce (Picea abies). Data from summer 2018 is analysed in this study, where diurnal cycles with variations up to more than 1 dB are identified for HH and VV polarizations. The HH cycle has its minimum at night, its maximum at noon and mainly occurs in the middle layers of the forest. Conversely, the VV cycle has its maximum at night, its minimum at noon and is most apparent in the upper and lower forest layers, especially during June and July. The cycles are compared with vapour-pressure deficit (VPD), an indicator of favourable transpiration conditions, and they are seen to co-vary closely. The VV cycle compares well to that seen for P-band in a previous study and is speculated to originate from stem water content variations. The HH cycle cannot be directly connected to stem water content, but is indicated to follow a currently unknown transpiration related phenomena.
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| 3. |
- Tebaldini, S., et al.
(author)
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TomoSense: A unique 3D dataset over temperate forest combining multi-frequency mono- and bi-static tomographic SAR with terrestrial, UAV and airborne lidar, and in-situ forest census
- 2023
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In: Remote Sensing of Environment. - : Elsevier BV. - 0034-4257. ; 290
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Journal article (peer-reviewed)abstract
- The TomoSense experiment was funded by the European Space Agency (ESA) to support research on remote sensing of forested areas by means of Synthetic Aperture Radar (SAR) data, with a special focus on the use of tomographic SAR (TomoSAR) to retrieve information about the vertical structure of the vegetation at different frequency bands. The illuminated scene is the temperate forest at the Eifel National Park, North-West Germany. Dominant species are beech and spruce trees. Forest height ranges roughly from 10 to 30 m, with peaks up to over 40 m. Forest Above Ground Biomass (AGB) ranges from 20 to 300 Mg/ha, with peaks up to over 400 Mg/ha. SAR data include P-, L-, and C-band surveys acquired by flying up to 30 trajectories in two headings to provide tomographic imaging capabilities. L- and C-band data were acquired by simultaneously flying two aircraft to gather bistatic data along different trajectories. The SAR dataset is complemented by 3D structural canopy measurements made via terrestrial laser scanning (TLS), Unoccupied Aerial Vehicle lidar (UAV-L) and airborne laser scanning (ALS), and in-situ forest census. This unique combination of SAR tomographic and multi-scale lidar data allows for direct comparison of canopy structural metrics across wavelength and scale, including vertical profiles of canopy wood and foliage density, and per-tree and plot-level above ground biomass (AGB). The resulting TomoSense data-set is free and openly available at ESA for any research purpose. The data-set includes ALS-derived maps of forest height and AGB, forest parameters at the level of single trees, TLS raw data, and plot-average TLS vertical profiles. The provided SAR data are coregistered, phase calibrated, and ground steered, to enable a direct implementation of any kind of interferometric or tomographic processing without having to deal with the subtleties of airborne SAR processing. Moreover, the data-base comprises SAR tomographic cubes representing forest scattering in 3D both in Radar and geographical coordinates, intended for use by non-Radar experts. For its unique features and completeness, the TomoSense data-set is intended to serve as an important basis for future research on microwave scattering from forested areas in the context of future Earth Observation missions.
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