SAR Tomographic imaging of tropical forests:

P and L-band

(1) Centre d’Ėtudes Spatiales de la Biosphère (CESBIO), Toulouse, France

(2) Dipartimento di Elettronica, Informazione e Bioingegneria, Politecnico di Milano, Italy

Dinh Ho Tong Minh1, Thuy Le Toan1, Stefano Tebaldini2, Fabio Rocca2

Frascati, PolInSAR 2015 – 1st BIOMASS Science workshop, 29 January

Airborne tomography TropiSAR campaign in Paracou and Nouragues

Data from TropiSAR 2009 – ESA

System Sethi - ONERA

Scene Tropical forests forest height : 20-40 m Biomass : 200-600 t/ha

Paracou

Nouragues

French Guiana

P-band SAR BIOMASS mission

P/L-Band 6 tracks Full-Pol 9-10 h, 24/08/2009

P-Band 5 tracks Full-Pol

TomoSAR (SAR Tomography)

x

y

z

o

New method Gain new knowledge

Ground spectrum

Heig

ht

[m]

Azimuth [m]

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Volume spectrum

Heig

ht

[m]

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0 0.5 1

vvgg

K

k

kkk RCRCRCW 1

Jointly interferometry - polarimetry to gain vertical (altimetry) information

Ho Tong Minh et al., TGRS 2014

P-band BIOMASS: TomoSAR

(Tropical forest : Nouragues )

SAR Tomography over forested areas

sin2 maxb

rz

SAR Tomography (TomoSAR)

3D imaging of the forest structure

P-band TomoSAR yields the forest vertical structure

even in the tropical forests.

Nouragues, French Guiana , HV

Heig

ht [m

]

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0

Paracou, French Guiana , HV

Range [m]

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ht [m

]

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5500 4500

db

rjxrPxry nn

4exp,,,

Complex reflectivity along cross-range () direction and signal along image index are related by a Fourier Transform.

P-band tomographic analysis

Significant scattering contributions are observed at the canopy level in HH polarization, whereas this volume scattering contribution is dominating in HV polarization.

P-band TomoSAR to understand Scattering Mechanisms

The forest height can be readily determined, and it matches with LiDAR height.

Algorithm: Coherent focusing

HH channel

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LiDAR height

HV channel

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LiDAR height

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Slant range [m]

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P-band

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Topography

Original image

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ange

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SAR resolution cell Intensity (dB) Intensity - biomass

Paracou

Original image, rP = 0.37, Slope = 0.51

1 2 3 4 5 6 -20

-15

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-5

0

g HV

[d

B]

100 t/ha

-20 -15 -10 -5 0 5 200 t/ha - 600 t/ha Best fit line in-situ Paracou in-situ Nouragues

a

0m

30m

TomoSAR resolution cell

30 m layer

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ange

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Height is always measured with respect to terrain elevation. Layer 30m, rP = 0.75, Slope = 1.84

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g HV

[d

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TomoSAR to understand how to retrieve biomass

Ho Tong Minh et al., TGRS 2014 - RSE 2015

200 t/ha - 600 t/ha

The effects of the ground contribution is minimized. Total AGB at large trees is well represented in 20-40 m layer.

3D Tomographic reconstruction

H ≈ 4000 m B = 125 MHz

Airborne data

1

2

BIOMASS data

H = 650 Km B = 6 MHz

3

Generation of 6 MHz data

2. Degrading the resolution of 125 MHz airborne data through linear filtering (ONERA)

1. Back projection of airborne tomographic data onto BIOMASS geometry.

Two different processing approaches have been considered :

Impulse Response Function

Simulated Orbits +

(6 MHz filtering airborne geometry)

(6 MHz simulation spaceborne geometry)

Implications : BIOMASS @ 6 MHz Simulation

6 MHz spaceborne geometry

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In situ above-ground biomass (t/ha)

Ret

riev

ed b

iom

ass

(t/h

a)

RMSE = 35.02 (t/ha) = 9.86 (%)

Observation 6 MHz airborne geometry

0 100 200 300 400 500 600 0

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In situ above-ground biomass (t/ha)

Re

trie

ved

bio

mas

s (t

/ha)

RMSE = 35.39 (t/ha)

= 9.97 (%)

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100

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RMSE = 18.74 (t/ha)

= 5.28 (%)

In situ above-ground biomass (t/ha)

Re

trie

ved

bio

mas

s (t

/ha)

Observation 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

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.53, no.2, pp.965,975, Feb. 2015

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.

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

Algorithm: Coherent focusing

L-band tomographic analysis

Penetration of L and P-band signal in a tropical forest

P band

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Co-polar phase HH-VV

Optical image at Paracou – tropical forest

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Tomogram P-band

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Tomogram L-band

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2D Intensity

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Algorithm: Capon spectrum

2D Intensity L-band

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Tomogram L-band

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L-band

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L and P-band tomographic profiles

Tropical forest : Paracou Boreal forest : Krycklan

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Intensity [dB]

Tree top height [m] Topographic slope [°]

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Original SLC Ground layer

Original SLC Ground layer L-band, HH

P-band,HH

Intensity [dB]

Imaging the Ground scattering

L-band

P-band

Imaging the Ground scattering

Co-polar phase HH-VV

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L-band P-band

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ol phase [°]

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Ground slope [°]

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Ground slope [°]

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Original SLC Ground layer

Conclusions

The vertical structure of the forest in a dense tropical forest:

• Ground scattering is strongly visible

• Volume scattering is significantly related to the biomass from 200 t/ha –

500t/ha (even @ 6 MHz, RMSE 10% at 6.25-ha plots).

• Forest height can be readily determined, and it matches with LiDAR height.

At P-band

• Ground scattering is partly visible

• The L-band tomographic profile is quite disturbed as compared to the P-band profile. In this condition, the use of tomographic imaging at L-band in tropical forests appears limited.

• However, when the forest top height is roughly below 20 m (e.g. in forest regrowth), the tomographic results are expected to be the same as in boreal forests.

At L-band