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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 coherences according to specific time scales of the BIOMASS tomographic and interferometric phases. Second, it provides key insights to explore new processing approaches with the combination of data from different orbit directions (ascending/descending) and different mission cycles separated by about seven months according to the current acquisition plan. For the first time, this study shows that 18-day coherences (corresponding to the time interval between the first and last acquisitions of the BIOMASS tomographic processing) can vary significantly according to rainy and dry seasons (medians from 0.3 to 0.9). The extension to time intervals of up to 90 days within both seasons and over two consecutive years puts forward the key role of the typical sporadic rainfalls occurring during dry periods in tropical rainforests, with a stronger impact on temporal coherence evolution compared to the more reproducible rainy seasons. Furthermore, outstanding values significantly above zero have been obtained for the 7- and 14-month coherences (medians of 0.35 and 0.2, respectively), opening the way to new methods of change detection. Overall, this study highlights the role of P-band temporal decorrelation not only as a disturbance factor for coherent applications but also as a relevant indicator of forest changes.

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 profiles of both radar backscattering coefficient and temporal coherence. The objective of this paper is to make a preliminary comparison between the temporal evolution of Sentinel-1 radar data and in situ radar measurements over a Mediterranean olive orchard located in Morocco. In situ radar data consist in quad polarizations measurements realized from a 20m high tower, every 15 minutes, for the period extending from May 2019 to October 2020.

The effects of dry and fresh matter on microwave remote sensing data represent a great challenge, for which vegetation permittivity models are essential. To develop this model, in situ permittivity sensors are needed. There are few commercial equipment dedicated to these in situ measurements and none of them are developed in order to leave the instrument on site for automatic measurements with specific constraints such as communication, wide frequency band, non-invasive measurements and low price. We developed an instrument addressing these challenges. In this article, we present the first collected measurements and the computed permittivity data.

This aims to compare the performance of different observations modes of the future BIOMASS mission for the characterization of tropical forests. In particular, it provides indicators of variability for different typical descriptors of the SAR response of a forest, and computes estimates using real data acquired at P band in the frame of the TropiSAR campaign. Results show that the best performance, reached in the Polarimetric and Tomographic mode, degrades slightly in the single-polarization case and significantly when the vertical tomographic resolution is reduced. Nevertheless, it is shown that in this latter case, the use of priors estimated in the high-resolution tomographic phase and consisting on the estimate of the wave extinction, leads to a spectacular improvement of the performance. This results reveals important in the frame of the BIOMASS missions which plans to phases with different acquisition configurations.

C-band radar observations are well known to have great potentials for the monitoring of crop hydric conditions. Recent studies suggested that the observed difference of backscattering coefficient ($\sigma_{o}$) between ascending and descending pass over tropical forest could be related to the physiological functioning of the trees. Likewise, the water movement within annual crops could lead to a daily cycle of both$\sigma_{o}$and temporal coherence ($\rho$). The objective of this paper is to present the preliminary results of an experiment carried out on a winter wheat field in Morocco that was instrumented with six C-band antennas during 2020 growing season. The preliminary results showed strong daily cycles of$\rho$and$\sigma_{o}$that are analyzed in relation to wind speed, surface soil moisture and evapotranspiration. This work open insights for the monitoring of the crop hydric status using C-band radar data acquired by Sentinel-1 and by potential future radar geostationary missions.

This paper deals with the retrieval of Digital Terrain Model (DTM) from P-band SAR, with a specific focus on the Paracou study case acquired during the TropiSAR airborne campaign. A technique based on the sum of Kronecker product (SKP) decomposition is used to compute a SAR-derived DTM. An analysis of the differences between this estimate and the one obtained for airborne LIDAR acquisitions is proposed. Moreover, a multi-scale adaptive filtering method, using local regression and morphological image processing, is also proposed to improve the DTM. The post-processed DTM show an improvement of the estimation of ground topography.

Scheduled for launch in 2023, ESA's seventh Earth Explorer Mission, BIOMASS, will carry the first P-band synthetic aperture radar (SAR) to be flown in space, to gather fully polarimetric acquisitions over forested areas worldwide in interferometric and tomographic modes. This paper presents the algorithms developed to estimate biophysical parameters from BIOMASS measurements and their implementation in the BIOMASS level 2 (L2) prototype processor. The L2 processor will generate global maps of forest Above Ground Biomass (AGB), Forest Height (FH), Forest disturbance (FD). Accurate generation of these products requires the L2 processor to be closely inter-linked with the BIOMASS interferometric processor, in order to produce phase-calibrated interferometric stacks, retrieve sub-canopy terrain topography, and generate a 3D representation of forest structure by use of SAR tomography. AGB estimation results are here shown using BIOMASS-like acquisitions derived from campaign data acquired over six tropical forests in South America and Equatorial Africa.

Polarimetric Synthetic Aperture Radar Tomography (TomoSAR) is a technology to image the three-dimensional (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. Several experimental studies by different research groups demonstrate that the use of the 3D information results in an accurate characterization of forested areas, providing access to a number of biophysical variables such as terrain topography below the vegetation, forest height, forest Above Ground Biomass (AGB), and forest classification. This paper is intended to provide the reader with an introduction to the use of TomoSAR for the characterization of forest areas, addressing basic imaging principles and methods, retrieval of biophysical parameters, and perspective for spaceborne missions.

This paper describes the rationale and development of the estimation techniques for the level-2 data products of the European Space Agency's 7th Earth Explorer BIOMASS mission. BIOMASS is planned for launch in 2023 and will carry the first-ever P-band synthetic aperture radar (SAR) onboard a satellite. It has been designed to produce consistent global maps of the Earth's forests during a nominal five-year lifetime. Fully polarimetric SAR data will be collected and the satellite orbit will be selected for repeat-pass interferometry and tomography in separated mission phases. Mission requirements call for three level-2 data products: above-ground biomass, forest height and forest disturbance. The paper also discusses the expected limitations of the estimation techniques and remaining problems to be addressed.

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 addresses the question of temporal decorrelation variability at several time scales using a period of 67 days dating back to early August 2018. Overall, C-band coherences have been found very dependent on the reference hour and higher than expected at night (15-min coherences above 0.75 and 1-h coherences above 0.5 75% of the time). Supported by meteorological data from the flux-tower, clear evidence about diurnal convective effects is shown. For the first time, the link with evapotranspiration is pointed out based on a drop of the 15-min coherences going from about 0.8 to 0.2 in less than 1 h (around 7h30) and before the rise of convective winds. Furthermore, the specific impacts of convective winds during the day have been demonstrated using very short timescales (with the spread of coherences at 1 s from 0 to 0.9), whereas it is more difficult to separate these effects from those of water transfer for longer timescales. In addition to providing new insights for the understanding of microwave interactions with dense vegetation, these results should also contribute to widen the capabilities of future companion or high revisit space-borne missions at C-band.

The ESA BIOMASS mission will be the 7th Earth Explorer measuring the above-ground biomass (AGB) in the world's forests. The current ESA Level-2 (L2) implementation study focuses on defining and implementing the main algorithms for forest parameter retrieval from BIOMASS data. After the first year of L2 study innovative results were achieved: the development of ground cancellation, in particular, has proved to be huge value, since it removes from the data the effects of environmental variability and contributions unrelated to the forest carried in the ground scattering. In this paper the current processor implementation and validation activities of the L2 team will be described.

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-band tomography allows us to retrieve the vertical structure of the forest, to better characterize the ground and/or volume scattering mechanisms and to provide a unique solution for the AGB retrieval over the full range of biomass. The relationship between AGB and tomography data was found to be highly similar for forests across continents and sites: Paracou (French Guiana), Lope, Rabi and Mondah (Gabon). The developed metrics derived from the tomographic data have been found highly correlated to reference in situ AGB estimates (R2=0.85) and the root mean square error was 16% (for AGB ranging from 0 to 500 t/ha). These results have strong implications for the tomographic phase of the BIOMASS spaceborne mission.

The objective of this paper is to provide a better understanding of tomographic capabilities in characterization of dense forested areas at P-band. The analysis is carried out on airborne data acquired by ONERA over the site in French Guyana, and in Gabon during the ESA campaign TropiSAR 2009 and AfriSAR 2015, respectively. The results shown support the idea that ground- and -volume interactions play a significant role at P-band. For a dense forest of 30 m and more, strong ground contribution at P-band can be visible in tomograms. P-band tomography allow us to retrieve the whole forest vertical structure, better characterizing of the ground and/or volume scatterings and providing an unique solution in high biomass ranging from 0–500 t/ha.

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-based scatterometers is the most appropriate method for characterizing how rapidly a specific scene decorrelates. Based on the TropiScat experiment that occurred in a tropical dense forest in French Guiana, this letter proposes a comparative analysis between temporal decorrelation at P-band and at higher frequencies in the range of 800-1000 MHz (the low end of the L-band). This letter aims to support the design of future repeat-pass spaceborne missions and to offer a better understanding of the physics behind temporal decorrelation. Beyond the expected lower values that are found and quantified at the low L-band compared with the P-band, similar decorrelation patterns related to rainy and dry periods are emphasized in addition to the critical impacts of acquisition time during the day.

This paper deals with AfriScat, a long-term tower-based scatterometer measurements experiment which will take place over the Ankasa forest in Ghana, an African tropical rain forest.

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 coherent processing of polarimetric synthetic aperture radar (SAR) interferometry and SAR tomography in repeat-pass acquisitions. During the experiment, data were continuously collected for six months during both the rainy and dry seasons. For the rain-free days in both seasons, the coherence exhibits a daily cycle showing a high decorrelation during daytime, which is likely due to motion in the canopy. Up to a 20-day baseline, the coherence is much higher in the dry season than in the rainy season (> 0.8). From 20 to 40 days, it presents the same order of magnitude in both seasons [0.6, 0.7]. For larger temporal baselines, it becomes lower in the dry season. The results can be used to assess the long-term coherence of repeat-pass observations over a tropical forest. However, an extension of this study to several years and over other forest spots would be necessary to draw more general conclusions.

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 Paracou, French Guiana, during the European Space Agency campaign TropiSAR. This high-resolution data set (125-MHz bandwidth) was reprocessed in order to generate a new data stack consistent with BIOMASS as for the bandwidth (6 MHz) and the azimuth resolution (about 12 m). To do this, two different processing approaches have been considered. One approach consisted of degrading the resolution of the airborne data through the linear filtering of raw data, followed by standard SAR processing. The other approach consisted of recovering the 3-D distribution of the scatterers at a high resolution, which was then reprojected onto the BIOMASS geometry. The latter procedure allows us to obtain a data stack that is the most realistic emulation of BIOMASS imaging capabilities. In both approaches, neither ionospheric disturbances nor temporal decorrelation has been considered. The connection to the forest biomass has been examined in both cases by investigating the correlation between the backscatter at different forest heights and the above-ground biomass (AGB) values from in situ data. As expected, the reduction of the system bandwidth to 6 MHz resulted in significant vertical resolution losses compared with the original airborne data. Nevertheless, it was possible to retrieve the forest height to within an accuracy of better than 4 m, whereas the backscattered power at the volume height (30 m above the ground) exhibited a correlation higher than 0.8 with the in situ data and no bias phenomena over the AGB values ranging from 250 to 450 t/ha.

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 tropical forests into two to three layers. A recent study using airborne data from the TropiSAR campaign at the site of Paracou, French Guiana, showed that this information significantly improves the retrieval of forest above-ground biomass (AGB), resulting in an accuracy of about 10% of AGB at a resolution of 1.5-ha. In this paper, we generalize this result, by applying the same algorithm to the Nouragues test site in central French Guiana. This site is characterized by a hilly terrain and an AGB ranging from 150 to 600 t/ha. The relationship between AGB and TomoSAR data at Nouragues was found to be highly similar to the one observed at Paracou. We found that the best correlation between the backscatter signal and AGB is held in the upper canopy layer (i.e. 20-40 m). Cross validation using training plots from Nouragues and validation plots from Paracou, and vice versa, resulted in an accuracy of about 16%-18% of AGB using 1-ha plots. This result suggests that the TomoSAR AGB retrieval method is generalizable to other study sites. In addition we show that, TomoSAR can be used to estimate the canopy height with an error of less than 4 m with forest height ranging from 20 m-40 m.

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 above a saturation point, typically around 100 t.ha−1, others assume a continuous positive correlation between SAR backscatter and biomass. The objective of this paper is to revisit the relationship between L-band SAR backscatter and dense tropical forest biomass for a large range of biomass values, using both theoretical and experimental approaches. Both approaches revealed that after reaching a maximum value, SAR backscatter correlates negatively with forest biomass. This phenomenon is interpreted as a signal attenuation from the forest canopy as the canopy becomes denser. This result has strong implication for L-band vegetation mapping as it can lead to a more-than-expected biomass under-estimation.

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 into two-three layers. The information provided by tomography has recently been shown to be strictly linked to above ground biomass (AGB) in tropical forest, therefore providing a valuable tool for ABG estimation. The aim of this paper is to present a bird-eye overview of BIOMASS Tomography, along with the main experimental results from airborne campaigns flown during Phase-A BIOMASS activities.

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 addressed. In boreal forest, multi polarization backscatter data can be used to mitigate much of the variability due to environment effects. In high biomass tropical forest, because of the low sensitivity of the backscatter to biomass, appropriate correction methods were developed to mitigate the disturbing effects. Also to enhance the retrieval results, a combination of PolSAR and PolInSAR methods was proposed.

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 station, French Guiana, to continuously produce height-range images of the forest with a temporal sampling of 15 min. The forest temporal behavior can be then captured by analyzing the interferometric coherence between the images gathered at different times, considering time scales on the order of hours, days, and months. The results indicate that the vegetation is likely to undergo a significant motion during day hours due to wind and temperature changes, whereas it appears to be definitively more stable during night hours. This result appears to provide a very useful input to the Biomass Monitoring Mission for Carbon Assessment (BIOMASS), as it suggests that the performance over a tropical forest could be optimized by gathering acquisitions in early morning or night hours. The long-term temporal decorrelation has been then evaluated by considering dawn-dawn acquisitions to minimize the impact of wind gusts and by excluding rainy days in order to not confuse forest and system decorrelation. As a result, the temporal coherence at the ground level was found to stay high at about 0.8 at 27 days, whereas the temporal coherence at the canopy height was found to be about 0.8 at 4 days and about 0.65 at 27 days, indicating coherence sensitivity to height.

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, the decrease of sensitivity can occur at biomass values higher than about 300${\rm t~ha}^{-1}$, e.g., those of many dense tropical forests. Another limiting factor is associated with the ground effects, as they can change significantly the magnitude of returns from vegetation–ground interactions. As a consequence, terrain topography or ground moisture status can determine the variations of the observed signal that are not due to forest biomass. A solution to reduce the ground effects is to have access to layers inside the forest canopy where the backscatter from vegetation–ground interactions is not significant. The study presented in this paper is an attempt to overcome the issues outlined above based on direct 3-D imaging of the forest volume, which is possible through multibaseline SAR tomography. In this way, forest biomass can be investigated by considering not only the backscattered power at each slant range and azimuth location but also its vertical distribution. The data analyzed in this paper are from the P-band airborne dataset acquired by Office National d'Études et de Recherches Aérospatiales (ONERA) over French Guiana in 2009, in the frame of the European Space Agency campaign TropiSAR. The dataset is characterized by a favorable baseline distribution, resulting in a vertical resolution less than half the forest height, which made it possible to decompose the vertical distribution of the backscattered power into a number of layers by coherent focusing, i.e., without assuming any prior knowledge about the forest vertical structure. For each layer, the relationship between the backscattered power and forest AGB was then analyzed. As expected, it was found that the power from the bottom layer is very weakly correlated to AGB, whereas the power from a layer at about 30 m above the ground yields the best correlation and sensitivity to AGB in all polarizations, for actual AGB values ranging from 250 to 450${\rm t~ha}^{-1}$. An interpretation of this result is also provided, based on a forest growth model simulation. Finally, the relevance of tomographic technique in P-band spaceborne mission is discussed.

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 complex scattering matrix coefficients. The experiment has been successfully set up and it is operating since October 2011. The main goal of this campaign is to investigate the evolution of the backscattering coefficient and the temporal coherence of the tropical forest at different time scales range. Data are calibrated in relative and processed to take advantage of the largest number of independent looks. Three months of data are exploited in terms of polarimetric temporal coherence and backscattering coefficient in the rainy season and about two months in the dry period. The temporal coherence exhibits daily cycles during the consecutive dry days, whatever the period, and these cycles are perturbed by the presence of rain. Its overall time series appear clearly dependent on the period, dry or rainy, and also on the polarization. The backscattering coefficient time series exhibit also a daily cycle during consecutive dry days, very clearly in the dry period but less pronounced or absent during the rainy period. The backscattering coefficient presents an overall relatively high stability over the full period.

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 produce height-range images of the forest below with a temporal sampling of 15 minutes. The forest temporal behaviour can then be captured by analyzing the interferometric coherence between images gathered at different times, considering time scales on the order of hours, days, and months. Temporal coherence at the ground level was found to be higher than 0.8 at 27 days in all polarimetric channels, whereas temporal coherence at canopy height was found to be about 0.8 at 4 days and about 0.65 at 27 days, witnessing coherence sensitivity to height.

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 layer. The experiment has been successfully set up and operated since October 2011 at the Paracou field station, French Guiana, where the equipment was installed on top of the 55-m high Guyaflux Tower to illuminate the forest below. The system consists of a vector network analyzer connected to 20 antennas through a switchbox, which allows the use of any of them either as a transmitter or as a receiver. Vertical imaging and fully polarimetric capabilities are achieved by operating the 20 antennas in a multistatic fashion, resulting in an equivalent monostatic array consisting of 15 phase centers displaced along the vertical direction in each polarization. Such a design allows unambiguous imaging of the vegetation while yielding a minimum distance between nearby antennas on the order of 0.8 m, so as to minimize coupling effects. The equipment allows the gathering of signals with the tomographic array within a few minutes, resulting in the possibility to produce a tomographic image of the forest with a temporal sampling of 15 min. System calibration and validation was performed by employing a 2-m trihedral reflector and a rotating dihedral reflector. This allowed the evaluation of the system pulse response in all polarizations and also assessment of the extent of tower motions. As a result, tomographic images have been generated from 500 (P-band) to 900 MHz in all polarizations. Results from real data acquired in Fall 2011 confirm the feasibility of carrying out reliable coherence measurements for the whole duration of the campaign.

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 out on the influence of scene description parameters and system ones on the relationship between polarimetric intensity and biomass. It has been shown that the most influent parameters are branches moisture content and branches orientation, the other parameters playing an important role but limited to some observables.

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-dimensional imaging of the forest volume, which is possible through multi-baseline SAR tomography. In this way, forest biomass can be investigated by considering not only the backscattered power at each slant range, azimuth location, but also its vertical distribution. It was found that the power from the a layer at 30m ± 10m above the ground yields the best correlation and best sensitivity with forest biomass in all polarizations, for biomass ranging from 250 to 450 tons/ha.

In this paper we present preliminary results from the ground-based ESA campaign TropiScat, aimed at evaluating the temporal coherence at P-band in a tropical forest in quad-polarization 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 the possibility to produce a tomographic image of the forest with a temporal sampling of 15 minutes. Concerning short term temporal decorrelation, the most relevant phenomenon is the coherence drop during daytime, due to the action of wind moving the forest canopy. This result appears to provide a very useful input concerning the BIOMASS mission, as it suggests that performance over tropical forest could be optimized by gathering acquisitions in the early morning or night hours. A diurnal motion along the vertical direction is observed to characterize the forest phase center. Studies are being carried out to evaluate whether this variation can be imputed to forest evapotranspiration phenomena.

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 periods.

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 various forests, linking γhvoto biomass, is inversed using a probabilistic bayesian approach. There is a mean of 52 t.ha-1of biomass in 12 553 km2which represents 32.63 Mt of carbon not taken into account in existing carbon budget assessments. Besides, forest savanna transition and changes in AGB from 2007 to 2010 are analysed. Changes are mainly due to shifting cultivation, grazing fires, degradation and deforestation.

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 therefore led to serious efforts to estimate biomass from space. P-band SAR, which is the longest wavelength radar systems possible appears the most suitable for biomass measurements, however, its performance in tropical forests still needed to be demonstrated. This was achieved by the design of the TropiSAR campaign in French Guiana, and the ensuing development of inversion algorithms, which will be presented in the paper.

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 obscured by perturbing effects. The most important effect is caused by topography, which needs to be corrected before biomass inversion. In this work, we develop a new backscatter coefficient called t0to account for a) the non reflection symmetry of the forests in case of slopes and b) the change in angular variation of the backscatter following the change of scattering mechanisms due to slopes. The method is tested on TropiSAR campaign data at two forest sites in French Guiana. The results shows the relevance of proposed correction methods to reduce the effects of topography in dense forests.

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 Phase 0 studies, and the data collection strategy was constructed accordingly. More specifically, a tropical forest data set was required in order to provide test data for the evaluation of the foreseen inversion algorithms and data products. The paper provides a description of the resulting data set which is now available through the European Space Agency website under the airborne campaign link. First results from the TropiSAR database analysis are presented with two in-depth analyses about both the temporal radiometric variation and temporal coherence at P-band. The temporal variations of the backscatter values are less than 0.5 dB throughout the campaign, and the coherence values are observed to stay high even after 22 days. These results are essential for the BIOMASS mission. The observed temporal stability of the backscatter is a good indicator of the expected robustness of the biomass estimation in tropical forests, from cross-polarized backscatter values as regarding environmental changes such as soil moisture. The high temporal coherence observed after a 22-day period is a prerequisite for SAR Polarimetric Interferometry and Tomographic applications in a single satellite configuration. The conclusion then summarizes the paper and identifies the next steps in the analysis.

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 purpose, electromagnetic simulations have been achieved using our model MIPERS which theoretical specificities accounting for the topographic effects are herein described. Its originality lies mainly in the 3D characterization of the ground and the volume, as well as the coupling effects between both. This description is followed by a sensitivity analysis in order to further-on quantify the possible consequences on biomass retrieval, conducted with the two standard approaches, namely the P-HV intensity technique and the Pol-InSAR one assuming the RVoG model. This investigation have been also undertaken for a better understanding of experimental data, particularly with the BioSAR and the TropiSAR airborne campaigns over boreal and tropical forests.

This paper comes within the context of exploring the potential of bistatic Pol-InSAR. Numerical simulations are carried out by means of MIPERS, multistatic interferometric polarimetric electromagnetic model for remote sensing which has already proved many abilities. The latter involves a electro-magnetic coherent approach of a discrete natural media description and enables us to model Pol-InSAR observables over forests in the bistatic configuration. Taking into account bistatic SAR constrains in terms of resolution, critical base-line, SNR, range and height ambiguities, new configurations for Pol-InSAR applications are envisioned.

This paper aims at presenting a model able to study the impact of the target presence under vegetation and to assess the detection feasibility. In a first part the model is described. We will then get at the analysis of results for several set of radar configurations to illustrate the potentiality of detection by means of shadows.

Simulating forest scattering by electromagnetic waves has been proposed with both coherent and incoherent models. However, to our best knowledge, most models consider a description of the forest as infinite in the horizontal plane and layered vertically. Such description is relevant in most cases, but fail when border effects are present near the boundaries : the shadowing effects as well as the reinforcement ones, which are present in true SAR images, are absent in the synthetic images. The purpose of this study is to present a coherent scattering model taking into account forest regions of finite extent and benefit from its polarimetric, interferometric and bistatic capabilities to bring out interesting outlooks about border effect in the bistatic case.