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Kartering av biomassa i skog med SAR- Förstudier för ESA:s kandidatmission
BIOMASSLars Ulander, Gustaf Sandberg och Leif ErikssonChalmers tekniska högskola
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BIOMASS: P-band polarimetric satellite SAR
• ESA has six Earth Explorers under development within the Living Earth Program– GOCE, SMOS, ADM-Aeolus, Cryosat-2, Swarm, Earthcare
• Biomass is a candidate for the seventh Earth Explorer – ESA core mission to be launched in 2016
• Selection mechanism in process– 2005: Call for Ideas and 24 proposals evaluated– 2006: BIOMASS selected as one of six candidates for phase 0– 2009: BIOMASS, Premiere, CoreH2O selected for phase A– 2011: One mission will be selected
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The global carbon cycle for the 1990s
Addressed by BIOMASS
Values from IPCC 2007. Units: Gigatons of carbon/ yr
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Importance of forest biomass in carbon cycle
• Biomass is a proxy for carbon (Carbon ~ 0.5 x Biomass)
• Forests account for most of the Earth’s vegetation biomass
• Changes in forest biomass with time correspond to carbon fluxes
– Loss = Emissions
– Growth = Uptake
• Forest biomass is very poorly known and is a major source of uncertainty in carbon flux estimation
Biomass = dry weight of woody matter + leaves (tons/ha)
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Calibration, Ionospheric correctionCalibration, Ionospheric correction
Retrieval algorithmRetrieval algorithm
Geophysical productsGeophysical products
• Forest biomass• Forest height• Forest biomass temporal change• Forest disturbance
• Forest biomass• Forest height• Forest biomass temporal change• Forest disturbance
Polarimetric radar intensity
Polarimetric InterferometricPhase
Orbit cycle n
Orbit cycle n+1
Observation concept – satellite radar
HH HV VV Phase
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Measurement campaign in Sweden: BioSAR-I
• Campaign to test the BIOMASS concept, funded by ESA.
• Fully polarimetric L- and P-band SAR images acquired .
• SAR-images from three occasions:– 9 Mars 2007– 31 Mars and 2 April 2007– 2 May 2007
• High resolution laser data acquired spring 2007.• In-situ data on forest properties were collected.
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Remningstorp: a much used test site
• Many kinds of remote sensing data available from Remningstorp:– SAR-data
(CARABAS, LORA, ESAR, PALSAR etc.)
– Laser data– Optical data– Ground
measurements
Sweden
Finland
Norway
Denmark
Remningstorp
30°E
20°E
20°E
10°E
10°E0°
70°N70°N
65°N65°N
60°N 60°N
55°N 55°N
Remningstorp is situated near Skara
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Example SAR image: clear difference between fields and forests
• SAR image (HV-backscatter) from 2 May 2007.
• Dark and blue areas = lakes, fields
• Green and red areas= forests
7 km
6 km
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Reflectors giving high backscatter were used
• Large reflectors were used to evaluate radiometric calibration.
• The reflectors were 5 m high, weighed 700 kg.
The image shows a reflector when directed towards PALSAR (an L-band SAR system onboard the satellite ALOS).
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Detailed ground measurements were made
• 17 forest stands were inventoried:– 7 á 20 m x 50 m– 10 á 80 m x 80 m.
• Measurements on all trees.
• Stem volume and biomass were estimated.
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Laser data was used as complement to in-situ data• High density laser
data acquired.• ~30 pulses/m2.• Individual canopies
easily seen.• Used to estimate
biomass for 58 stands.
125 m
125 m
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L-band backscatter saturates for high biomass
• L-band backscatter (HV, 2 May) plotted against biomass for 68 stands.
• Backscatter increase with increasing biomass.
• Saturation for high biomass.
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Blue crosses: biomass estimated using laser data.Green circles: biomass estimated using in-situ data.
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Less saturation for P-band backscatter
• Same plot for P-band backscatter (HV, 2 May)
• Near linear dependence.
• Contrary to L-band, only weak saturation effects were observed.
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Blue crosses: biomass estimated using laser data.Green circles: biomass estimated using in-situ data.
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A regression model for biomass estimation
• A regression model for biomass estimation was developed for the two frequency bands.
• The same model was used for both L- and P-band data (HV and HH).
• All regression coefficients were significant.• 10 in-situ stands were used as validation stands.
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W is the biomass, γ0 is the backscatter in decibels.
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Biomass estimated using L-band data (HV)
• Good results for stands with low biomass.
• Underestimation for stands with high biomass.
• RMSE about 30% (blue crosses), higher for validation stands.
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Blue crosses: biomass estimated using laser data (with error bars).Green circles: biomass estimated using in-situ data (small errors).
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Biomass estimated using P-band data (HV)
• Good results over whole range of biomass.
• RMSE about 20-25 % for all stands (including validation stands).
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Blue crosses: biomass estimated using laser data (with error bars).Green circles: biomass estimated using in-situ data (small errors).
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Conclusions from the BioSAR-I campaign
• Both L- and P-band backscatter depend on biomass.
• Saturation at high biomass for L-band.
• Biomass estimation using L-band good for low biomass, large errors for high biomass.
• Using P-band biomass can be estimated even for stands with high biomass.
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