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Tomo_20.pdf

  1. 1. P-Band Penetration in Tropical and Boreal Forests: Tomographical ResultsStefano Tebaldini, Mauro Mariotti d’Alessandro, Ho Tong Minh Dinh, Fabio Rocca Politecnico di Milano Dipartimento di Elettronica e Informazione
  2. 2. IntroductionLonger wavelength SARs  precious tool for forestry remote sensing• Under foliage penetration capabilities• Mitigate saturation in backscatter vs forest biomass law From Le Toan et al., 2004
  3. 3. IntroductionLonger wavelength SARs  precious tool for forestry remote sensing• Under foliage penetration capabilities• Mitigate saturation in backscatter vs forest biomass lawSensitivity to the whole forest structure  many different scattering mechanisms• Back scatter from the canopy• Back scatter from the ground (Bragg)• Trunk-Ground forward scatter• Canopy-Ground forward scatterSignal interpretation requires physical models• One passage  coherent or incoherent polarimetric decomposition• Two passages  PolInSAR (i.e: RVoG)
  4. 4. Introduction Multi-baseline SAR Tomography  Direct imaging of the forest vertical structure Tomogram - HH Track n cross 60 range 50 40 Height [m] ReferenceTrack (Master) 30 π/2 20 10 Track 1 0 θ -10 slant 400 600 800 1000 1200 1400 range Slant range [m] elevation Tomography is a fundamental tool to: • investigate the phenomenology of Radar scattering from forested areas • help physical modeling to be used with non interferometric data and single baseline data
  5. 5. Introduction Multi-baseline SAR Tomography  Direct imaging of the forest vertical structure Tomogram - HH Track n cross 60 range 50 40 Height [m] ReferenceTrack (Master) 30 π/2 20 10 Track 1 0 θ The BIOMASS Tomographic phase -10 slant 400 600 800 1000 1200 1400 range Features: Slant range [m] elevation • 55 days (3% of mission lifetime) • ≤ 4 day repeat pass time Main goal: Help improve forest biomass and height retrieval methods by addressing three questions: • What are the main scattering mechanisms (SMs) at forest and ground level • How do the SMs vary as a function of polarization • How do the SMs vary over the global forest biomes
  6. 6. Investigated sitesBIOSAR 2007Site Remningstorp, Southern SwedenPeriod Spring 2007Scene Semi-boreal forestTopography FlatCarrier frequency P-BandVertical resolution 10 m (near range) to 40 m (far range) BIOSAR 2008 Site Krycklan, Northern Sweden Period Fall 2008 Scene Boreal forest Topography Hilly Carrier frequency P-Band and L-Band Vertical resolution P-Band: 20 m (near range) to 80 m (far range) L-Band: 6 m (near range) to 25 m (far range)TROPISAR – data courtesy of ONERASite Paracou, French GuyanaPeriod August 2009Scene Tropical forestCarrier frequency P-BandVertical resolution ≈15 m
  7. 7. Investigated sitesBIOSAR 2007Site Remningstorp, Southern SwedenPeriod Spring 2007Scene BIOSAR 2007,forest Semi-boreal BIOSAR 2008: Vertical resolution ≥ forest heightTopography  Tomographic imaging: Capon spectrum FlatCarrier frequency • Greatly P-Band enhances vertical resolutionVertical resolution Requires multilooking  10•m (near range) to 40 m (far range) horizontal resolution loss • Not radiometrically accurate BIOSAR 2008  Quantitative measurements by assuming ground + volume scattering Site Krycklan, Northern Sweden • Parametric models Period Fall 2008 Scene Boreal forest • Algebraic Synthesis Topography Hilly Carrier frequency P-Band and L-Band Vertical resolution P-Band: 20 m (near range) to 80 m (far range) TROPISAR: Vertical resolution < forest height L-Band: 6 m (near range) to 25 m (far range)  Tomographic imaging: coherent focusing at pixel levelTROPISARSite • no need for multilooking Paracou, French GuyanaPeriod • model free August 2009Scene • radiometrically Tropical forest accurateCarrier frequency P-BandVertical resolution ≈15 m
  8. 8. Results from BIOSAR 2007Campaign BioSAR 2007 - ESASystem E-SAR - DLRPeriod Spring 2007Site Remningstorp, South SwedenScene Semi-boreal forest Norway spruce, Scots pine, BirchTopography FlatTomographic 9 – Fully PolarimetrictracksCarrier 350 MHzfrequencySlant range 2mresolutionAzimuth 1.6 mresolutionVertical 10 m (near range) to 40 mresolution (far range)
  9. 9. BIOSAR 2007 HHVV phase HHVV coherence +180° 2 slant range [Km] • Phase: 1.6 0 1.2 Forest: φHH - φVV ≈ 80° 0.8 Open areas: φHH - φVV ≈ 0° 0.4 -180° 1 2 3 4 5 HHVV coherence amplitude 1 slant range [Km] 2 • Amplitude: 0.8 1.6 Forest: |γHHVV | ≈ 0.45 0.6 1.2 0.4 Open areas: |γHHVV | ≈ 0.8 0.2 0.8 0.4 0 1 2 3 4 5 Mean reflectivity - HH Amplitude Stability Analysis 2 slant range [Km] • Presence of a high number of 1.6 amplitude stable points in the 1.2 co-polar channels 0.8 0.4 1 2 3 4 5 azimuth [Km]
  10. 10. BIOSAR 2007 – Tomographic profiles Tomographic reconstruction of an azimuth cut: Reflectivity (HH) – Average on 9 tracks 50 azimuth [m] Reflectivity (HH) – Average on 9 tracks 40 30 20 10 slant range 200 600 1000 1400 1800 2200 Capon Spectrum - HH 60 azimuth 50 height [m] The analyzed profile is almost totally forested, 40 30 except for the dark areas 20 10 0 HH: -10 200 600 1000 1400 1800 2200 Dominant phase center is ground locked Vegetation is barely visible Capon Spectrum - HV 60 LIDAR Terrain Height 50 LIDAR Forest Height height [m] 40 Similar conclusions for VV 30 20 10 HV: 0 -10 Dominant phase center is ground locked 200 600 1000 1400 1800 2200 Vegetation is much more visible slant range [m]
  11. 11. BIOSAR 2007 – Tomographic profiles Tomographic reconstruction Physical interpretation: of an azimuth cut: Reflectivity (HH) – Average on 9 tracks • Scattering from ground level is determined by an imperfect dihedral 50 azimuth [m] Reflectivity (HH) – Average on 9 tracks 40 contribution from ground-trunk interactions, perturbed by understory and 30 20 topography oscillations 10 slant range Possible presence of canopy-ground interactions600 200 1000 1400 1800 2200 • Scattering from above the ground, due to canopy backscattering, HH 60 Capon Spectrum - is extremely weak azimuth 50 height [m] The analyzed profile is almost totally forested, 40 30 except for the dark areas 20 10 0 HH: -10 200 600 1000 1400 1800 2200 Dominant phase center is ground locked Vegetation is barely visible Capon Spectrum - HV 60 LIDAR Terrain Height 50 LIDAR Forest Height height [m] 40 Similar conclusions for VV 30 20 10 HV: 0 -10 Dominant phase center is ground locked 200 600 1000 1400 1800 2200 Vegetation is much more visible slant range [m]
  12. 12. Results from BIOSAR 2008Campaign BioSAR 2008 - ESASystem E-SAR - DLRSite Krycklan river catchment, Northern SwedenScene Boreal forest Pine, Spruce, Birch, Mixed standTopography HillyTomographic 6 + 6 – Fully PolarimetricTracks (South-West and North-East)Carrier P-Band and L-BandFrequencySlant range 1.5 mresolutionAzimuth 1.6 mresolutionVertical resolution 20 m (near range) to >80 m (far range)(P-Band)Vertical resolution 6 m (near range) to 25 m (far range)(L-Band)
  13. 13. BIOSAR 2008 – Tomographic profiles Tomographic reconstruction of P-Band SW - HV 30 an azimuth cut: Height [m] 20 Polarization: HV 10 Method: Capon Spectrum 0• Results are geocoded onto the same ground range, height grid -10 2000 2500 3000 3500 4000 4500 5000 P-Band NE - HV• All panels have been re-interpolated such that 30 the ground level corresponds to 0 m Height [m] 20• Loss of resolution from near to far range, 10 especially at P-Band (Δz > 80 m at far ranges) 0 -10• Relevant contributions from the ground level 5000 4500 4000 3500 3000 2500 2000 below the forest are found at P-Band L-Band SW - HV 30 30 LIDAR DEM 250 Height [m] 20 20 Height [m] Height [m] 10 10 200 00 -10 -10 2000 2000 2500 2500 3000 3000 3500 3500 4000 4000 4500 4500 5000 5000 150 Ground range [m] 2000 2500 3000 3500 4000 4500 5000 Ground range [m] Ground range [m]
  14. 14. BIOSAR 2008 – ground/volume decomposition Ground to Volume Ratio: P-Band SW P-Band SW Ratio between the HH HV backscattered powers associated with ground-only and volume-contributions 15 15 0 0 -15 -15 L-Band SW L-Band SW HH HV 15 15 0 0 -15 -15
  15. 15. BIOSAR 2008 – ground/volume decomposition HV GVR vs. LIDAR H100 HV GVR vs. Terrain slope P-Band SW P-Band SW• At both wavelengths it is 15 15 15 observed that the HV GVR 10 10 10 HV GVR [dB] HV GVR [dB] decreases with forest height, HV GVR [dB] 55 5 consistently with the 00 0 enlargement of volumetric -5 -5 -5 structures. -10 -10 -10 -15 -15 -15• HV GVR exhibits a 10 10 15 15 20 20 25 25 Forest Height [m] 30 30 0 5 10 15 dependence on terrain slope L-Band SW L-Band SW 15 15 15 at P-Band but not at L-Band 10 10 10 This result indicates that HV HV GVR [dB] HV GVR [dB] HV GVR [dB] 55 5 ground contributions are 00 0 due to double bounce -5 -5 -5 contributions at P-Band, but -10 -10 -10 not at L-Band -15 -15 -15 1010 1515 2020 2525 3030 0 5 10 15 Forest Height [m] Absolute Ground Slope [deg] LIDAR [m]
  16. 16. Results from TropiSARCampaign TropiSAR- ESA data courtesy of ONERASystem Sethi- ONERAPeriod August 2009Site (among Paracou, French Guyanaothers)Scene Tropical forest estimated 150 species per hectare Dominant families: Lecythidaceae, Leguminoseae, Chrysobalanaceae, Euphorbiaceae.Tomographic 6 – Fully PolarimetrictracksCarrier P-BandfrequencySlant range ≈1 mresolutionAzimuth ≈1 mresolutionVertical 15 mresolution
  17. 17. Processing of TropiSAR Goal: generation of a stack of multi-layer SLC SAR images out of a stack of multi-baseline SLC SAR images height Tomographic ProcessorSlant range azimuth Layer N SAR Tomography resolution cell SAR resolution Layer 1 cell
  18. 18. TROPISAR – Tomographic profiles Tomographic reconstruction of two azimuth cuts: Polarization = HH - azimuth bin = 455 60Method: coherent focusing Height [m] 40 20 All panels have been re-interpolated 0 such that the ground level corresponds 400 600 800 1000 1200 1400 to 0 m Polarization = HV - azimuth bin = 455 60 Height [m] 40 20 HH 0 Visible contribution from the 400 600 800 1000 1200 1400 ground level beneath the forest Slant range [m] Polarization = HH - azimuth bin = 1455 60 Vegetation is well visible Height [m] 40 20 0 HV 400 600 800 1000 1200 1400 Poor contributions from the Polarization = HV - azimuth bin = 1455 ground level beneath the forest 60 Height [m] 40 20 Vegetation is well visible 0 400 600 800 1000 1200 1400 Slant range [m]
  19. 19. TROPISAR – Tomographic sectionsTomographic reconstruction of radar scattering from fourdifferent heights Ground level Ground level + 10 mMethod: coherent focusing 20 20 15 15 Polarization: HH Slant range Slant range 10 10 5 5• The strongest dependence on 0 0 terrain topograpy is found at the -5 -5 ground level• The most uniform tomographic Azimuth -10 Azimuth -10 layer is found at about15-20 m above the ground Ground level + 20 m Ground level + 35 m 20 20• Highest layers exhibit a 15 15 dependence on terrain topography, Slant range Slant range similarly to the ground layer 10 10 5 5 0 0 -5 -5 Tomographic data exhibit a more -10 -10 complex dependence of terrain Azimuth Azimuth topography than traditional SAR data.
  20. 20. Dependence on TopographyA closer look…
  21. 21. Dependence on TopographyA closer look… This resolution cell gathers contributions from terrain only. => Signal intensity in this cell is affected by terrain slope the same way as in traditional SAR images of bare surfaces
  22. 22. Dependence on TopographyA closer look… This cell is completely within the volume layer, independently on volume orientation w.r.t. the Radar LOS. => Signal intensity in this cell is independent of terrain slope This resolution cell gathers contributions from terrain only. => Signal intensity in this cell is affected by terrain slope the same way as in traditional SAR images of bare surfaces
  23. 23. Dependence on TopographyA closer look… The scattering volume within cells at the boundaries of the vegetation layer depends on volume orientation w.r.t. the Radar LOS. => Signal intensity in this cell is affected by terrain slope in a similar way as the cell corresponding to the ground layer. This cell is completely within the volume layer, independently on volume orientation w.r.t. the Radar LOS. => Signal intensity in this cell is independent of terrain slope This resolution cell gathers contributions from terrain only. => Signal intensity in this cell is affected by terrain slope the same way as in traditional SAR images of bare surfaces
  24. 24. TROPISAR – Polarimetric analysis
  25. 25. TROPISAR – Polarimetric analysis Clear trunk-ground signature in the copolar channels at ground level
  26. 26. TropiSAR:connections to forest biomass
  27. 27. Tomographic layering vs in situ biomass measurements HV – 0 m HV – 15 m HV – single passStandard E HV [dB] HV – 30 m HV – 45 m 2000 0 -5 2500 12 10 -10 6 3000 11 9 -15 15 5 Ground range [m] 8 7 3500 4 -20 14 -25 3 13 4000 2 1 -30 4500 -35 -40 500 1000 1500 2000 2500 3000 Azimuth [pixel]
  28. 28. Tomographic layering vs in situ biomass measurements Polarization = HV Polarization = HV Spatial resolution = 125 m Spatial resolution = 250 m Layer 0 [m], R = 0.31, Slope m = -0.6% Layer 0 [m], R = 0.46, Slope m = -0.7% -15 -15 -20 -20 E HV [dB] E HV [dB] -25 -25 8 4 5123 10 9 2 1 7 11 13 15 16 14 6 -30 -30 -35 -35 250 300 350 400 450 250 300 350 400 450 Layer 15 [m], R = 0.12, Slope m = 0.1% Layer 15 [m], R = 0.48, Slope m = 0.3% -15 -15 -20 -20 E HV [dB] E HV [dB] -25 -25 13 15 8 4 5123 10 9 2 1 7 1114 16 6 -30 -30 -35 -35 250 300 350 400 450 250 300 350 400 450 Layer 30 [m], R = 0.82, Slope m = 2% Layer 30 [m], R = 0.93, Slope m = 1.9% -15 -15 -20 -20 E HV [dB] E HV [dB] -25 -25 13 15 16 7 11 6 9 2 1 14 -30 -30 8 4 5123 10 -35 -35 250 300 350 400 450 250 300 350 400 450 in situ biomass [T/ha] in situ biomass [T/ha] -10° Color coding = ground slope +10°
  29. 29. Tomographic layering vs in situ biomass measurements Polarization = HH Polarization = VV Spatial resolution = 250 m Spatial resolution = 250 m Layer 0 [m], R = 0.3, Slope m = -0.5% Layer 0 [m], R = 0.2, Slope m = -0.4% -15 -15 -20 -20 EVV [dB]E HH [dB] 8 4 5123 10 9 2 7 11 16 13 6 8 4 5123 10 9 7 11 16 13 15 6 1 14 15 2 1 14 -25 -25 -30 -30 -35 -35 250 300 350 400 450 250 300 350 400 450 Layer 15 [m], R = 0.32, Slope m = 0.2% Layer 15 [m], R = 0.12, Slope m = 0.1% -15 -15 -20 -20EHH [dB] E VV [dB] 13 15 6 8 4 5123 10 9 2 7 13 15 1114 6 8 4 5123 10 9 2 1 7 1114 16 1 16 -25 -25 -30 -30 -35 -35 250 300 350 400 450 250 300 350 400 450 Layer 30 [m], R = 0.86, Slope m = 1.6% Layer 30 [m], R = 0.9, Slope m = 1.8% -15 -15 -20 -20E HH [dB] E VV [dB] 6 7 13 15 16 11 6 7 13 15 16 1 14 -25 4 12 10 5 3 9 2 1 1114 -25 8 4 5123 10 9 2 8 -30 -30 -35 -35 250 300 350 400 450 250 300 350 400 450 in situ biomass [T/ha] in situ biomass [T/ha] -10° Color coding = ground slope +10°
  30. 30. Biomass prediction from tomography – linear regression biomass = a*(HV at 30 m ) + b HV, Power layer 30m 600 10 RMSD = 35.14 (t.ha -1) 8 = 9.59 (%) 500 MPE = 1.33 (%) 6 rP = 0.82 Retrieved biomass (t.ha-1) rS = 0.77 4 400 2 300 0 -2 200 -4 -6 100 -8 0 -10 0 100 200 300 400 500 600 in-situ biomass (t.ha-1) Processing parameters: 15x4 = 60 sample plots 125 m (plots 1 to 15) Color coding = ground slope and 1x25 = 25 sample plots 100m (plot 16) N_training = 10 samples -10° +10° N_validation = 75 samples
  31. 31. Tomography @ BIOMASS resolution
  32. 32. Tomography @ BIOMASS resolutionResolution Loss Factor w.r.t. E-SAR = 100/6 •12.5/1.6 > 100 !•At 30° a 60 x 60 estimation window contains just 5 independent looks !  less robuststatistics• Slant range resolution loss further causes a spread of the of the backscattered powerdistribution, resulting in a vertical resolution loss E-SAR - HVTheoretical vertical resolution limit due to pulse 30bandwidth is ≈ 10 m at θ = 30° 20 Height [m] 10• Nevertheless, Tomographic profiles 0provide information about the foreststructure that is consistent with the -10 2000 2500 3000 3500 4000 4500 5000airborne case BioMass – HV 30 30 20 20 Height [m] BioMass data-set derived by Elevation [m] DLR from BIOSAR 2008 10 10 Pulse Bandwidth = 6 MHz 00 Azimuth resolution = 12.5 m -10 -10 2000 2000 2500 2500 3000 3000 3500 3500 4000 4000 –4500 5000 LIDAR TOP 4500 5000 Ground range [m] HEIGHT
  33. 33. BioMass: Forest Height RetrievalForest height has been retrievedthrough a direct investigation of the Forest Relativeshape of the retrieved tomographic height errorprofilesRising trend due to the very large 30 1variation of baseline aperture resultingfrom flight geometry 15 0.5 0 0Good match with LIDAR • Standard Deviation < 4 m w.r.t. 2D Histogram Normalized 2D Histogram LIDAR by exploiting a 1 hectare 30 30 30 30 BioMass Forest Height [m] BioMass Forest Height [m] estimation window 25 25 25 25 • No significant bias beyond 10 m 20 20 20 20 SAR [m] SAR [m] 15 15 15 15Estimation loses reliability for forest 10 10 10 10lower than 10 m, consistently with the 55 55theoretical resolution limit 0 0 00 0 0 5 5 10 10 15 15 20 20 25 25 30 30 00 5 5 10 10 15 15 20 20 25 25 30 30 LIDAR [m] LIDAR [m] LIDAR [m] LIDAR [m]
  34. 34. ConclusionsTomography is highly sensitive to forest structure: • Double bounce contributions from ground-trunk interactions have clearly been observed at the Paracou site, despite the presence of a tropical forest 40 m high • Boreal and semi-boreal forest have shown an almost ground-locked vertical structure in both in co and cross polarization, suggesting specular reflections play a non negligible role at P-BandDifferent tomographic layers connect differently to forest biomass • Best correlation factor observed at 30 m in HV (R = .82 @ 125 m , R = .93 @ 250 m) • Preliminary biomass inversion results are very encouraging. Final assessment needs accurate comparison to existing inversion techniques (Intensity, Intensity + PolInSAR height, LIDAR)Forest imaging @ BIOMASS resolution is a challenging problem. Measurements from BIOSAR 2007 and BIOSAR 2008 show that: • Tomographic imaging consistent with the airborne case • Forest height retrieved within an accuracy of 20% with a 1 ha spatial resolution • No significant bias observed for forests higher than 10 m, consistently with the theoretical limit Assessment of tomography capabilities @ BIOMASS resolution in tropical forests is yet to be done

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