Multipath in automotive MIMO SAR imaging Manzoni, Marco; Tebaldini, Stefano; Monti-Guarnieri, Andrea Virgilio ...
IEEE transactions on geoscience and remote sensing,
01/2023, Volume:
61
Journal Article
Peer reviewed
This paper discusses the effect of multipath in automotive radar imaging under different sensor configurations. The study is motivated by the fact that radar technologies are becoming indispensable ...in the automotive scenario. Many applications such as collision avoidance systems, assisted parking, and driving assistance systems take advantage of radar technologies to accomplish their task. However, one of the main concerns about automotive radars is the possibility of detecting false targets due to multiple signal reflections. In this paper, we show how different sensor layouts experience multipath differently. In particular, we demonstrate that with Multiple-Input Multiple-Output (MIMO) radars, what really matters is the physical positions of the transmitting and receiving antennas. The monostatic/bistatic equivalent configurations cannot be used to design a system and to simulate an acquisition in the presence of multipath. We also demonstrate how vehicle-based MIMO-Synthetic Aperture Radar (MIMO-SAR) imaging can generate a bi-dimensional aperture which significantly reduces multipath effects in the focused image, avoiding the detection of false targets. All the theoretical analyses are supported by several simulations where different sensor layouts are tested, and the capability of MIMO-SAR to reject multipath is validated.
Mitigating decorrelation effects on interferometric synthetic aperture radar (InSAR) time series data is challenging. The phase linking (PL) algorithm has been the key to handling signal ...decorrelations in the past 15 years. Numerous studies have been carried out to enhance its precision and computational efficiency. Different PL algorithms have been proposed, each with unique phase optimization approaches, such as the quasi-Newton method, equal-weighted and coherence-weighted factors, component extraction and selection SAR (CAESAR), and eigendecomposition-based algorithm (EMI). The differences among the PL algorithms can be attributed to the weight criteria adopted in each algorithm, which can be coherence-based, sparsity-based, or other forms of regularization. The PL algorithm has multiple applications, including SAR tomography (TomoSAR), enhancing distributed scatterers (DSs) to combine with persistent scatterers (PS) in PS and DS (PSDS) techniques, and compressed PSDS InSAR (ComSAR), where it facilitates the retrieval of the optimal phase from all possible measurements. This article aims to review PL techniques developed in the past 15 years. The review also underscores the importance of the PL technique in various SAR applications (TomoSAR, PSDS, and ComSAR). Finally, the deep learning (DL) approach is discussed as a valuable tool to improve the accuracy and efficiency of the PL process.
This paper illustrates the potential of correlation SAR tomography for monitoring forested areas. Correlation SAR tomography requires several pairs of interferometric images rather than a full stack ...as for standard SAR tomography. As a consequence, constrains for the survey are relaxed and cheaper space-borne system can be designed. A promising configuration is represented by a space-borne SAR platform supported by a receiver-only satellite providing the second image. Experiments were carried out by means of two airborne SAR sensors flying in close formation in the framework of the new ESA TomoSense campaign. Data were acquired over the Kermeter forest (Germany) and pre-processed by MetaSensing. TomoSense campaign features multi frequency, multi polarimetric, opposite views tomographic SAR acquisitions. Ad-hoc calibration and tomographic algorithms were successfully developed and applied to this dataset. They are here presented together with some estimates of physical parameters of the forest under observation resulting from L-band data. Estimates are compared to external LiDAR measurements showing very good agreement.
The primary objective of the European Space Agency's 7th Earth Explorer mission, BIOMASS, is to determine the worldwide distribution of forest above-ground biomass (AGB) in order to reduce the major ...uncertainties in calculations of carbon stocks and fluxes associated with the terrestrial biosphere, including carbon fluxes associated with Land Use Change, forest degradation and forest regrowth. To meet this objective it will carry, for the first time in space, a fully polarimetric P-band synthetic aperture radar (SAR). Three main products will be provided: global maps of both AGB and forest height, with a spatial resolution of 200 m, and maps of severe forest disturbance at 50 m resolution (where “global” is to be understood as subject to Space Object tracking radar restrictions). After launch in 2022, there will be a 3-month commissioning phase, followed by a 14-month phase during which there will be global coverage by SAR tomography. In the succeeding interferometric phase, global polarimetric interferometry Pol-InSAR coverage will be achieved every 7 months up to the end of the 5-year mission. Both Pol-InSAR and TomoSAR will be used to eliminate scattering from the ground (both direct and double bounce backscatter) in forests. In dense tropical forests AGB can then be estimated from the remaining volume scattering using non-linear inversion of a backscattering model. Airborne campaigns in the tropics also indicate that AGB is highly correlated with the backscatter from around 30 m above the ground, as measured by tomography. In contrast, double bounce scattering appears to carry important information about the AGB of boreal forests, so ground cancellation may not be appropriate and the best approach for such forests remains to be finalized. Several methods to exploit these new data in carbon cycle calculations have already been demonstrated. In addition, major mutual gains will be made by combining BIOMASS data with data from other missions that will measure forest biomass, structure, height and change, including the NASA Global Ecosystem Dynamics Investigation lidar deployed on the International Space Station after its launch in December 2018, and the NASA-ISRO NISAR L- and S-band SAR, due for launch in 2022. More generally, space-based measurements of biomass are a core component of a carbon cycle observation and modelling strategy developed by the Group on Earth Observations. Secondary objectives of the mission include imaging of sub-surface geological structures in arid environments, generation of a true Digital Terrain Model without biases caused by forest cover, and measurement of glacier and icesheet velocities. In addition, the operations needed for ionospheric correction of the data will allow very sensitive estimates of ionospheric Total Electron Content and its changes along the dawn-dusk orbit of the mission.
•BIOMASS will be the first spaceborne P-band mission.•Global estimates of forest biomass and height, subject to US DoD restrictions•The first systematic use of Pol-InSAR to measure forest height from space•The first systematic use of spaceborne SAR tomography•Sub-surface imaging, icesheet motion estimation and a bias-free DTM
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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 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 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.
Coherent multistatic radio imaging represents a pivotal opportunity for forthcoming wireless networks, which involves distributed nodes cooperating to achieve accurate sensing resolution and ...robustness. This paper delves into cooperative coherent imaging for vehicular radar networks. Herein, multiple radar-equipped vehicles cooperate to improve collective sensing capabilities and address the fundamental issue of distinguishing weak targets in close proximity to strong ones, a critical challenge for vulnerable road users' protection. We prove the significant benefits of cooperative coherent imaging in the considered automotive scenario in terms of both probability of correct detection, evaluated considering several system parameters, as well as resolution capabilities, showcased by a dedicated experimental campaign wherein the collaboration between two vehicles enables the detection of the legs of a pedestrian close to a parked car. Moreover, as coherent processing of several sensors' data requires very tight accuracy on clock synchronization and sensor's positioning-referred to as phase synchronization -(such that to predict sensor-target distances up to a fraction of the carrier wavelength), we present a general three-step cooperative multistatic phase synchronization procedure, detailing the required information exchange among vehicles in the specific automotive radar context and assessing its feasibility and performance by hybrid Cramér-Rao bound.
P-band synthetic aperture radar (SAR) is sensitive to above-ground biomass (AGB) but retrieval accuracy has been shown to deteriorate in topographic areas. In boreal forest, the signal penetrates ...through the canopy to interact with the ground producing variations in backscatter depending on ground topography, forest structure, and soil moisture. Tomographic processing of multiple SAR images Tomographic SAR (TomoSAR) provides information about the vertical backscatter distribution. This article evaluates the use of P-band TomoSAR data to improve AGB retrievals from backscattered intensity by suppressing the backscattered signal from the ground. This approach can be used even when the tomographic resolution is insufficient to resolve the vertical backscatter profile. The analysis is based on P-band data from two campaigns: BioSAR-1 (2007) in Remingstorp, southern Sweden, and BioSAR-2 (2008) in Krycklan (KR), northern Sweden. BioSAR airborne data were also processed to correspond as closely as possible to future BIOMASS TomoSAR acquisitions, with BioSAR-2-based results shown. A power law AGB model using volumetric HV polarized backscatter performs best in KR, with training residual root mean-squared error (RMSE) of 30%-36% (27-33 t/ha) for airborne data and 38%-39% for simulated BIOMASS data. Airborne TomoSAR data suggest that both vertical and horizontal tomographic resolution are of importance and that it is possible to greatly reduce AGB retrieval bias when compared with airborne P-band SAR backscatter intensity-based retrievals. A lack of significant ground slopes in Remningstorp reduces the benefit of using TomoSAR data which performs similar to retrievals based solely on P-band SAR backscatter intensity.
This paper introduces the CASINO (CAnopy backscatter estimation, Subsampling, and Inhibited Nonlinear Optimisation) algorithm for above-ground biomass (AGB) estimation in tropical forests using ...P-band (435 MHz) synthetic aperture radar (SAR) data. The algorithm has been implemented in a prototype processor for European Space Agency's (ESA's) 7th Earth Explorer Mission BIOMASS, scheduled for launch in 2023. CASINO employs an interferometric ground cancellation technique to estimate canopy backscatter (CB) intensity. A power law model (PLM) is then used to model the dependence of CB on AGB for a large number of systematically distributed SAR data samples and a small number of calibration areas with a known AGB. The PLM parameters and AGB for the samples are estimated simultaneously within pre-defined intervals using nonlinear minimisation of a cost function. The performance of CASINO is assessed over six tropical forest sites on two continents: two in French Guiana, South America and four in Gabon, Africa, using SAR data acquired during airborne ESA campaigns and processed to simulate BIOMASS acquisitions. Multiple tests with only two randomly selected calibration areas with AGB > 100 t/ha are conducted to assess AGB estimation performance given limited reference data. At 2.25 ha scale and using a single flight heading, the root-mean-square difference (RMSD) is ≤ 27% for at least 50% of all tests in each test site and using as reference AGB maps derived from airborne laser scanning data. An improvement is observed when two flight headings are used in combination. The most consistent AGB estimation (lowest RMSD variation across different calibration sets) is observed for test sites with a large AGB interval and average AGB around 200–250 t/ha. The most challenging conditions are in areas with AGB < 200 t/ha and large topographic variations. A comparison with 142 1 ha plots distributed across all six test sites and with AGB estimated from in situ measurements gives an RMSD of 20% (66 t/ha).
•Algorithm for above-ground biomass (AGB) estimation with limited reference data.•Ground cancellation removes topographic effects and soil moisture variations.•Power law model explains dependence of P-band canopy backscatter on biomass.•New fitting approach exploits spatial and polarisation trends in SAR data.•Estimation error at 1 ha scale for six test sites in Gabon and French Guiana: 20%.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
This work deals with 3D structure characterization and permittivity profile retrieval of snowpacks by tomographic SAR data processing. The acquisition system is a very high resolution ground based ...SAR system, developed and operated by the SAPHIR team, of IETR, University of Rennes-1 (France). It consists mainly of a vector network analyser and a multi-static antenna system, moving along two orthogonal directions, so as to obtain a two-dimensional synthetic array. Data were acquired during the AlpSAR campaign carried by the European Space Agency and led by ENVEO. In this study, tomographic imaging is performed using Time Domain Back Projection and consists in coherently combining the different recorded backscatter contributions. The assumption of free-space propagation during the focusing process is discussed and illustrated by focusing experimental data. An iterative method for estimating true refractive indices of the snow layers is presented. The antenna pattern is also compensated for. The obtained tomograms after refractive index correction are compared to the stratigraphy of the observed snowpack.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
This paper presents new experimental results of 3-D imaging using tomographic techniques over a snow covered sea ice medium, sensed with an X-band radar system. The available data are from a ...ground-based synthetic aperture radar data collection campaign carried out over Kattfjord, Tromsø, Norway. Direct imaging of the vertical structures of the radar reflectivity of the snow and sea ice layers is achieved by focusing the signal from a 2-D synthetic array in the 3-D space. The effect of a change in propagation velocity of the wave inside the considered medium is investigated in the focusing process, and the tomograms are effectively corrected for this effect. The distribution of the scattering contributions in the vertical direction reveals a strong response from the sea ice cover. Tomograms at two different polarizations are investigated and compared. The results and the interpretations are also supported by the simulated data from the same system.
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