| 1. |
- Bennet, Patrik, 1995, et al.
(författare)
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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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Ingår i: International Geoscience and Remote Sensing Symposium (IGARSS). ; 2023-July, s. 2195-2198
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Konferensbidrag (refereegranskat)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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| 2. |
- Fransson, Johan E.S., et al.
(författare)
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Estimation of Forest Stem Volume using ALOS-2 PALSAR-2 Satellite Images
- 2016
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Ingår i: Proceedings of 36th IEEE International Geoscience and Remote Sensing Symposium, IGARSS 2016; Beijing; China; 10-15 July 2016. - 9781509033324 ; 2016-November, s. Art no 7730388, Pages 5327-5330
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Konferensbidrag (refereegranskat)abstract
- © 2016 IEEE. A first evaluation of ALOS-2 PALSAR-2 data for forest stem volume estimation has been performed at a coniferous dominated test site in southern Sweden. Both the Fine Beam Dual (FBD) polarization and the Quad-polarimetric mode were investigated. Forest plots with stem volume reaching up to a maximum of about 620 m 3 ha -1 (corresponding to 370 tons ha -1 ) were analyzed by relating backscatter intensity to field data using an exponential model derived from the Water Cloud Model. The estimation accuracy of stem volume at plot level (0.5 ha) was calculated in terms of Root Mean Square Error (RMSE). For the best case investigated an RMSE of 39.8% was obtained using one of the FBD HV-polarized images. The corresponding RMSE for the FBD HH-polarized images was 43.9%. In the Quad-polarimetric mode the lowest RMSE at HV- and HH-polarization was found to be 43.1% and 66.1%, respectively.
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| 3. |
- Leistner, Theresa, 1996, et al.
(författare)
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COMPARISON OF TOWER-BASED AND SATELLITE L- AND C-BAND RADAR BACKSCATTER FROM A BOREAL FOREST
- 2023
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Ingår i: IGARSS 2023 - 2023 IEEE INTERNATIONAL GEOSCIENCE AND REMOTE SENSING SYMPOSIUM. - 2153-6996. - 9798350320107 ; , s. 8319-8322
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Konferensbidrag (refereegranskat)abstract
- In this study, spaceborne SAR observations were compared to the tower-based radar observations of a hemiboreal forest site in southern Sweden. BorealScat radar tower observations at L- and C-band are verified using Sentinel-1 and ALOS-2 PALSAR-2 products. For L-band, a 14-month backscatter time series was produced. For C-band, a 3.5 year backscatter time series that includes tree deaths due to a bark beetle infestation was produced. Tomographic capabilities of the tower-based BorealScat allowed both canopy-only and full forest backscatter to be analyzed. For a healthy forest, the agreement of tower-based and spaceborne SAR observations was within 0.9 dB for C-band and within 0.6 dB for L-band. Sentinel-1 backscatter was found to be insensitive to tree deaths, whereas variations were clearly visible in the tomographic tower-based backscatter. These results show that tower-based radar observations agree with those of spaceborne SARs and can be used to understand their shortcomings and identify new mission needs.
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| 4. |
- Monteith, Albert, 1990, et al.
(författare)
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A Tower-Based Radar Study of Temporal Coherence of a Boreal Forest at P-, L-, and C-Bands and Linear Cross Polarization
- 2022
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Ingår i: IEEE Transactions on Geoscience and Remote Sensing. - 0196-2892 .- 1558-0644. ; 60
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Tidskriftsartikel (refereegranskat)abstract
- Cross-polarized temporal coherence observations of a boreal forest, acquired using a tower-based radar, are presented in this article. Temporal coherence is analyzed with respect to frequency, temporal baseline, time of day of observation, season, meteorological variables, and biophysical variables. During the summer, P- and L-band temporal coherence exhibited diurnal cycles, which appeared to be due to high rates of transpiration and convective winds during the day. During the winter, freeze-thaw cycles and precipitation resulted in decorrelation. At temporal baselines of seconds to hours, a high temporal coherence was observed even at C-band. The best observation times of the day were midnight and dawn. Temporal coherence is the main limitation of accuracy in interferometric and tomographic forest applications. The observations from this experiment will allow for better spaceborne SAR mission designs for forest applications, better temporal decorrelation modeling, and more accurate forest parameter estimation algorithms using interferometric and tomographic SAR data.
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| 5. |
- Monteith, Albert, 1990, et al.
(författare)
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BorealScat: A Tower-Based Tomographic and Polarimetric Radar Experiment in the Boreal Forest at P- and L-Band
- 2016
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Ingår i: International Geoscience and Remote Sensing Symposium (IGARSS). - 9781509033324 ; , s. 7458 - 7461
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Konferensbidrag (refereegranskat)abstract
- This paper describes BorealScat, a tower-based radar campaign for acquiring multitemporal polarimetric, tomographic and Doppler radar measurements at P-, L- and C-band over a hemi-boreal forest site in Remningstorp, Sweden. The facility consists of a 50-m high tower equipped with a radar system including an array of 30 antennas. The site will also be equipped with meteorological instruments and moisture sensors. The aim of the experiment is the temporal survey of radar signatures over time scales ranging from sub-seconds to years in varying environmental conditions. This experiment will provide fundamental information about the electromagnetic scattering mechanisms in boreal forests at P-, L- and C-band.
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| 6. |
- Monteith, Albert, 1990, et al.
(författare)
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Calibration of a Ground-Based Array Radar for Tomographic Imaging of Natural Media
- 2019
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Ingår i: Remote Sensing. - : MDPI AG. - 2072-4292. ; 11:24, s. 1-22
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Tidskriftsartikel (refereegranskat)abstract
- Ground-based tomographic radar measurements provide valuable knowledge about the electromagnetic scattering mechanisms and temporal variations of an observed scene and are essential in preparation for space-borne tomographic synthetic aperture radar (SAR) missions. Due to the short range between the radar antennas and a scene being observed, the tomographic radar observations are affected by several systematic errors. This article deals with the modelling and calibration of three systematic errors: mutual antenna coupling, magnitude and phase errors and the pixel-variant impulse response of the tomographic image. These errors must be compensated for so that the tomographic images represent an undistorted rendering of the scene reflectivity. New calibration methods were described, modelled and validated using experimental data. The proposed methods will be useful for future ground-based tomographic radar experiments in preparation for space-borne SAR missions.
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| 7. |
- Monteith, Albert, 1990, et al.
(författare)
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LONG-TERM P-BAND TOMOSAR OBSERVATIONS FROM THE BOREALSCAT TOWER EXPERIMENT
- 2018
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Ingår i: International Geoscience and Remote Sensing Symposium (IGARSS). - 9781538671504 ; , s. 8594-8597
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Konferensbidrag (refereegranskat)abstract
- SAR tomography at P-band allows the separation of scatterers throughout the vertical extent of a forest canopy, offering a SAR observable that can be used for biomass estimation. But the vertical backscattering distribution of forests are sensitive to changes in the weather and seasons, effects which are poorly understood. In this study, a tower-based radar is used to produce fully-polarimetric tomographic images of a boreal forest at P-band which are analysed over a period of one year. The largest variations seen were due to sub-zero temperatures, causing a drop in the effective scattering height. Seasonal changes in soil moisture and temperature caused a drop in ground-level backscatter at HH-polarisation and a drop in canopy-level cross-polarised backscatter during the summer.
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| 8. |
- Monteith, Albert, 1990
(författare)
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Radar Measurements of Temporal Variation in a Boreal Forest
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Synthetic aperture radar (SAR) on a satellite platform is a suitable technique for all-weather global monitoring of forest parameters such as biomass. This is important for increasing the accuracy of the global terrestrial carbon flux, the largest uncertainty in our current understanding of the Earth’s carbon cycle. In recent decades there has been increasing interest in relating SAR observables from current and future spaceborne SAR missions to forest parameters. Ideally, these SAR observables are only dependent on the forest parameters of interest, but they are also affected by temporal variations in the forest due to weather, diurnal and seasonal changes. These effects are not properly accounted for in current forest parameter estimation models using SAR data due to shortcomings in our understanding of these effects.To fill this knowledge gap, a tower-based radar has been developed for measuring temporal variation of radar signatures in a boreal forest site in southern Sweden. The instrument allows monitoring of radar signatures from the forest site over timescales ranging from less than a second to years. The experiment consists of a 50-m high tower equipped with 30 antennas, allowing fully-polarimetric tomographic imaging at microwave frequencies of P-band (420 - 450 MHz), L-band (1240 - 1375 MHz) and C-band (5250 - 5570 MHz). Results from an on-site weather station assists in interpretation of the radar results. The work in this thesis involves the design and implementation of the experimental setup and analysis of the first results. The results have provided new information about variations in forest radar backscatter during freeze-thaw cycles at P- and L-band, causing drops in backscatter of 4 to 10 dB. An effect where strong winds cause a drop in co-polarised backscatter at P-band has been directly observed for the first time. Finally, the tomographic capabilities of the instrument were demonstrated. Tomographic imaging quality was shown to be better than for measurement protocols used by previous tower-based radars when acquired while the trees were moving due to wind.The results of this experiment are important for understanding and modelling temporal variations in radar measurements over boreal forests such that these unwanted variations can be compensated for in forest parameter estimation algorithms using SAR data.
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| 9. |
- Monteith, Albert, 1990
(författare)
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Temporal Characteristics of Boreal Forest Radar Measurements
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Radar observations of forests are sensitive to seasonal changes, meteorological variables and variations in soil and tree water content. These phenomena cause temporal variations in radar measurements, limiting the accuracy of tree height and biomass estimates using radar data. The temporal characteristics of radar measurements of forests, especially boreal forests, are not well understood. To fill this knowledge gap, a tower-based radar experiment was established for studying temporal variations in radar measurements of a boreal forest site in southern Sweden. The work in this thesis involves the design and implementation of the experiment and the analysis of data acquired. The instrument allowed radar signatures from the forest to be monitored over timescales ranging from less than a second to years. A purpose-built, 50 m high tower was equipped with 30 antennas for tomographic imaging at microwave frequencies of P-band (420-450 MHz), L-band (1240-1375 MHz) and C-band (5250-5570 MHz) for multiple polarisation combinations. Parallel measurements using a 20-port vector network analyser resulted in significantly shorter measurement times and better tomographic image quality than previous tower-based radars. A new method was developed for suppressing mutual antenna coupling without affecting the range resolution. Algorithms were developed for compensating for phase errors using an array radar and for correcting for pixel-variant impulse responses in tomographic images. Time series results showed large freeze/thaw backscatter variations due to freezing moisture in trees. P-band canopy backscatter variations of up to 10 dB occurred near instantaneously as the air temperature crossed 0⁰C, with ground backscatter responding over longer timescales. During nonfrozen conditions, the canopy backscatter was very stable with time. Evidence of backscatter variations due to tree water content were observed during hot summer periods only. A high vapour pressure deficit and strong winds increased the rate of transpiration fast enough to reduce the tree water content, which was visible as 0.5-2 dB backscatter drops during the day. Ground backscatter for cross-polarised observations increased during strong winds due to bending tree stems. Significant temporal decorrelation was only seen at P-band during freezing, thawing and strong winds. Suitable conditions for repeat-pass L-band interferometry were only seen during the summer. C-band temporal coherence was high over timescales of seconds and occasionally for several hours for night-time observations during the summer. Decorrelation coinciding with high transpiration rates was observed at L- and C-band, suggesting sensitivity to tree water dynamics. The observations from this experiment are important for understanding, modelling and mitigating temporal variations in radar observables in forest parameter estimation algorithms. The results also are also useful in the design of spaceborne synthetic aperture radar missions with interferometric and tomographic capabilities. The results motivate the implementation of single-pass interferometric synthetic aperture radars for forest applications at P-, L- and C-band.
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| 10. |
- Monteith, Albert, 1990, et al.
(författare)
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Temporal Characteristics of P-band Tomographic Radar Backscatter of a Boreal Forest
- 2021
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Ingår i: IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing. - 2151-1535 .- 1939-1404. ; 14, s. 1967-1984
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Tidskriftsartikel (refereegranskat)abstract
- Temporal variations in synthetic aperture radar (SAR) backscatter over forests are of concern for any SAR mission with the goal of estimating forest parameters from SAR data. In this article, a densely sampled, two-year long time series of P-band (420 to 450 MHz) boreal forest backscatter, acquired by a tower-based radar, is analyzed. The experimental setup provides time series data at multiple polarizations. Tomographic capabilities allow the separation of backscatter at different heights within the forest. Temporal variations of these multi-polarimetric, tomographic radar observations are characterized and quantified. The mechanisms studied are seasonal variations, effects of freezing conditions, diurnal variations, effects of wind and the effects of rainfall on backscatter. An emphasis is placed on upper-canopy backscatter, which has been shown to be a robust proxy for forest biomass. The canopy backscatter was most sensitive to freezing conditions but was more stable than ground-level backscatter and full-forest backscatter during non-frozen conditions. The analysis connects tree water transport mechanisms and P-band radar backscatter for the first time. The presented results are useful for designing boreal forest parameter estimation algorithms, using data from P-band SARs, that are robust to temporal variations in backscatter. The results also present new forest remote sensing opportunities using P-band radars.
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