Use of ERS1/2 interferometric data for the retrieval of forest information

Nicolas FLOURY, Jérôme BRUNIQUEL, Thuy LE TOAN
Centre d’Etudes Spatiales de la Biosphère
CNES-CNRS-Université Paul Sabatier
bpi 2801, 18 avenue Edouard Belin
31401 Toulouse Cedex 4
France
Tel: (33) 5 61 55 85 84
Fax: (33) 5 61 55 85 00
E-mail: Nicolas.Floury@cesbio.cnes.fr


Until now, most of the results using ERS data for forest observations have
been obtained by using multitemporal data for forest / non-forest mapping.
Recent results have pointed out the usefulness of interferometric data for
forest mapping based on the low coherence of forested areas and for the
retrieval of forest parameters (biomass from coherence and height from phase
differeence analysis). However, the conditions for a successful inversion of
interferometric data into forest information have to be understood prior to
the effective application.

The objective of the present study is twofold: 1/ to extend the existing
experimental results with both repeat pass ERS-1 and tandem ERS-1/2 data on
a well controlled forest site for different seasonal conditions and system
configurations (baseline, time interval). 2/ to better understand the
physical meaning of the interferometric measurements by using a thoretical
model.

Interferometric data have been acquired over the Landes forest. Due to its
structure of large and homogeneous forest stands and its flat topography,
this forest is well suited to such methodological study. Repeat pass ERS-1
data with a variable time interval and several ERS-1 / ERS-2 tandem data
have been studied. The degree of coherence and the phase difference have
been analysed as a function respectively of forest biomass and forest
height, for different configurations of acquisition.

The theoretical model describes the forest as an homogeneous layer of
cylindric dielectric scatterers. The caracteristics of the medium are
provided by ground-truth data. The coherent scattered field is computed. The
model has been used to simulate the effect of changes in dielectric and
geometric properties of the scatterers on the interferometric measurements.
The relative importance of the different sources of decorrelation has been
determined, including seasonal change of vegetation (moisture content,
growth) and scatterers movement due to the wind.

The results will contribute to a better understanding of the conditions of
using interferometric data for retrieving forest parameters.

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry