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- Eriksson, Leif, 1970, et al.
(författare)
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ALOS PALSAR Calibration and Validation Results from Sweden
- 2007
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Ingår i: IEEE Geoscience and Remote Sensing Symposium (IGARSS 2007), Barcelona, Spain, July 23-27, 2007. ; DOI: 10.1109/IGARSS.2007.4423115, s. 1589-1592
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In 2006 calibration activities for ALOS PALSAR wereconducted in Sweden. Four five-metre trihedral corner reflectorsand three smaller dihedral reflectors were deployed and operatedduring eight months. 23 PALSAR scenes were acquired over thecalibration site allowing an evaluation of the quality and temporalstability of the data. Results show that the co-polarized datahave been stable during the whole calibration period with variationsin the trihedral responses lower than 0.7 dB. The measuredresolution in azimuth was 4.4 m and in slant range 4.7 m forsingle polarization images and 9.5 m for polarimetric data. Forthe cross-polarized data large variations in the dihedral responseswere found. It is assumed that this is caused by a larger sensitivityto pointing errors. For the polarimetric data, estimation ofFaraday rotation gave values ranging from 0.1º to 3º.
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| 5. |
- Folkesson, Klas, 1976, et al.
(författare)
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Model-based compensation of topography effects for improved stem volume retrieval from CARABAS-II VHF-band SAR images
- 2009
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Ingår i: IEEE Transactions on Geoscience and Remote Sensing. - 0196-2892 .- 1558-0644. ; 47:4, s. 1045-1055
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Tidskriftsartikel (refereegranskat)abstract
- A limiting factor that has been identified for stem-volume retrieval in coniferous forests using VHF synthetic aperture radar is that the backscatter varies depending on ground topography. On sloping ground, the backscatter from a forest is reduced, since the dominant groundtrunk double-bounce scattering mechanism is changed. This leads to underestimation of stem volume, and the variations caused by topography can obscure real variations in stem volume. By using multiple images acquired with different flight headings and combining the image information with ground-topography data in a model-based inversion method, we are able to compensate for the ground-topography influence on the backscatter. The inversion method is based on image segmentation and the optimal estimation method. Using four or more images from the CARABAS-II system and a coarse digital elevation model with 50-m horizontal grid, the stem volume can be retrieved with an average root-mean-square error (rmse) of less than 60 $\hbox{m}^{3}\hbox{ha}^{-1}$ for stem volumes in range of 80700 $\hbox{m}^{3}\hbox{ha}^{-1}$ (in terms of above-ground biomass, this is equivalent to an rmse of less than 40 $\hbox{ton} \cdot\hbox{ha}^{-1}$ over the range of 50400 $\hbox{ton} \cdot\hbox{ha}^{-1}$). The retrieval accuracy is similar to that previously obtained for similar forests standing on flat and horizontal ground.
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| 6. |
- Folkesson, Klas, 1976
(författare)
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Model-Based Stem Volume Retrieval and Windthrow Detection using CARABAS and P-band SAR
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Radar remote sensing is an efficient method for monitoring forests over large areas. Radar waves transmitted from a system carried by a satellite or an aircraft interact with trees on the ground, and the reflected waves are used to form images which are independent of weather and light conditions. Information in the images is then used to retrieve forest parameters.CARABAS is an airborne synthetic aperture radar (SAR) system. Due to the long wavelengths of the system (3-15 m), the radar signal is able to penetrate the forest canopies and for boreal forests the main backscattering component is the ground-trunk double-bounce. The scattering strength is directly related to the stem volume and hence stem volume retrieval with CARABAS has higher accuracy than SAR systems using shorter wavelengths. However, the ground-trunk double-bounce is very sensitive to ground slope and if not corrected for, ground slope will cause the stem volume to be underestimated.By using multiple images acquired with different flight headings and combining the image information with ground topography data in a model-based inversion method, most of the ground topography influence on the retrieved stem volume is removed. With four CARABAS images and a coarse digital elevation model with 50 m horizontal grid, the stem volume can be retrieved with a root mean square error(RMSE) of 48 m3/ha for stem volumes in the range 80-700 m3/ha. The proposed retrieval method gives better accuracy and is more robust than existing heuristic methods. By using this method, the retrieval accuracy in areas with ground topography is similar to that of similar forests standing on flat and horizontal ground.Another forestry application where CARABAS and other airborne SAR systems with long-wavelengths and high resolution can provide timely and useful information is in the detection of windthrown trees. A method for detecting windthrows in CARABAS images and a discussion about when windthrows can be detected is presented in this thesis.
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| 7. |
- Folkesson, Klas, 1976
(författare)
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Segmentation of Radar Images for Remote Sensing of Forests
- 2005
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Radar remote sensing is an efficient method for monitoring forests over large areas. Radar waves transmitted from a system carried by a satellite or an aircraft interact with trees on the ground, and the reflected waves are used to form images which are independent of weather and light conditions. Information in the images is then used to retrieve forest parameters.
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| 8. |
- Folkesson, Klas, 1976, et al.
(författare)
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Stem volume retrieval at stand level using multiple low-frequency SAR images
- 2003
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Ingår i: IEEE Geoscience and Remote Sensing Symposium (IGARSS), Toulouse, France, 21-25 July. - 0780379292 ; IV, s. 2556-8
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In this paper the ground slope effect on stem volume retrieval from VHF-band SAR images has been studied using multiple co-registered images acquired from different flight headings. Accurate measurements of the topography were used as additional input data to a model of the backscatter amplitude variation with ground slope and aspect angle. Modeling this topographical effect on the radar backscatter provides a base for improvement of the stem volume retrieval
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| 10. |
- Fransson, J.E.S., et al.
(författare)
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Detection of clear-cuts using ALOS PALSAR satellite images
- 2008
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Ingår i: Proceedings of the European Conference on Synthetic Aperture Radar, EUSAR. - 2197-4403. ; 1-4
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Konferensbidrag (refereegranskat)abstract
- The objective of this study is to make a first evaluation of the possibilities to detect forest clear-cuts using high-resolution ALOS PALSAR FBD (Fine Beam Dual polarization) satellite images. New operational applications for mapping of changes in forest cover are of interest for government authorities in Sweden and in other countries with similar needs. The study was conducted in southern Sweden and included seven old coniferous stands located on flat terrain. Three of the stands were clear-felled and the remaining stands were left untreated for reference. Altogether, six PALSAR FBD images (look angle 34.3°, HH- and HV-polarization) acquired during the summer and fall seasons were analyzed. The difference in backscattering coefficient between the reference and the clear-felled stands was on average 2.4 dB and 2.9 dB for the HH- and HV-polarization, respectively. When comparing the backscattering coefficient before and after clear-felling the drop was found to be 1.7 dB and 2.3 dB for the HH- and HV-polarization, respectively.
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