Brix workshop. Mauro Mariotti d Alessandro, Stefano Tebaldini ESRIN

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1 Brix workshop Mauro Mariotti d Alessandro, Stefano Tebaldini ESRIN Dipartimento di Elettronica, Informazione e Bioingegneria Politecnico di Milano

2 Outline A. SAR Tomography 1. How does it work? 2. 3D imaging on forests 3. Correlation with AGB on tropical forests B. Why 3m backscatter works in estimating AGB on tropical forests? 1. Biophysical explanation: 3m as key elevation 2. Tomographic explanation: ground rejection C. Ground notching 1. How does it work? 2. The effect of system geometry 3. Preliminary achievements D. Conclusions

3 Outline A. SAR Tomography 1. How does it work? 2. 3D imaging on forests 3. Correlation with AGB on tropical forests B. Why 3m backscatter works in estimating AGB on tropical forests? 1. Biophysical explanation: 3m as key elevation 2. Tomographic explanation: ground rejection C. Ground notching 1. How does it work? 2. The effect of system geometry 3. Preliminary achievements D. Conclusions

4 Standard SAR imaging SAR systems employ a RADAR sensor flown onboard a moving platform to synthesize an antenna aperture as long as several kilometers o o Accurate measurement of the travel time of the Radar echoes backscattered from the targets as the system is flown along platform trajectory Image formation by Digital Processing techniques Two-dimensional map of Radar intensity at a given wavelength θ height

5 From 2D to 3D: SAR tomography The target is bound to lie on the circles: Centered on each trajectory Perpendicular to the trajectory, Only 1 solution in the 3D space! 3D localization Of radius R 1 R n. R N S N S N τ S n R N S 1 R N Sn τ S 1 τ z R n R 1! z y R 1 R n! x y

6 TomoSAR imaging TomoSAR systems employ a RADAR sensor flown along multiple trajectories o Image formation by Signal Processing techniques Three dimensional representation of Radar intensity at a given wavelength Track n elevation SAR produces pixels TomoSAR produces voxels! Track 2 Track 1 π/2 θ range height

7 height [m] height [m] slant range azimuth [m] BioSAR 27 Tomographic reconstruction of an azimuth cut: Reflectivity (HH) Average on 9 tracks Reflectivity (HH) Average on 9 tracks azimuth The analyzed profile is almost totally forested, except for the dark areas HH: Dominant phase center is ground locked Vegetation is barely visible Similar conclusions for VV HV: Vegetation is much more visible Dominant phase center is ground locked Normalized Vertical Section - HH Normalized Vertical Section - HV LIDAR Terrain Height LIDAR Forest Height slant range [m]

8 TropiSAR 29 Campaign System TropiSAR- ESA Sethi- ONERA Period August 29 Sites Scene Tomographic tracks Carrier frequency Slant range resolution Azimuth resolution Vertical resolution Paracou & Nourages, French Guyana Tropical forest estimated 15 species per hectare 6 Fully Polarimetric P-Band 1 m 1 m 15 m 3D Imaging of the Guyaflux Tower

9 AfriSAR 216 Multi-look beamforming Single-look back-projection: --- TanDEM-X Tomo DTM

10 6 images 3m power [db] slc(vv) 2 [db] 6 images m power [db] Correlation with AGB TropiSAR Paracou Single image TomoSAR Ground truth AGB [T/Ha*1] TomoSAR provides 2 db per 1 T/Ha AGB Very high correlation:.97 D. Ho Tong Minh et al., Relating P-band SAR tomography to tropical forest biomass, TGRS, Feb. 214.

11 Outline A. SAR Tomography 1. How does it work? 2. 3D imaging on forests 3. Estimating AGB on tropical forests B. Why 3m backscatter works in estimating AGB on tropical forests? 1. Biophysical explanation: 3m as key elevation 2. Tomographic explanation: ground rejection C. Ground notching 1. How does it work? 2. The effect of system geometry 3. Preliminary achievements D. Conclusions

12 3m layer in tropical forests Biophysical issues The canopy layer: o containing a major part of the leaves that convert sunlight to energy through photosynthesis, o being a principal site for the interchange of heat, water vapor, and atmospheric gases o contains a large proportion of the woody elements, including trunks and most of the branches (primary, secondary, and higher order) that contribute to the total AGB

13 3m layer in tropical forests Tree modeling o Why is the best correlation observed at 3 m? o How the biomass in this layer is related to AGB? o Is 3 m a general feature of tropical forests? Assessment using the TROLL model o The biomass contained in the 2-4m layer vs the total AGB, both derived using the TROLL model. o The biomass proportion is about 33%. o A linear fit to the relationship gives a correlation coefficient of.92. o An interesting feature is that it implies that this relation holds independent. of biomass, from 25 to 7 t/ha, including the highest AGB plots with emergent trees D. Ho Tong Minh et al., Relating P-band SAR tomography to tropical forest biomass, TGRS, Feb Chave, Jérôme. "Study of structural, successional and spatial patterns in tropical rain forests using TROLL, a spatially explicit forest model." Ecological modelling (1999):

14 3m layer in tropical forests LiDAR observations Assessment by LIDAR metrics o The specific feature of the 3 m layer in tropical rainforests has also been noticed in a recent study of 9 tropical rainforest in South America by Meyer et al. o Correlation between AGB and the area occupied at different heights by large trees (as derived from Lidar) o Correlation (R 2 ) has been found to be maximum at height of 27-3 m, irrespectively of the 9 study sites. Meyer V. Saatchi S.. et al., Large canopy area explains landscape variation of Above Ground Biomass Submitted to Ecological Applications, 217. Also in De la canopée à la biomasse thèse de l Université Paul Sabatier

15 Tomography as a tool to remove ground echo Truong-Loi model: σ PQ = A PQ W α PQ cos θ i 1 exp B PQW β PQ + cos θ i Volume scattering z C PQ W δ PQΓ PQ θ i, ε, k, s sin θ i exp B PQW β PQ cos θ i + S PQ θ i, ε, k, s exp z B PQW β PQ cos θ i z Double bounce scattering more AGB more strength but......more extinction so less strength surface bounce unrelated to AGB Surface scattering decreasing with AGB dependent on ground roughness, dielectric z constant, slope, etc.. = ( + + ) Power Power Power

16 Tomography as a tool to remove ground echo Truong-Loi model: σ PQ = A PQ W α PQ cos θ i 1 exp B PQW β PQ + cos θ i Volume scattering z C PQ W δ PQΓ PQ θ i, ε, k, s sin θ i exp B PQW β PQ cos θ i + S PQ θ i, ε, k, s exp B PQW β PQ z cos θ i z Double bounce scattering more AGB more strength but......more extinction so less strength surface bounce unrelated to AGB Surface scattering decreasing with AGB dependent on ground roughness, dielectric z constant, slope, etc.. = ( + + ) Power Power Power Tomo filter

17 Outline A. SAR Tomography 1. How does it work? 2. 3D imaging on forests 3. Estimating AGB on tropical forests B. Why 3m backscatter works in estimating AGB on tropical forests? 1. Biophysical explanation: 3m as key elevation 2. Tomographic explanation: ground rejection C. Ground notching 1. How does it work? 2. The effect of system geometry 3. Preliminary achievements D. Conclusions

18 Slant range SAR interferometry I 2 I 1 δr π 2 Interferometric phase (Δφ) Azimuth True topography π 2 φ 1 = 4π λ r 1 + φ sc Interferometric coherence: φ 2 = 4π λ r 2 + φ sc Δφ φ 1 φ 2 = 4π λ r 1 r 2 Ε I 1 I 2 Ε I 1 2 Ε I 2 2 = ρ ej Δφ

19 Slant range SAR interferometry I 2 I 1 δr π 2 Interferometric phase (Δφ) Azimuth True topography Removed topography π 2 Interferometric coherence: Ε I 1 I 2 Ε I 1 2 Ε I 2 2 = ρ ej Δφ φ 1 = 4π λ r 1 + φ sc 4π λ r DEM,1 Δφ φ 1 φ 2 = 4π λ r 1 r 2 4π λ r DEM,1 r DEM,2 φ 2 = 4π λ r 2 + φ sc 4π λ r DEM,2

20 Slant range SAR interferometry I 2 I 1 δr π 2 Interferometric phase (Δφ) Azimuth True topography Removed topography π 2 Interferometric coherence: Ε I 1 I 2 Ε I 1 2 Ε I 2 2 = ρ ej Δφ φ 1 = 4π λ r 1 + φ sc 4π λ r DEM,1 Δφ φ 1 φ 2 = 4π λ r 1 r 2 4π λ r DEM,1 r DEM,2 φ 2 = 4π λ r 2 + φ sc 4π λ r DEM,2

21 Slant range SAR interferometry I 2 I 1 δr π 2 Interferometric phase (Δφ) Azimuth True topography Removed topography π 2 Interferometric coherence: Ε I 1 I 2 Ε I 1 2 Ε I 2 2 = ρ ej Δφ φ 1 = 4π λ r 1 + φ sc 4π λ r DEM,1 Δφ φ 1 φ 2 = 4π λ r 1 r 2 4π λ r DEM,1 r DEM,2 φ 2 = 4π λ r 2 + φ sc 4π λ r DEM,2 Ground steered images zero interferometric phase at the ground level!

22 Ground steered images I 1 Δφ I 2 2π z 2π 7πΤ4 3πΤ2 5πΤ4 π 3πΤ4 πτ2 πτ4 Zero interferometric phase means same phase for image 1 (I 1 ) and image 2 (I 2 ).

23 Elevation above the ground Ground notching I 1 Δφ I 2 2π z 2π 7πΤ4 3πΤ2 5πΤ4 π 3πΤ4 πτ2 πτ4 I notch Power Zero interferometric phase means same phase for image 1 (I 1 ) and image 2 (I 2 ). I notch = I 1 I 2 cancels out echoes coming from m (±n z 2π ) emphasizes echoes coming from z 2π Τ2m (±n z 2π ) Ground notching Mariotti d Alessandro M., Tebaldini S., Interferometric Ground Notching of SAR Images for Estimating Forest Above Ground Biomass, IGARSS 218, accepted.

24 Sensitivity to above-ground features SLC HV power Notch power LIDAR CHM Weak correlation between SLC power and canopy height Stronger correlation between notch power and canopy height

25 Ground features rejection 25 Copolar phase (m tomo) Double bounce Surface backscattering or volumetric scattering Copolar phase (ground notch) Double bounce Double bounce revealed by a copolar phase value close to 18 Almost zero copolar phase after ground notching

26 Notch varying filter order Number of images: 2 images 3 images 4 images z 2π z 2π 2 z

27 Notch varying baseline Normal baseline: B 1.5 B 2 B z 2π z 2π 2 z

28 Slant range Space-varying vertical shaping Azimuth z 2π m 2 3m m I notch Power m I notch Power

29 InSAR notch power [db] slc(hv) 2 [db] 6 images 3m power [db] Comparison Single image TomoSAR Ground truth AGB [T/Ha*1] Sensitivity to the AGB of the interferometric notch is comparable to the TomoSAR focusing: about 2 db per 1 T/Ha AGB Correlation is.74 vs.97

30 Conclusions A. SAR Tomography 1. Several images are needed (about 4 at least) 2. A wave long enough is needed to penetrate the forest layer 3. A good DTM is needed to perform ground steering 4. It is possible to focus on a certain height above the ground level 5. Very good correlation of 3m power with AGB up to 45 T/Ha 6. Sensitivity of about 1 db per 5 T/Ha AGB B. Ground notching Future work 1. Two images are sufficient 2. A good DTM is needed to perform ground steering 3. The ground echo is cancelled 4. The tree echo is locally attenuated or amplified depending on the considered height 5. The vertical shaping depends on the height of ambiguity (hence on the baseline) 6. Airborne notches show power fluctuations as a consequence of baseline oscillations 7. Good correlation and sensitivity from the preliminary results A. SAR Tomography and ground notch with narrow bandwidths B. Equalized ground notch C. Triplets notch