Ludovic Villard's research while affiliated with University of Toulouse and other places
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Publications (85)
The penetration capability of P-band radar waves through dense vegetation, along with the ability of tomography to separate the contributions of different layers in a vertical reflectivity profile, make P-band radar tomography a promising tool for digital terrain modeling in forested areas, specifically in dense tropical forests under which terrain...
Forests are one of the key elements in ecological transition policies in Europe. Sustainable forest management is needed in order to optimise wood harvesting, while preserving carbon storage, biodiversity and other ecological functions. Forest managers and public bodies need improved and cost-effective forest monitoring tools. Research studies have...
Most land surface models can, depending on the simulation experiment, calculate the vegetation distribution and dynamics internally by making use of
biogeographical principles or use vegetation maps to prescribe spatial and temporal changes in vegetation distribution. Irrespective of whether
vegetation dynamics are simulated or prescribed, it is no...
C-band radar remote sensing is a suitable tool for monitoring agricultural areas on a large scale, providing access to information on vegetation such as plant biomass, or on the surface water content of the soil. Recent studies suggest that the water state and the physiological functioning of trees influence radar response leading to marked daily p...
Fostered by the upcoming BIOMASS mission, this article explores long-term trends of P-band temporal decorrelation over a tropical forest due to a time series of 617 days acquired during the TropiScat-2 experiment. The interest in this unique time series is twofold. First, it provides consistent statistics to monitor the yearly evolution of temporal...
Most land surface models can either calculate the vegetation distribution and dynamics internally by making use of biogeographical principles or use vegetation maps to prescribe spatial and temporal changes in vegetation distribution. Irrespective of whether vegetation dynamics are simulated or prescribed, it is not practical to represent vegetatio...
The purpose of this paper is twofold, considering first the generalization of a multichannel speckle filter in order to handle temporal stacks of polarimetric SLC SAR data, and secondly the development of an ad hoc performance indicator based on the Polarimetric Orientation Angle (POA) in order to better estimate the resulting speckle reduction tha...
Spaceborne remote sensing can track ecosystems changes thanks to continuous and systematic coverage at short revisit intervals. Active remote sensing from synthetic aperture radar (SAR) sensors allows day and night imaging as they are not affected by cloud cover and solar illumination and can capture unique information about its targets. However, S...
BIOMASS is ESA’s seventh Earth Explorer mission, scheduled for launch in 2022. The satellite will be the first P-band SAR sensor in space and will be operated in fully polarimetric interferometric and tomographic modes. The mission aim is to map forest above-ground biomass (AGB), forest height (FH) and severe forest disturbance (FD) globally with a...
Following the past TropiScat and AfriScat tower-based scatterometer experiments conducted as part of the European Space Agency (ESA) BIOMASS mission preparation activities, a follow-up experiment referred to as TropiScat-2 has been undertaken since March 2018 in French Guiana. The main objectives of this new campaign consist in extending the previo...
Following the past TropiScat and AfriScat tower-based scatterometer experiments conducted as part of the European Space Agency (ESA) BIOMASS mission preparation activities, a follow-on experiment referred to as TropiScat-2 has been undertaken in French Guiana since March 2018. Based on new capabilities including C-band acquisitions, this letter add...
Synthetic aperture radar (SAR) tomography (TomoSAR) is an emerging technology to image the 3D structure of the illuminated media. TomoSAR exploits the key feature of microwaves to penetrate into vegetation, snow, and ice, hence providing the possibility to see features that are hidden to optical and hyper-spectral systems. The research on the use o...
Natural and anthropogenic disturbances pose a significant threat to forest condition. Continuous, reliable and accurate forest monitoring systems are needed to provide early warning of potential declines in forest condition. To address that need, state-of-the-art simulations models were used to evaluate the utility of C-, L- and P-band synthetic ap...
Temperate forests are under climatic and economic pressures. Public bodies, NGOs and the wood industry are looking for accurate, current and affordable data driven solutions to intensify wood production while maintaining or improving long term sustainability of the production, biodiversity, and carbon sequestration. Free tools and open access data...
New technologies are urgently needed to observe forest biomass stocks globally, providing data for efforts to mitigate greenhouse gas emissions by reducing deforestation and forest degradation. To meet this challenge, the next Earth Explorer BIOMASS will collect P-band SAR data to produce three-dimensional maps of most of the world's forests by usi...
Following the TropiScat and AfriScat experiments that took place in French Guiana and
Ghana respectively between 2011 and 2017 in the framework of the preparation phase of BIOMASS mission, a new campaign TropiScat-2 has been set up since March 2018. This campaign, located in the experimental site of Paracou in French Guiana, has three major interre...
The aim of this paper is to map the aboveground biomass (AGB) in Gabon. First, a random forest (RF) model that relates reference AGB values to remote sensing (RS)-derived variables (mainly radar and optical images) was built, and the significant predictive variables were determined. Second, the built RF model was applied to the significant RS-deriv...
In this study, the potential of Sentinel-1 data to seasonally monitor temperate forests was investigated by analyzing radar signatures observed from plots in the Fontainebleau Forest of the Ile de France region, France, for the period extending from March 2015 to January 2016. Radar backscattering coefficients, σ0 and the amplitude of temporal inte...
The Biomass satellite mission will be launched by the European Space Agency in 2022 with a P-band synthetic aperture radar. Due to the penetration capability of P-band radar waves through dense vegetation cover like forest and the ability of tomography to separate the contributions of different layers in a vertical backscattering profile, the Bioma...
The objective of this paper is to provide a better understanding
of tomographic capabilities to estimate above ground biomass
(AGB) in dense forested areas at P-band. The analysis is
carried out on airborne data acquired over sites in French
Guyana and in Gabon during the ESA campaigns TropiSAR
and AfriSAR 2015, respectively. Over both sites, P-ban...
Tropical forests are a key component of the global carbon cycle. Yet, there are still high uncertainties in forest carbon stock and flux estimates, notably because of their spatial and temporal variability across the tropics. Several upcoming spaceborne missions have been designed to address this gap. High-quality ground data are essential for accu...
Forest monitoring is a major issue to carry out energetic and environmental policies. Actual context in spaceborne remote sensing data is very promising. Our study aims to test the ability of SAR, optical and textural data to estimate forest parameters (biomass, height, diameter and density), and to evaluate the improvement of combining these remot...
La mission spatiale Biomass sera lancé par l’Agence spatiale européenne en 2021 avec un radar à synthèse d’ouverture en bande P dans le but de cartographier la biomasse forestière à l’échelle planétaire, ainsi que la hauteur et les dégradations du couvert forestier. Plusieurs produits secondaires sont aussi envisagés, notamment la cartographie 3D d...
Savannahs and woodlands are among the most important biomes in Africa: they cover half of sub-Saharan Africa, provide vital ecosystem services to the rural communities, and play a major part in the carbon budget. Despite their importance and their fragility, they are much less studied than other ecosystems like rainforests. In particular, the distr...
Temporal decorrelation is a critical parameter for repeat-pass coherent radar processing, including many advanced techniques such as polarimetric SAR interferometry (PolInSAR) and SAR tomography (TomoSAR). Given the multifactorial and unpredictable causes of temporal decorrelation, statistical analysis of long time series of measurements from tower...
The potential of Sentinel-1 radar data for vegetation monitoring is assessed. Both intensity and interferometric products are assessed over temperate forests and agricultural fields.
Forests play a primordial role for life on Earth. Beyond their contribution as a major source of raw materials and renewable energy, they also hold an inestimable treasure of biodiversity. They ensure the protection of arable land, are a continuous source of water and contribute to improved air quality. Whether for food or pharmacopoeia, forests ar...
In Radar remote sensing, the electromagnetic (em) modeling of scattering contributions from natural or man-made scatterers has often to deal with coupling scattering mechanisms due to the underlying reflecting surface (ground or water). For the monostatic configuration, this contribution is commonly associated with the so-called double bounce scatt...
In this letter, the temporal coherence of tropical forest scattering at P-band is addressed by means of a ground-based experiment. The study is based on the TropiScat campaign in French Guiana, designed to support the Biomass mission, which will be the ESA 7th Earth Explorer mission. For Biomass, temporal coherence is a crucial parameter for cohere...
The objective of this paper is to provide a better understanding of the capabilities of the BIOMASS tomography concerning the retrieval of forest biomass and height in tropical areas. The analysis presented in this paper is carried out on airborne data acquired by Office National d'Etudes et de Recherches Aérospatiales (ONERA) over the site of Para...
Synthetic aperture radar (SAR) is one of the most promising remote sensors to map forest carbon. The unique spaceborne and long-wavelength SAR data currently available are L-band data, but their relationship with forest biomass is still controversial, particularly for high biomass values. While many studies assume a complete loss of sensitivity abo...
Forest biomass retrieval using P-band synthetic aperture radar (SAR) data has been studied during the past two decades. However, much progress remains necessary to recover biomass for tropical dense forests, where above-ground biomass (AGB) values are often found between 250 and 400 t/ha. In this high range, the sensitivity of radar backscatter to...
In this letter, we present the results from the ground-based European Space Agency campaign TropiScat, which is aimed at evaluating the temporal coherence at P-band in a tropical forest in all polarizations and at different heights within the vegetation layers. The TropiScat equipment has been operated since October 2011 at the Paracou field statio...
This paper deals with the temporal survey of the tropical forest electromagnetic scattering with a ground-based radar equipment. Installed on the top of a 55 m flux tower overlooking the Paracou forest in French Guiana, a dense primary tropical forest, the radar system uses a vertical antenna array and it is able to provide every 15 minutes P-band...
In the frame of the Biomass mission activities, this paper presents the challenges and recent results in the retrieval of forest biomass from polarimetric (PolSAR) and interferometric (PolInSAR) P-band SAR data. During the mission Phase A, critical issues in the biomass retrieval algorithms in boreal and tropical forests have been identified and ad...
The 7-th ESA Earth Explorer, BIOMASS is a synthetic aperture radar (SAR) which will collect data from employing a multiple baseline orbit during the initial phase of its lifetime. This data can be used for tomographic SAR (TomoSAR) processing resulting in a vertical resolution of about 20 m, sufficient to decompose the backscatter from most tropica...
Synthetic aperture radar (SAR) is one of the most promising remote sensor to map forest carbon. The unique spaceborne and long-wavelength SAR data currently available are L-band data, but their relationship with forest biomass is still under controversy, particularly for high biomass values. While many studies assume a complete loss of sensitivity...
The next ESA Earth Explorer Core Mission BIOMASS is envisaged to collect multiple baselines on selected areas during the initial phase of its lifetime. Such data will allow to image the vertical structure of the vegetation layer to within a vertical resolution of about 20 m, sufficient to decompose the backscattered power from a tropical forest int...
This paper deals with the modelling of time series of polarimetric backscattering and temporal coherence of a tropical forest at P Band. The experimental data show that in the dry season daily cycles of both observables are present and that they are linked to the wind and to the dielectric changes of the canopy. To retrieve these observations, a de...
In this paper we discuss the design concepts and preliminary results relating to the European Space Agency's ground-based campaign TropiScat, whose main goal is to evaluate temporal coherence at P-band in a tropical forest in quad-polarization, considering temporal lags ranging from hours to months and at different heights within the vegetation lay...
In this paper we present results from the ground-based ESA campaign TropiScat, aimed at evaluating the temporal coherence at P-band in a tropical forest in all polarizations and at different heights within the vegetation layers. The TropiScat equipment has been operated since October 2011 at the Paracou field station, French Guiana, to continuously...
The retrieval of above-ground biomass (AGB) in dense tropical forests using synthetic aperture radar (SAR) images is widely recognized as a challenging task. The first difficulty arises from the decrease of sensitivity of the backscattered intensity to biomass at high biomass values, often referred to as the backscatter saturation effect. At P-band...
This work aims at studying day-night phenomenology in a tropical forest illuminated by P-band radar signals. The analysis is carried out based on data set from the ground-based ESA campaign TropiScat, which aimed at evaluating the temporal coherence in a tropical forest in quad-polarization and at different heights within the vegetation layer. The...
The TropiSAR campaign has been conducted in August 2009 in French Guiana with the ONERA airborne radar system SETHI. The main objective of this campaign was to collect data to support the Phase A of the 7th Earth Explorer candidate mission, BIOMASS. Several specific questions needed to be addressed to consolidate the mission concept following the P...
Forest biomass retrieval using P-band SAR data has been demonstrated during the past 2 decades. However, little effort has been made to recover biomass in tropical forests, with biomass values typically ranges from 250 to 500 t/ha. In this high range of biomass, the radar backscatter coefficients exhibit poor sensitivity to biomass, which is often...
This paper deals with the modelling of forest pine scattering at P Band. A growth model for forest description and an electromagnetic model for the scattering are used. First, canonical studies are carried out about single and multiple branches scattering behaviour with the goal of highlighting further results. Then, a sensitivity study is carried...
In terms of biomass information, the most critical forest regions of the Earth are in the tropical belt, where knowledge on biomass and biomass changes is poor and where biomass information is the most needed, for carbon flux calculations and for national scale inventory. The difficulty of making in situ inventories of biomass in the tropics, has t...
This paper deals with a radar ground experiment dedicated to tropical forest backscattering at P band. With polarimetric and tomographic capabilities, this system is able to provide long-term radar data over a dense tropical forest. These data will be use to improve our comprehension of backscattering mechanisms and their evolution over long period...
In this paper we present preliminary results from the groundbased ESA campaign TropiScat, aimed at evaluating the temporal coherence at P-band in a tropical forest in quadpolarization and at different heights within the vegetation layer. The TropiScat equipment allows to gather the signal with a multistatic array within few minutes, resulting in th...
The purpose of this work is to assess biomass in forest savanna and to analyse changes in biomass from 2007 to 2010 in Cameroon, by using ALOS PALSAR data. The methodology includes an inter-calibration step, and multi-image filtering for noise reduction and preservation of the resolution. An existing direct model previously developed by CESBIO over...
The retrieval of above-ground biomass in dense tropical forests using Synthetic Aperture Radar (SAR) images is widely recognized as a challenging task, because of the backscatter saturation effect at high biomass values and the ground topography effect. The study presented in this paper is an attempt to overcome these issues based on direct three-d...
This paper presents a preliminary assessment of ALOS/PALSAR data from the Kyoto and Carbon Initiative program for monitoring of forest status and mapping of biomass in Vietnam. The changes in forest status (deforestation, reforestation, growth) occurred during three years (2007-2008-2009) were detected in a region of intensive rubber plantation pro...
This paper deals with a ground experiment and its modelization, related to the future spaceborne BIOMASS mission for global forest biomass estimation.
Ground topography under vegetated area is liable to bring significant changes on radar backscattering and thereby on the associated standard retrieval algorithms dedicated to forest biomass. Within the framework of the ESA BIOMASS mission, this paper evinces the evolution of P-band polarimetric intensities with a tilted underlying ground. For that...