1) P-band SAR tomography can provide 3D images of tropical forests with resolution along the vertical direction, exploiting the relationship between reflectivity and multi-baseline signals. 2) Analysis of SAR tomography data from French Guiana showed scattering mechanisms are linked to the ground level and canopy level, with HV polarization dominating volume scattering at the canopy. 3) Relating scattering to independently measured biomass at two sites showed significant correlation between biomass and backscattering from layers between 20-40m above the ground, indicating sensitivity to total above-ground biomass.

1. Relating Tropical Forest Biomass to
P-Band SAR Tomography
D. Ho Tong Minh - IRSTEA, UMR TETIS
With
T. Le Toan1 , F. Rocca2, S. Tebaldini2
(1) Centre d'Ėtudes Spatiales de la Biosphère (CESBIO),
Toulouse, France
(2)Dipartimento di Elettronica e Informazione
Politecnico di Milano, Italy

2. 2
P-band SAR BIOMASS mission
What can tomography bring to BIOMASS ?
Operation concept Baseline Option
Tomographic phase
Nominal phase
Coverage
3 months 1 year
4.7 years 4 years
Optional concepts are ~90 kg heavier than the baseline.
“ESAC commends the innovation embodied in the tomographic aspect of the mission,
and considers this capability a highly desirable feature. ESAC therefore recommends
that the optional tomographic mode be operated for as long as is allowed by the need
to keep some contingencies in the design and operation of the satellite.”
Comments of ESAC on 22nd April 2013:
Selected as 7th Earth Explorer Core Mission of ESA on 7 May 2013‫‏‬
Selected transects Global

3. 3
Introduction
• Even at P-Band, Radar intensity tends to saturate for very high biomass density (
> 300 t/ha)  Information about forest structure becomes crucial
=> 3D P-band SAR Tomography for forests

4. 4
P-band SAR tomography
key tool to SEE through the forest
good resolution along the three spatial dimensions
suitable long wavelength to penetrate the dense forest layer
Model independent polarimetric SAR Tomography
not relying on any particular assumption about the observed scene
providing resolution along the vertical direction
exploiting the relationship (a Fourier transform) linking reflectivity and multi-baseline signal
Introduction

5. 5
Preliminary issues
✕
PSF
elevation
ground range
✕
PSF
ground range
elevation
elevation
ground range
reference
height
height
above a
fixed
reference
Airborne SAR surveys are often
characterized by deviations of the
platform from the nominal trajectory.
A non regular sampling of the total
baseline aperture follows.
As a consequence, the Point Spread
Function (PSF) along the cross-range
direction exhibits undesired side-lobes.
Standard interferometric processing
removes the phases associated with a
constant elevation along the images.
The local topography is not taken into
account so that height measurements are
not referred to the ground level.
Being the goal the exploration of the
forest layer, the topographic contribution
shall be removed.
✕
✕
✕
✕
✕
✕
✕
✕
✕
✕
✕✕
✕
✕
1.Terrain topography
2. Baseline sampled irregularly

6. 6
Terrain flattening
Volume Contributions tomography
Ground Contributions tomography
200 600 1000 1400 1800 2200
-10
0
10
20
30
40
50
60
200 600 1000 1400 1800 2200
-10
0
10
20
30
40
50
60
SAR
data
Decomposition
In Sum of
Kronecker
Products
Volume only
covariance
matrix
Rvolume
Ground only
covariance
matrix
Rground
Estimations of the
interferometric
phases associated
with the ground
Phase calibrated
The removal of the interferometric phases
associated with the ground level makes the local
elevation of the terrain the reference height.
Hereinafter, 0m always refers to the ground level
regardless of the actual topography.
The phases are determined by the optical
wavepath so that the effects due to
uncompensated platform motion are removed too.
elevation
ground range
space-varying
reference height
height
above
the
ground
✕

7. 7
Baseline interpolation
Performing linear interpolation:
The distortion is minimized if, before
the interpolation, the reflectivity profile
is shifted around 0 m.
reflectivity
profile
(original
signal)
elevation [m]
0
40
Fourier
domain
(original
domain)
→
→
0
40
profile
shift
(signal
demodulation)
0
40
✕
spectra
multiplication
(interpolation)
0
40
profile shifted
back
(interpolated
signal)
0
40
(interpolated
baseband signal)
resulting
profile
elevation [m]

8. 8
From multi-baseline to multi-layer
q
elevation
0
1
2
n b1
b2
    





 


 db
r
jxrPxry nn
4
exp,,,
q

sin
2 maxb
r
z 
Complex reflectivity along cross-range () direction and signal along
image index are related by a Fourier Transform.
The baseline distribution determines the vertical resolution
z
The Guyaflux tower (camera )
Spatial frequencies along the
baseline axis correspond to
above ground elevations.
SAR Tomography

9. 9
Investigated site
Paracou
Nourgues
French Guiana
Data from TropiSAR 2009 – ESA
System Sethi - ONERA
Scene Tropical forests
forest height ≈ 30 m
Biomass : 200-600 t/ha
Carrier frequency P-Band with 125 MHz bandwidth
Data 5/6 tracks Full-Pol

10. 10
Ground spectrum
Height[m]
Azimuth [m]
1000 1500 2000 2500 3000 3500 4000 4500 5000
-20
0
20
40
60
Volume spectrum
Height[m]
Azimuth [m]
1000 1500 2000 2500 3000 3500 4000 4500 5000
-20
0
20
40
60
vvgg
K
k
kkk RCRCRCW  1

Multi-polarimetric multi-baseline covariance matrix can be expressed :
0 20 40 60
LiDAR top height
Algorithm: Capon
Ground-only contribution decomposition

11. 11
0 20 40 60
LiDAR top height
HH spectrum
Height[m]
Azimuth [m]
1000 1500 2000 2500 3000 3500 4000 4500 5000
-20
0
20
40
60
HV spectrum
Height[m]
Azimuth [m]
1000 1500 2000 2500 3000 3500 4000 4500 5000
-20
0
20
40
60
0 0.5 1
TomoSAR to understand Scattering Mechanisms
Contributions from the ground
level beneath the forest are
observed.
However, significant scattering
contributions are observed at
the canopy level in HH
polarization, whereas this
volume scattering contribution is
dominating in HV polarization.
ESA BIOSAR 2008 campaign (DLR)
Boreal forest : Krycklan, Northern Sweden
forest‫‏‬height‫‏51‏≈‏‬m
The scattering mechanisms are
dominately linked to the ground level.
Algorithm: Capon
HV spectrum
Height[m]
Slant range [m]
4500 5000 5500 6000 6500
0
20
40
60

12. 12
Multi-layer
Note:
Height is always measured with respect to terrain elevation
a
0m
10m
20m
40m
30m
SAR Tomography resolution cell
a
0m
10m
20m
40m
30m
SAR resolution cell

13. 13
Terrain topography [m]
-10
0
10
20
30
12
3
4
5
6
78
9
10
11
12
13
14
15
16
15 m layer
45 m layer
Azimuth [m]
500 1000 1500 2000 2500 3000 3500
30 m layer
Azimuth [m]
Groundrange[m]
500 1000 1500 2000 2500 3000 3500
2000
4500
Ground layer
Groundrange[m]
2000
4500
-20
-15
-10
-5
0
5
-20
-15
-10
-5
0
5
-20
-15
-10
-5
0
5
-20
-15
-10
-5
0
5
-20
-15
-10
-5
0
5
Original image
Groundrange[m]
2000
4500
A
A’
Backscattered power HV
Multi-layer
The ground and the top
(45 m) layers show strong
topographic effects.
The middle layer images
appear much less affected
by topography.
Cells inside the canopy are
always filled up by trunk and
woody branches
irrespective of the ground
slope, resulting in the
topographic slope to have a
minor effect on signal
power.

14. 14
TomoSAR to understand how to retrieve biomass
Original image
Groundrange[m]
2000
4500
12
3
4
5
6
78
9
10
11
12
13
14
15
16
a
0m
SAR resolution cell Intensity (dB) Intensity - biomass
Paracou
15m
30m Original image, rP = 0.37, Slope = 0.51
1 2 3 4 5 6
-20
-15
-10
-5
0
gHV
[dB]
100 t/ha
-20 -15 -10 -5 0 5 200 t/ha - 600 t/haBest fit line
in-situ Paracou
in-situ Nouragues

15. 15
a
0m
30m
TomoSAR resolution cell
30 m layer
Azimuth [m]
Groundrange[m]
500 3500
2000
4500
Ground layer (0 m)
Groundrange[m]
2000
4500
-20 -15 -10 -5 0 5
Original image
Groundrange[m]
2000
4500
12
3
4
5
6
78
9
10
11
12
13
14
15
16
200 t/ha - 600 t/ha
a
0m
15m
30m
SAR resolution cell
Height is always measured with respect
to terrain elevation.
Intensity (dB) Intensity - biomass
TomoSAR to understand how to retrieve biomass
Layer 0m, rP = -0.2, Slope = -0.29
1 2 3 4 5 6
-20
-15
-10
-5
0
gHV
[dB]
Layer 30m, rP = 0.75, Slope = 1.84
1 2 3 4 5 6
-20
-15
-10
-5
0
gHV
[dB]
Original image, rP = 0.37, Slope = 0.51
1 2 3 4 5 6
-20
-15
-10
-5
0
gHV
[dB]
Paracou
Best fit line
in-situ Paracou
in-situ Nouragues
100 t/ha
15m

16. 16
Discussion
For the layers below 15 m, the correlation is very
weak (and negative). This can be explained by: i)
extinction; and ii) double bounces that dominate
whenever the topography is flat (about 10° lobe
width)
The 15 m layer shows almost no correlation (and no sensitivity) (<0.15). One
hypothesis is that this layer is constituted by trunks, and presents the same
characteristics among the 85 forest plots under study.
For layers between 20 m and 40 m, the correlation
becomes highly significant, implying that: i) the
perturbing effect of the ground contribution is
minimized; and ii) there is a strong correlation
between the biomass contained in this layer and the
total above ground biomass (0.92 by TROLL).
-10 -5 0 5 10
-100
-50
0
Ground slope [degree]
[degreee]
Co-polar phase SHH
SVV

layer ground
layer 15 m
The correlation between the backscattered power and biomass
J. Chave, “Study of structural, successional and spatial patterns in tropical rain forests
using TROLL, a spatially explicit forest model,” Ecological Modelling, pp. 233–254, 1999
200 300 400 500 600 700 800
0
50
100
150
200
250
300
350
400
Above-ground biomass (t.ha-1
)
Biomassofthe20-40mlayer(t.ha-1)
rP
= 0.92

17. 17
Training on stratified subset of
Nouragues data. Performance
assessed on data from Paracou.
Plot size 100 m x 100 m
Transferability : Cross-validation site
Training on stratified subset of
Paracou data. Performance
assessed on data from
Nouragues.
Plot size 100 m x 100 m
Training: Nouragues
Validation: Paracou
Training: Paracou
Validation: Nouragues
(a) (b)
0 100 200 300 400 500 600
0
100
200
300
400
500
600
Reference biomass (t/ha)
Retrievedbiomass(t/ha)
RMSE = 64.83 (t/ha)
= 18.35 (%)
rP
= 0.8
0 100 200 300 400 500 600
0
100
200
300
400
500
600
Reference biomass (t/ha)
Retrievedbiomass(t/ha)
RMSE = 58.66 (t/ha)
= 16.6 (%)
rP
= 0.72
D. Ho Tong Minh, T. Le Toan, F. Rocca, S. Tebaldini, L. Villard, M. Rejou-Mechain, J. Chave, K. Scipal. "SAR tomography for the
retrieval of forest biomass and height : cross-validation at two tropical forest sites in French Guiana". Remote Sensing of
Environment. under revision.
D. Ho Tong Minh, T. Le Toan, F. Rocca, S. Tebaldini, M. Mariotti d’Alessandro, and L. Villard, “Relating P-band SAR tomography to
tropical forest biomass”, IEEE Transactions on Geoscience and Remote Sensing, Vol. 52, No. 2, pp. 967-979, Feb. 2014.

18. 18
D. Ho Tong Minh, S. Tebaldini, F. Rocca, T. Le Toan, L. Villard, and P. Dubois-Fernandez, "Capabilities of BIOMASS Tomography
for Investigating Tropical Forests," Geoscience and Remote Sensing, IEEE Transactions on , vol.PP, no.99, pp.1-11,
doi: 10.1109/TGRS.2014.2331142.
Implications : BIOMASS @ 6 MHz
6 MHz spaceborne geometry
0 100 200 300 400 500 600
0
100
200
300
400
500
600
In situ above-ground biomass (t/ha)
Retrievedbiomass(t/ha)
RMSE = 35.02 (t/ha)
= 9.86 (%)
6 MHz airborne geometry
0 100 200 300 400 500 600
0
100
200
300
400
500
600
In situ above-ground biomass (t/ha)
Retrievedbiomass(t/ha) RMSE = 35.39 (t/ha)
= 9.97 (%)
0 100 200 300 400 500 600
0
100
200
300
400
500
600
RMSE = 18.74 (t/ha)
= 5.28 (%)
In situ above-ground biomass (t/ha)
Retrievedbiomass(t/ha)
125 MHz
rP = 0.94
R2 = 0.89
p << 0.00001
rP = 0.83
R2 = 0.69
p = 0.00008
rP = 0.84
R2 = 0.70
p = 0.00005
Results at 6 MHz in both the spaceborne geometry and the airborne geometry appear
to be well consistent with those observed in the 125-MHz case, indicating that the 30-
m layer appears to be the most informative about the AGB.
Paracou forest: Plot size 250 m x 250 m
Observation Observation Simulation

19. 19
Forest biomass prediction
Colors on the map represent the
amount of biomass density in a
continuum fashion.
This is consistent with the 16
permanent plots available.
The map helps to point out many
places which have been never
been measured and reported.
0 200 400 600

20. 20
BIOMASS MISSION : TOMOSAR L2/L3 PROCESSOR
Product Resolution
TomoSAR cube
for all
polarization pairs
( T3(x,y,z) )
 The same as SLC spatial
resolution
 Approx. 20 m vertical
resolution
Note
 Linear focusing by Fourier
Transform
 Radiometric accuracy preserver
 x is latitude (in WGS 84 datum)
 y is longitude (in WGS 84 datum)
 z is height above the ground

21. 21
Conclusions
SAR tomography allows to map not only vertical forest
structure but also biomass.
The scattering mechanisms at P-band in a very dense
tropical forest:
It was found that scattering contributions from about 30 m
above ground exhibit high sensitivity to forest biomass
value ranging from 250 t/ha to 450 t/ha.
Ground scattering is strongly visible and double bounces in flat
terrain topography are visible everywhere.
Volume scattering is significantly related to the high range biomass