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

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.

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

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

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).

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.

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

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.

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.

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.

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).

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).

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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