The SLOPE estimator has the particularity of having null components (sparsity) and components that are equal in absolute value (clustering). The number of clusters depends on the regularization parameter of the estimator. This parameter can be chosen as a trade-off between interpretability (with a small number of clusters) and accuracy (with a small mean squared error or a small prediction error). Finding such a compromise requires to compute the solution path, that is the function mapping the regularization parameter to the estimator. We provide in this article an algorithm to compute the solution path of SLOPE and show how it can be used to adjust the regularization parameter.

ONERA, the French aerospace lab, has been developping a drone-embedded SAR imaging tool, SAR-Light, over the past three years. Now that it has been fully characterized in both X- and C-band and in full-polar, and demonstrated to reach similar imaging capabilities as larger carriers despite a 6kg payload, we focus on more operational use cases. We use flight trajectories analogous to those of larger carriers, but the ability of the drone to fly close to the scenery suggests more drone-specific applications. In this paper we display recent results for target characterization and scenery inspection, which demonstrate the high repetability of image projection from one flight path to another.

ONERA, the French aerospace lab, has been developping over the past two years SAR-Light, a drone-embedded SAR tool. Our aim is to achieve equivalent performances to those of SETHI, ONERA’s plane embedded SAR tool [1], while enabling more flexible acquisitions. SAR-Light is now able to reach an 0.08 meter resolution (in X-band) without the need for autofocus, and has proven capable of performing advanced SAR modes, both in X- and C-band, and in full-polar. Here we display milestones of the development of SAR-Light, along with a few of its recent achievements during flight test campaigns, and more operational use cases.

In this paper, we propose an extension to an existing permittivity inversion scheme by taking into account double bounce scattering from a rotated dihedral geometry. By considering the rotational aspect, we aim to broaden the applicability of the permittivity inversion scheme for rotated double bounce signatures present in Synthetic Aperture Radar (SAR) imagery. The validity is first demonstrated by invoking to the characteristics of the co-pol ratio through simulations. The inversion scheme is then tested on real airborne SAR data to further validate its effectiveness in real-world scenarios.

This paper presents a simplified scheme for complex permittivity inversion of a double bounce scattering using polarimetric measurements. A forward scattering model is employed to establish the relationship between the observed far field and the underlying permittivity profiles. An inversion scheme based on an approximation is then developed to separately obtain the complex permittivity values corresponding to the two surfaces that constitute a dihedral structure. Electromagnetic simulations demonstrate the effectiveness of the proposed method in accurately recovering the complex permittivity values. Using real airborne data provided by ONERA, the proposed scheme was then utilized to test the possibility of extracting the complex permittivity profiles from double bounce scattering signatures in PolSAR imagery. The obtained complex permittivity profiles exhibit a correlation with previous studies, indicating the reliability of the inversion scheme to study the inverse double bounce scattering problem.

Remote sensing data are usually collected by SAR instruments on-board airplanes or satellites and many applications have been developed in a wide range of scientific subjects. Developments in technologies permit radar sensors to be miniaturized, which now allow them to be embedded on-board smaller platforms such as UAVs. ONERA is currently developing a new line of SAR instruments on-board UAV, called SAR-Light. In its first version, it is a mono-polarized (VV) SAR operating in X-band at high spatial resolution (0.25 m). This paper presents the SAR-Light instrument and describes the performance obtained during a flight tests campaign carried out in April 2021 in the South of France.

We extend a technique to retrieve effective permittivities developed for real aperture radar to be used in a Synthetic Aperture Radar (SAR). It can remotely retrieve complex permittivities using polarimetric radar signals from a double bounce. We include the technical hurdles in the extension and then present the first results of a test using airborne X-band data obtained by ONERA.

This work addresses the potential of multi-band polarimetric SAR imaging for terrains and vegetation classification. A classic supervised Wishart classifier is adapted to polarimetric multi-band datasets, and is applied on the X-, L- and UHF-band acquisitions done during the NAOMI campaign (ONERA-Total) in Gabon (Africa) in 2015. The contributions of the different frequencies are shown and discussed. It is shown that the use of the multi-band dataset improves significantly the classification result.

AfriSAR is an ESA-funded airborne P-band SAR campaign over the African tropical forests of Gabon that is being carried out by ONERA (July 2015) and DLR (February 2016) in support of the development of the geophysical algorithms of the future BIOMASS mission. Multibaseline fully-polarimetric acquisitions have been designed over four test sites in order to further develop and validate algorithms for the estimation of forest upper canopy height, 3-D structure and biomass by implementing PolSAR (SAR Polarimetry), Pol-InSAR (Polarimetric SAR Interferometry) and TomoSAR (SAR Tomography) P-band measurements. In this paper first data processing results will be shown.

This paper presents a new acquisition capability of the ONERA RAMSES-NG SAR sensor consisting in circular imaging, and an associated experiment. Target recognition or area monitoring is tricky using classical SAR imaging because only a part of the object shape is visible. The backscattering is strongly dependant of the relative heading angle between the radar line of sight and the target. Then, circular imaging mode is very efficient to overcome this problem since 360° data are acquired in a single pass.

In 2012, a large airborne campaign dedicated to radar backscattering measurements over natural surfaces has been undertaken by ONERA. This campaign, KaSAR, was funded by ESA in the frame of the Living Planet Program. Different surface types have been addressed, such as water surfaces (lakes and sea), continental surfaces (agricultural areas, forests, bare surfaces) and coastal zones. The DRIVE Ka band SAR payload was installed on board the ONERA BUSARD platform.. A large Ka band radar dataset has been processed, leading to backscattering measurements that have been synthesized into a sigma0 data base. Results from this campaign are now available to compare with published values and to validate simulations.

The potential of polarimetric synthetic aperture radar data for the soil surface characterization of bare agricultural soils was investigated by using air- and spaceborne data acquired by Radar Aéroporté Multi-Spectral d'Etude des Signatures (RAMSES), Système Expérimental de Télédétection Hyperfréquence Imageur (SETHI), and RADARSAT-2 sensors over several study sites in France. Fully polarimetric data at ultrahigh frequency, X-, C-, L-, and P-bands were compared. The results show that the main polarimetric parameters studied (entropy, α angle, and anisotropy) are not very sensitive to the variation of the soil surface parameters. Low correlations are observed between the polarimetric and soil parameters (moisture content and surface roughness). Thus, the polarimetric parameters are not very relevant to the characterization of the soil surface over bare agricultural areas.

To support studies on urban SAR imaging and Non Line Of Sight (NLOS) phenomenon mitigation at Ka band, a measurement campaign has been performed, involving the ONERA BUSARD motor glider equipped with the DRIVE radar. One validation flight and six measurements flights have been done, leading to the acquisition of a large radar dataset. First results highlight some NLOS radar echoes and measured distances from wall direct echo to NLOS observations correspond to theoretical evaluations.

The ESA funded campaign BioSAR 2010 was carried out at the forestry test site Remningstorp in southern Sweden, in support to the BIOMASS satellite mission under study. Fully polarimetric SAR data were successfully acquired at Land P-band using ONERA's multi-frequency system SETHI. In addition with other data types gathered, e.g. LiDAR and in-situ measurements, the compiled data set will be used for analyses and comparisons with biomass estimation results obtained at the same test site in the campaign BioSAR 2007, in which DLR's E-SAR made the SAR imaging. Detection of forest changes, robustness of biomass retrieval algorithms and long-term P-band coherence will be in focus as well as cross-validations between the two SAR sensors.

We analyze the application of Shannon entropy and Bhattacharyya distance for the characterization of polarimetric interferometric synthetic aperture radar (PolInSAR) images. We discuss the relevance of the decomposition of the Shannon entropy of a homogeneous region into the sum of three physically meaningful terms that depend respectively on intensity, polarimetry, and interferometry. We also discuss an analogous decomposition property for the Bhattacharyya distance, which is an efficient measure of the contrast between two regions. We illustrate the application of this approach for analyzing different contributions in the information content of PolInSAR images.

This paper provides an overview of the new airborne SAR system developed by the French Aerospace Lab ONERA over the last two years. This system, called SETHI, was developed according to the standard FAR25 applied to civil application. The main improvement compared to the previous ONERA airborne radar system RAMSES is that the antennas are located in two pods compatible with small aircrafts like the Falcon 20. This pod-based configuration allows the easy integration of any kind of payloads under the single certification of the pods by authorities. In the first section a quick introduction presents the aircraft configuration (cabin and pods) with an overview of SETHI development. The second section describes the SETHI concept with the installation on board the falcon 20 cabin, the hardware development to control and command all ONERA racks, the pods with their control and command systems. The last section illustrates the validation campaign which took place in September 2007 at Nîmes Garons Airport in France.

The ONERA RAMSES system is a flexible SAR system in constant evolution, developed mainly as a test bench for new technologies and to provide specific data for TDRI (Target Detection, Recognition and Identification) algorithm evaluation. It is flown on a Transall C160 platform operated by the CEV (Centre d'Essais en Vol). This paper presents the multi-pass interferometric acquisitions in very high resolution acquired over Salon de Provence, in south of France. The interferometric and coherence information capabilities are analyzed for this resolution.

We propose a new approach for evaluating the contribution of the different channels of polarimetric and interferometric synthetic aperture radar (PolInSAR) images. For that purpose, we demonstrate that the Bhattacharyya distance between the probability density functions of neighboring regions in the image provides an efficient scalar contrast measure. We show that the analysis of this contrast measure allows one to precisely characterize the contribution of each channel for different system configurations, including intensity, polarimetric, and interferometric images. We illustrate this approach using a real synthetic aperture radar image to compare several polarimetric system architectures. Since PolInSAR imaging configurations can correspond to complex and expensive systems, the proposed method can be helpful in system imaging optimization.

The ONERA RAMSES system is a flexible SAR system in constant evolution, developed mainly as a test bench for new technologies and to provide specific data for TDRI (Target Detection, Recognition and Identification) algorithm evaluation. It is flown on a Transall C160 platform operated by the CEV (Centre d'Essais en Vol). This paper gives an overview of the latest upgrading to acquire large data set without on board deramp-on-receive mode and then present the acquisition campaign at X band for very high resolution single pass interferometric multi-baseline mode. The multi- baseline mode used is a ping-pong mode where four antennas are used. The goal of this campaign was to acquire interferometric data in very high resolution mode, better than 0.3 m along slant range axis with a large slant range swath of 2400 m.

In this paper, we show that polarimetric and interferometric SAR (PolInSAR) images can be efficiently partitioned into homogeneous regions with a statistical technique based on minimization of a parameter-free criterion. This technique consists of finding a polygonal partition of the image that minimizes the stochastic complexity, assuming that the image is made of a tessellation of statistically homogeneous regions. The obtained results demonstrate that a global partition in statistically homogeneous regions of PolInSAR images can provide better results than a partition based on a single characteristic such as polarimetry or interferometry only.

This paper investigates the polarimetric and polarimetric interferometric synthetic aperture radar (PolInSAR) information contained in the high-resolution X-band data acquired by the RAMSES airborne SAR system over an area around Avignon, France containing bare surfaces, vegetation, and urban areas. The interferometric coherences are computed over natural and urban areas for all possible baseline copolar polarizations. In the complex plane, the obtained regions of coherence corresponding to most vegetation areas display small angular extents, meaning that if penetration occurs in the foliage, it is shallower than the system height accuracy. To quantify the PolInSAR information, an analysis of the interferometric height accuracy is first performed, and the results are compared with those associated with a theoretical and an empirical model. Concerning vegetation, a 6-m height difference is measured between the different polarimetric phase centers over a sparse pine forest, probably due to the presence of holes in the canopy. Crop study reveals also that wheat-type fields present oriented media properties at X-band due to their vertical structure. Over urban areas, in most cases, building height can be accurately obtained by using Pauli polarimetric phase center information.

The ONERA RAMSES system is a flexible SAR system in constant evolution developed mainly as a test bench for new technologies and to provide specific data for target detection, recognition and identification (TDRI) algorithm evaluation. It is flown on a Transall C160, a twin engine transport aircraft, operated by the French test-flight centre (CEV). This paper gives an overview of the system, lists the latest upgrading (electronic and SAR processor) and presents recent acquisition campaign over an urban area in very high (around 10 cm) resolution mode and the first analysis of these SAR images. I. SYSTEM OVERVIEW The ONERA RAMSES system is an experimental synthetic aperture radar system designed for signal acquisition and recording under a wide spectrum of modes and frequencies. Signal processing and image synthesis is done off-line allowing for a large flexibility in the waveforms and operating modes (no processing hardware constraints). At present, available frequency bands range from 400 MHz (P-band) up to 95 GHz (W-band). Full polarimetric (switched transmit) modes are available in P, L, C and X bands. High resolution (10cm) in X, Ka and soon Ku bands. Along-track interferometry have been demonstrated in P- band (2 channels), L-band (3 channels), X-band (4 channels). Across-track (with passive or alternatively transmitting channels) is available in X and Ka bands. Simultaneous acquisitions (such as dual frequency bands) are possible (up to 4 to 8 channel × 2 alternating recording, depending on data rates) Signal can be recorded as ramp demodulated, frequency shifted and even direct digitising in the lower bands This impressive choice of operation mode is the consequence of the modular design of the system, in which high frequency front-ends are made for each frequency band, and a common signal generator, intermediate frequency and digitising/recording back-end is used. Antennae are stacked and positioned at an open side door of the aircraft after take-off, and pulled back before landing. Trajectory acquisition involves an inertial navigation unit (INU) used for quicklook synthesis and system guidance and a differential GPS hybridized inertial measurement unit (IMU)used for off-line post-processing of the final trajectory for image synthesis. Along with the GPS ground reference station, the ground team also deploys calibrating targets (corner reflectors, dihedrals, transponders or Luenberg lenses).

International audience

The resolution degradation impact on the polarimetric information has not been extensively studied as the other system parameters. This work proposes a preliminary study of the resolution decrease effects on bare surfaces and complex media, as cities. A pure surface component is filtered on bare surfaces and Scattering mechanism ranging from surface scattering to double bounce scattering are removed when resolution is degraded.

The ONERA RAMSES system (Radar Aeroporte Multi-spectral d'Etude des Signatures) is a flexible SAR system in constant evolution developed mainly as a test bench for new technologies and to provide specific data for TDRI (Target Detection, Recognition and Identification) algorithm evaluation. It is flown on a Transall C160 platform operated by the CEV (Centre d'Essais en Vol). Recently, the system was upgraded to include a very high resolution mode in X, Ku and Ka bands. The resolution cell is then only a few wavelength wide and the usual hypothesis underlying the speckle phenomena, i.e., a large number of scatterers per resolution cell, is not verified. This work presents the analysis made on the clutter statistical behavior and contrasts the change of behavior observed when shifting from medium resolution to high resolution to very high resolution. It is based on actual radar data acquired with the RAMSES radar system. A new parameter identifying the largest spatial resolution for which the natural surface can no longer be considered homogeneous (in a speckle based criteria) is proposed. This parameter characterizes the scale of the contributing scatterers.

The ONERA RAMSES system (Radar Aeroporte Multi-spectral d'Etude des Signatures) is a flexible SAR system in constant evolution developed mainly as a test bench for new technologies and to provide specific data for TDRI (Target Detection, Recognition and Identification) algorithm evaluation. It is flown on a Transall C160 platform operated by the CEV (Centre d'Essais en Vol). Recently a POLINSAR (polarimetric interferometric) mode has been added where the system is able to acquire simultaneous interferometric polarimetric data in X band. This paper relates the preliminary analysis of data acquired over an agricultural test site around Avignon in Southern France. The polarimetry associated with interferometry provides the necessary elements for understanding the different scattering mechanisms. Several areas display a significant difference in polarimetric elevation as a function of polarimetry. This is due to the presence inside the resolution cell of multiple responses associated with different scattering elements.

The ONERA RAMSES system (Radar Aeroporte Multi-spectral d'Etude des Signatures) is a flexible SAR system in constant evolution developed mainly as a test bench for new technologies and to provide specific data for TDRI (Target Detection, Recognition and Identification) algorithm evaluation. It is flown on a Transall C160 platform operated by the CEV (Centre d'Essais en Vol). This paper gives an overview of the system and its recent upgradings.

This paper deals with the supervised classification of synthetic aperture radar (SAR) images. Our approach is based on two criteria, which explicitly take into account the intensity of the SAR image and the neighborhood classes, similarly to the Pots model, but weighted by a discontinuity map. The high level of noise involves numerous classification errors. We classify a restored image filtered with a well-adapted algorithm to clustering. Moreover, we isolate the texture of SAR images in order to help the classification. Finally, we present results on real SAR images.

Coherent imagery emerges as one of the major domains in image processing and includes topics as diversified as radar, medical and surface analysis. Whatever the application, the resulting image is noisy corrupted. In coherent imaging, images suffer from speckle noise, whose main characteristic is to be multiplicative. The proposed method takes explicitly into account the multiplicative property of the noise while preserving discontinuities in the restored image. Moreover, in a second step our algorithm estimates the noise, thus the information contained in the speckle still remains usable.