The document summarizes the results of tomographic analysis of P-band SAR data collected over tropical and boreal forests. Three key findings are:
1) Analysis of BIOSAR 2007 data over a semi-boreal forest showed dominant scattering from the ground level in HH and VV polarizations, with weak scattering from the vegetation canopy. HV polarization showed stronger returns from the vegetation.
2) BIOSAR 2008 data over a boreal forest showed decreasing ground-to-volume ratio with increasing forest height, consistent with larger volumetric scattering structures in taller forests. Ground-to-volume ratio also depended on terrain slope for P-band but not L-band data.
3) Preliminary
A microwave active filter for nanosatellite’s receiver front-ends at s-bandsIJECEIAES
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A microwave active filter for nanosatellite’s receiver front-ends at s-bandsIJECEIAES
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Tomo_20.pdf
1. P-Band Penetration in Tropical and
Boreal Forests: Tomographical Results
Stefano Tebaldini, Mauro Mariotti d’Alessandro, Ho Tong Minh Dinh, Fabio Rocca
Politecnico di Milano
Dipartimento di Elettronica e Informazione
2. Introduction
Longer 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. Introduction
Longer wavelength SARs precious tool for forestry remote sensing
• Under foliage penetration capabilities
• Mitigate saturation in backscatter vs forest biomass law
Sensitivity 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 scatter
Signal interpretation requires physical models
• One passage coherent or incoherent polarimetric decomposition
• Two passages PolInSAR (i.e: RVoG)
4. Introduction
Multi-baseline SAR Tomography Direct imaging of the forest vertical structure
Tomogram - HH
Track n cross 60
range 50
40
Height [m]
Reference
Track (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. Introduction
Multi-baseline SAR Tomography Direct imaging of the forest vertical structure
Tomogram - HH
Track n cross 60
range 50
40
Height [m]
Reference
Track (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. Investigated sites
BIOSAR 2007
Site Remningstorp, Southern Sweden
Period Spring 2007
Scene Semi-boreal forest
Topography Flat
Carrier frequency P-Band
Vertical 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 ONERA
Site Paracou, French Guyana
Period August 2009
Scene Tropical forest
Carrier frequency P-Band
Vertical resolution ≈15 m
7. Investigated sites
BIOSAR 2007
Site Remningstorp, Southern Sweden
Period Spring 2007
Scene
BIOSAR 2007,forest
Semi-boreal
BIOSAR 2008: Vertical resolution ≥ forest height
Topography Tomographic imaging: Capon spectrum
Flat
Carrier frequency • Greatly
P-Band enhances vertical resolution
Vertical 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 level
TROPISAR
Site
• no need for multilooking
Paracou, French Guyana
Period
• model free
August 2009
Scene • radiometrically
Tropical forest accurate
Carrier frequency P-Band
Vertical resolution ≈15 m
8. Results from BIOSAR 2007
Campaign BioSAR 2007 - ESA
System E-SAR - DLR
Period Spring 2007
Site Remningstorp, South Sweden
Scene Semi-boreal forest
Norway spruce, Scots pine, Birch
Topography Flat
Tomographic 9 – Fully Polarimetric
tracks
Carrier 350 MHz
frequency
Slant range 2m
resolution
Azimuth 1.6 m
resolution
Vertical 10 m (near range) to 40 m
resolution (far range)
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. 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. 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. Results from BIOSAR 2008
Campaign BioSAR 2008 - ESA
System E-SAR - DLR
Site Krycklan river catchment,
Northern Sweden
Scene Boreal forest
Pine, Spruce, Birch, Mixed stand
Topography Hilly
Tomographic 6 + 6 – Fully Polarimetric
Tracks (South-West and North-East)
Carrier P-Band and L-Band
Frequency
Slant range 1.5 m
resolution
Azimuth 1.6 m
resolution
Vertical 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. 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. 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. 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. Results from TropiSAR
Campaign TropiSAR- ESA
data courtesy of ONERA
System Sethi- ONERA
Period August 2009
Site (among Paracou, French Guyana
others)
Scene Tropical forest
estimated 150 species per hectare
Dominant families:
Lecythidaceae, Leguminoseae,
Chrysobalanaceae, Euphorbiaceae.
Tomographic 6 – Fully Polarimetric
tracks
Carrier P-Band
frequency
Slant range ≈1 m
resolution
Azimuth ≈1 m
resolution
Vertical 15 m
resolution
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
Processor
Slant range
azimuth
Layer N
SAR Tomography
resolution cell
SAR resolution Layer 1
cell
18. TROPISAR – Tomographic profiles
Tomographic reconstruction of two azimuth cuts:
Polarization = HH - azimuth bin = 455
60
Method: 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. TROPISAR – Tomographic sections
Tomographic reconstruction of radar scattering from four
different heights
Ground level Ground level + 10 m
Method: 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.
21. Dependence on Topography
A 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. Dependence on Topography
A 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. Dependence on Topography
A 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
32. Tomography @ BIOMASS resolution
Resolution 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 robust
statistics
• Slant range resolution loss further causes a spread of the of the backscattered power
distribution, resulting in a vertical resolution loss
E-SAR - HV
Theoretical vertical resolution limit due to pulse 30
bandwidth is ≈ 10 m at θ = 30°
20
Height [m]
10
• Nevertheless, Tomographic profiles
0
provide information about the forest
structure that is consistent with the -10
2000 2500 3000 3500 4000 4500 5000
airborne 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. BioMass: Forest Height Retrieval
Forest height has been retrieved
through a direct investigation of the Forest Relative
shape of the retrieved tomographic height error
profiles
Rising trend due to the very large 30 1
variation of baseline aperture resulting
from flight geometry 15 0.5
0 0
Good 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
15
Estimation loses reliability for forest 10
10 10
10
lower than 10 m, consistently with the 55 55
theoretical 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. Conclusions
Tomography 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-Band
Different 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