| 1. |
- Vu, Viet Thuy, et al.
(författare)
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RFI Suppression in Ultrawideband SAR Using an Adaptive Line Enhancer
- 2010
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Ingår i: IEEE Geoscience and Remote Sensing Letters. - : IEEE. - 1545-598X .- 1558-0571. ; 7:4, s. 694-698
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Tidskriftsartikel (refereegranskat)abstract
- In this letter, we propose an approach to suppress radio-frequency interference (RFI) in ultrawideband (UWB) low-frequency synthetic aperture radar (SAR). According to the proposal, RFI is suppressed by using an adaptive line enhancer controlled by the normalized least mean square algorithm. The approach is tested successfully on real UWB low-frequency SAR data. In order to keep the computational burden down, possible ways to integrate the RFI suppression approach into SAR imaging algorithms are also suggested.
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| 2. |
- Askne, Jan, 1936, et al.
(författare)
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Model-Based Biomass Estimation of a Hemi-Boreal Forest from Multitemporal TanDEM-X Acquisitions
- 2013
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Ingår i: Remote Sensing. - : MDPI AG. - 2072-4292. ; 5:11, s. 5574-5597
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Tidskriftsartikel (refereegranskat)abstract
- Above-ground forest biomass is a significant variable in the terrestrial carbon budget, but is still estimated with relatively large uncertainty. Remote sensing methods can improve the characterization of the spatial distribution and estimation accuracy of biomass; in this respect, it is important to examine the potential offered by new sensors. To assess the contribution of the TanDEM-X mission, eighteen interferometric Synthetic Aperture Radar (SAR) image pairs acquired over the hemi-boreal test site of Remningstorp in Sweden were investigated. Three models were used for interpretation of TanDEM-X signatures and above-ground biomass retrieval: Interferometric Water Cloud Model (IWCM), Random Volume over Ground (RVoG) model, and a simple model based on penetration depth (PD). All use an allometric expression to relate above-ground biomass to forest height measured by TanDEM-X. The retrieval was assessed on 201 forest stands with a minimum size of 1 ha, and ranging from 6 to 267 Mg/ha (mean biomass of 105 Mg/ha) equally divided into a model training dataset and a validation test dataset. Biomass retrieved using the IWCM resulted in a Root Mean Square Error (RMSE) between 17% and 33%, depending on acquisition date and image acquisition geometry (angle of incidence, interferometric baseline, and orbit type). The RMSE in the case of the RVoG and the PD models were slightly higher. A multitemporal estimate of the above-ground biomass using all eighteen acquisitions resulted in an RMSE of 16% with R-2 = 0.93. These results prove the capability of TanDEM-X interferometric data to estimate forest aboveground biomass in the boreal zone.
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| 3. |
- Blomberg, Erik, 1987, et al.
(författare)
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P-band Polarimetric Model of Vertical Tree Stems on Sloping Ground
- 2014
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Ingår i: Joint 2014 IEEE International Geoscience and Remote Sensing Symposium, IGARSS 2014 and the 35th Canadian Symposium on Remote Sensing, CSRS 2014; Quebec Convention CentreQuebec City; Canada; 13 July 2014 through 18 July 2014. - 9781479957750 ; , s. 80-83
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Konferensbidrag (refereegranskat)abstract
- A fully polarimetric model is implemented to simulate backscatter from boreal forest. The aim is to correct for topography on a sub-stand level during the recovery of forest properties from P-band SAR data, specifically directed at the upcoming BIOMASS mission. The model incorporates the direct, double-bounce and triple-bounce scattering from tree stems on a sloping ground. Topography is introduced by assigning an independent and arbitrarily oriented local ground plane to each tree. Simulated SAR images are produced from in-situ measurements and a high resolution digital topography model (DTM) based on LiDAR data. These were obtained during BioSAR 2010 together with the corresponding SAR images, which are used for validation.
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| 4. |
- Carvajal, Gisela, 1983, et al.
(författare)
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Comparison between current fields detected with infrared radiometry and modeled currents around Sweden
- 2013
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Ingår i: 33rd IEEE International Geoscience and Remote Sensing Symposium (IGARSS); 2013, Melbourne. - 9781479911141 ; , s. 1270-1273
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Konferensbidrag (refereegranskat)abstract
- The understanding of the relationship between surface currents derived from weather models and remote sensing data is essential in order to produce an improved and integrated surface current information of high quality and resolution. The large availability of satellite derived infrared radiometer data at the high latitudes of the coastal waters surrounding Sweden makes the Maximum Cross Correlation (MCC) method a feasible alternative to produce real time measurements of surface currents. This work compares current retrievals from the MCC method and model data around Sweden. Our results indicate a similar magnitude for both sources of current fields in most of the locations. However, there exist small discrepancies in the localization of the larger current values. Also, the MCC retrievals generally present more features than the modeled ones. Possible reasons for these discrepancies might be the MCC detection of circulation patterns not predicted by the model, or the depreciation in the MCC performance due to the influence of diurnal variability of the sea surface temperature, wind driven mixing due to upwelling or tides.
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| 5. |
- Carvajal, Gisela, 1983, et al.
(författare)
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Correlation between Synthetic Aperture Radar Surface Winds and Deep Water Velocity in the Amundsen Sea, Antarctica
- 2013
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Ingår i: Remote Sensing. - : MDPI AG. - 2072-4292. ; 5:8, s. 4088-4106
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Tidskriftsartikel (refereegranskat)abstract
- The recent observed thinning of the glacier ice shelves in the Amundsen Sea (Antarctica) has been attributed to warm deep currents, possibly induced by along-coast winds in the vicinity of the glacial ice sheet. Here, high resolution maps of wind fields derived from Synthetic Aperture Radar (SAR) data have been studied and correlated with subsurface measurements of the deep water velocities in the Amundsen Sea area. Focus is on periods with low ice coverage in 2010 and 2011. In 2010, which had comparatively low ice coverage, the results indicate a more rapid response to wind forcing in the deep currents than in 2011. The SAR wind speed maps have better spatial resolution than available reanalysis data, and higher maximum correlation was obtained with SAR data than with reanalysis data despite the lower temporal resolution. The maximum correlation was R = 0.71, in a direction that is consistent with wind-driven Ekman theory. This is significantly larger than in previous studies. The larger correlation could be due to the better spatial resolution or the restriction to months with minimum ice coverage. The results indicate that SAR is a useful complement to infer the subsurface variability of the ocean circulation in remote areas in polar oceans.
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| 6. |
- Carvajal, Gisela, 1983, et al.
(författare)
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Retrieval and assessment of sub-mesoscalewind velocity vectors with Synthetic Aperture Radar
- 2011
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Ingår i: IEEE International Geoscience and Remote Sensing Symposium, IGARSS 2011. Vancouver, 24-29 July 2011. - 9781457710056 ; , s. 2041-2044
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Konferensbidrag (refereegranskat)abstract
- Wind vector fields are currently available from different sources at mesoscale resolutions (2 km to 200 km). The commonly provided wind resolution on 25 km imposes a limit in coastal areas and in the study of small-scale phenomena occurring in the ocean surface. Since wind is the driving force of the ocean, the study of its behavior in sub-mesoscale (
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| 7. |
- Carvajal, Gisela, 1983, et al.
(författare)
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Retrieval and Quality Assessment of Wind Velocity Vectors on the Ocean With C-Band SAR
- 2014
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Ingår i: IEEE Transactions on Geoscience and Remote Sensing. - : Institute of Electrical and Electronics Engineers (IEEE). - 0196-2892 .- 1558-0644. ; 52:5, s. 2519-2537
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Tidskriftsartikel (refereegranskat)abstract
- Wind vector fields derived from synthetic aperture radar (SAR) sensors show variations at smaller scales than most other globally available surface wind sources. However, few studies have been devoted to the investigation of the accuracy of SAR-derived wind fields at different scales and how they compare with other wind data. In order to investigate these issues, an algorithm for the retrieval of SAR-derived wind vectors has been developed, and a quality assessment between the retrievals and in situ, scatterometer, and numerical weather model (NWM) wind data has been performed. The implemented wind retrieval algorithm detects streak features in the SAR image to estimate wind directions and inverts wind speeds using CMOD-IFR2, CMOD5, or CMOD5.N geophysical model functions. In addition, a regularization method for filtering outliers in the wind direction retrievals is used. Retrievals compared with in situ data indicated better performance at offshore locations for wind speed inversions with CMOD5.N. The bias and standard deviation for offshore regularized wind directions and CMOD5.N speeds are 9 degrees and 25 degrees and -0.1 and 1.4 m/s, respectively. The comparison with the scatterometer and NWM wind data has been performed for retrievals at 5-, 10-, and 20-km resolution. The results indicate a better agreement of the coarser retrievals with the reference data. Nevertheless, mapping of smaller scale features requires wind directions from the SAR image itself.
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| 8. |
- Fransson, Johan E.S., et al.
(författare)
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Estimation of stem volume in hemi-boreal forests using airborne low-frequency Synthetic Aperture Radar and lidar data
- 2013
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Ingår i: International Geoscience and Remote Sensing Symposium (IGARSS). - 2153-6996 .- 2153-7003. - 9781479911141 ; , s. 161-164
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Konferensbidrag (refereegranskat)abstract
- Synthetic Aperture Radar (SAR) backscatter data from the Swedish airborne CARABAS-II and LORA systems were used to estimate stem volume at stand level. The study was performed in hemi-boreal forests at the Remningstorp test site, located in southern Sweden. In total, ten 80 m × 80 m stands, where all trees were measured in situ, with stem volumes in the range of 70-530 m 3 ha-1 (on average 347 m3 ha-1) were analyzed. SAR data from CARABAS-II and LORA were acquired from two different years, with nine unique flight headings that were repeated for each system and year. Regression analysis was used to estimate stem volume and the accuracy was assessed in terms of Root Mean Square Error (RMSE). As a first step, stem volume was estimated for each flight heading separately. The accuracy assessment was then performed by weighting the separate estimates for each system and year inversely proportionally to the variance about the regression function. The best results for CARABAS-II and LORA showed a relative RMSE of 7% and 24% of the mean stem volume, respectively. In a previous study, stem volume was estimated using LiDAR data and the same forest stands, resulting in an RMSE of about 12%. In conclusion, the estimation accuracy of stem volume using combined low-frequency CARABAS SAR images was found to be superior to that from using LiDAR data for the stands investigated.
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| 9. |
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| 10. |
- Hellsten, Hans, et al.
(författare)
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Evaluation of a geometrical autofocus algorithm within the framework of Fast Factorized Back-Projection
- 2012
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Ingår i: Proceedings of IET RADAR 2012, 22-25 October, Glasgow, Scotland. - : Institution of Engineering and Technology. - 9781849196765 ; 2012:603 CP
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Konferensbidrag (refereegranskat)abstract
- This paper introduces a new autofocus algorithm, specifically suitable for UWB SAR systems. The strategy is integrated in FFBP and relies on varying track parameters stage by stage to obtain a sharp image. Focus measures are provided by an object function (intensity correlation). The algorithm has been tried out successfully on a synthetic data set.
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