Vital information wireless sensing is an interesting alternative to conventional techniques based on contact sensors. The evolution of radar technologies enables the proposal of cost-effective and compact radar sensors designed for diverse health monitoring, diagnostics, and Internet of Medical Things (IoMT) applications. It is possible to observe respiratory rate (RR), heart rate (HR), cardiac rhythm, blood pressure waveform (BPW), and emerging biometrics from the phase of radar echo signals. However, recognition and separation of the signals of interest from other interfering motion is a challenge, taking into account the diversity of signal patterns depending on subjects, health and observation conditions. In this context, we propose a novel wireless vital sensing system with a self-designed 120 GHz Frequency-Modulated Continuous Wave (FMCW) Radar. The developed sensor, based on a Radar System-on-Chip (RSoC), uses a real-time Repetitive Waveform Adaptive Matched Filter (RWAMF) maximizing Signal-to-Interference-plus-Noise Ratio (SINR) for reliable cardiac rhythm and BPW observations. The radar design has been optimized for vital sensing with a very short wavelength providing excellent sensitivity to micro-motion and a centimetric range and transversal resolutions to reject clutter and unwanted motions. The superior performance of our solution has been validated and evaluated experimentally on different subjects and measurement conditions with our own acquired vital signal database.

Fully focused (FF) synthetic aperture radar (SAR) back-projection algorithms for radar altimetry have gained attention in the altimetry community due to their improved azimuth resolution compared to traditional methods, such as the delay/Doppler algorithm. However, the significant computational cost of back-projection has limited its use in operational applications. This article presents an accelerated back-projection algorithm that reduces the runtime of the classic back-projection by a factor of 28 on a CPU-based architecture. When implemented on a GPU-based architecture, the accelerated back-projection achieves a speedup ranging from 8 to 13 times compared to the classic back-projection on the same GPU. In comparison to the classic back-projection running on a CPU, the combination of the accelerated back-projection and GPU processing results in an overall improvement of up to 1570 times. The accelerated algorithm maintains the accuracy of classic time-domain algorithms and allows for the processing of data in the order of the sensing time under a GPU-based architecture. The algorithm has been extensively validated using transponder and open ocean data. These results indicate that operational back-projection algorithms are feasible for current radar altimetry missions, such as Sentinel-6, and upcoming missions like CRISTAL and Sentinel-3 Next Generation.

Data obtained during a ground-based SAR experiment and an associated field campaign have been exploited to study the rate and sources of decorrelation in an agricultural test site in the conditions of observation of a geosynchronous SAR. It was found that the scene is less affected by temporal decorrelation when the primary image is acquired during night time or early morning. Additionally, a periodic oscillation on a sub-daily scale was observed when creating coherence time series with increasing temporal baseline. Two factors which strongly contribute to these oscillations are the daily cycles of soil moisture and evapotranspiration.

The two-dimensional frequency-based omega-K method is known to be a suitable algorithm for Fully-Focused SAR (FF-SAR) radar altimeter processors, as its computational efficiency is much higher than equivalent time-based alternatives without much performance degradation. In this paper we provide a closed-form description of a two-dimensional frequency domain omega-K algorithm specific for instruments such as Poseidon-4 onboard Sentinel-6. The processor is validated with real data from point targets and over open ocean. Applications such as ocean swell retrieval and lead detection are demonstrated, showing the potentiality of the processor for future operational global-scale products.

Low Earth Orbit (LEO) remote sensing missions present a main limitation regarding their revisit time of several days or weeks. They cannot provide continuous monitoring over the same area of the planet. The on-going studies on Geosynchronous Synthetic Aperture Radar (GEOSAR), which have a revisit time of less than 24 h, aim to mitigate this limitation [1] . Since high altitude GEO missions are beyond MEO navigation satellites coverage, a ground based interferometric system (GEODE) has been developed for GEO Precise Orbit Determination, a requirement to form focused GEOSAR synthetic apertures [2] . Since no operational GEOSAR missions are available yet, present Geostationary telecommunications satellites can be used as transmitters of opportunity for GEODE experimental testing and validation. Recent results show the convenience of using ground reflectors to extend interferometric baselines lengths up to kilometers to achieve the required GEOSAR orbit precision. Locating stable reflectors of opportunity for interferometric orbit determination in urban areas is possible by complementing the ground interferometer with a multistatic SAR imaging processor capable to map the surrounding scattering centers.

HydroSoil is a measurement campaign, funded by the European Space Agency (ESA), where the temporal evolutions of two crops, barley and corn, have been continuously monitored during the whole crop period by means of a C-band Fully Polarimetric C-band Ground-Based Synthetic Aperture Radar (GB-SAR). SAR data has been collected together with ancillary data, and both are being processed to demonstrate the retrieval of soil moisture and vegetation parameters in an agricultural field under controlled conditions, to simulate the frequent acquisitions of GeoSAR missions.

The advent of geosynchronous remote sensing missions requires the development of precise orbit determination techniques at high orbits. Geosynchronous satellites require station-keeping manoeuvres to be performed periodically. Dynamical models do not consider artificial forces. Therefore, the tracking system must be able to detect them in order to keep track of the spacecraft autonomously. A compact baseline interferometer is deployed in order to track the signals of opportunity from the ASTRA 19.2°E geostationary constellation. The system is capable of autonomously estimate the trajectory of the satellite for a period of 22 days. Even after artificial orbital manoeuvres are performed. These results show that interferometry is a valid alternative for future geosynchronous remote sensing missions which require precise orbit determination.

In the framework of a feasibility study in support of the Hydroterra mission, a high resolution radar has been setup to monitor continuously a crop field with high resolution synthetic aperture radar images. In parallel, ground-truth data is acquired such as soil roughness, soil moisture and crop biological parameters. From this experimental data set the possibility to monitor soil moisture and crop biological parameters from back-scattering radar measurements will be assessed. To use the phase information of SAR images it is interesting to compensate the atmospheric phase screen induced by tropospheric refractive index. The paper presents a study of the atmosphetric phase drift prediction and compensation based on the local measurement of meteorological parameters.

The orbit accuracy requirements for a Geosynchronous Synthetic Aperture Radar (GEOSAR) are studied. Compared to telecommunication GEO satellites' station keeping needs, focusing the GEOSAR synthetic aperture demands a more accurate orbital determination. For this reason, the most suitable Precise Orbit Observation (POB) techniques are considered.Several radar tracking techniques fulfil the accuracy requirements while presenting a trade-off problem in terms of cost an operation. GNSS allow for precise autonomous navigation even at high orbits like GEO. Optical observation techniques perform well in order to calibrate other tracking systems.Interferometry stands out since it can be used taking advantage of present illuminators of opportunity. A ground interferometer is being designed and built in order to provide experimental measurements which will serve as proof of concept.

Technological developments in low-power mmW radar allow the measurement of deformations in the order of few micrometers and monitoring dynamic processes like structural vibration. This paper presents a fast acquisition FMCW radar operating at 94 GHz and recent experimental results of remote observation of micrometric vibration motion for bridge structural health monitoring. The presented approach offers cost reduction and applicability advantages compared to conventional techniques.

Orbit determination errors are expected to be a major threat for obtaining focused images on Geostationary Synthetic Aperture Radar (GeoSAR) missions. A ground-based interferometer system is presently being under research to determine the satellite orbit with the required precision. However, from that system, it must be addressed which orbit determination techniques offer better results for that purpose. This paper provides a comparison between two differential correction techniques: the least squares (LS) and the extended Kalman filter (EKF) techniques.

This paper proposes a method to solve the absolute phase ambiguity introduced by an orbit determination interferometer system, suitable for GeoSAR missions. This method will be complemented with simulated results in order to validate its reliability.

The paper analyzes the problem of achieving a coherent operation in Geostationary SAR (GEOSAR) missions intended for continuous monitoring of land surfaces. In contrast to LEOSAR missions, GEOSAR uses very long SAR integration times (from minutes to hours). Accordingly phase errors due to orbit perturbations, radar master oscillator drift, atmosphere propagation and other sources must be conveniently compensated during SAR processing to avoid image defocusing. A network of Active Radar Calibrators (ARC) distributed on the observed scene is proposed, providing echo envelope and phase observations before Synthetic Aperture Processing. In this way the radar antenna phase center trajectory and other phase error sources can be continuously tracked and compensated using a pyramidal sub-aperture processing approach.

This paper presents a detailed characterization of sea/ocean clutter returns at X-band, imaged by TerraSAR-X (TSX) mission from the radiometric, statistical, and polarimetric standpoints. Different TSX data takes, covering the typical spaceborne incidence angle region (20°-45°), are analyzed: dual-polarized (dual-pol) and experimental quad-polarized (quad-pol) data have been used. The thermal noise of the receiver for quad-pol data turns out to be an important limitation in the sea characterization from TSX, particularly at high incidence angles (above 36°), where low signal-to-noise ratios (SNRs) can impair the proper data distribution fitting and the polarimetric backscattering description. Statistical analysis shows large deviation from Gaussianity, indicating presence of texture, mainly in the small incidence region (20°-36°), with favorable SNR conditions. The different polarimetric features revealed contribution of nonpolarized scattering, related to the presence of breaking waves. This paper also proposes and evaluates an extension of the well-known X-Bragg model, named X2-Bragg (extended-extended Bragg), which properly accounts for the impact of thermal noise and sea clutter temporal decorrelation, due to the dual receive antenna (DRA) acquisition mode. Such additional decorrelation sources, if not properly analyzed and accounted for in the physical-based model description, could lead to an incorrect interpretation of the polarimetric properties and the related erroneous geophysical parametric inversion from the real data. In this sense, the X2-Bragg proves its fitting to the experimental data, quantifying accordingly the presence of nonpolarized scattering in terms of the roughness parameter β1.

We discuss the effects of the clutter on geosynchronous SAR systems exploiting long integration times (from minutes to hours) to counteract for two-way propagation losses and increase azimuth resolution. Only stable targets will be correctly focused whereas unstable targets will spread their energy along azimuth direction. We derive here a generic model for the spreading of the clutter energy based on the power spectral density of the clutter itself. We then assume the Billingsley Intrinsic Clutter Motion model, representing the clutter power spectrum as an exponential decay, and derive the expected GEOSAR signal-to-clutter ratio. We also provide some results from a Ground Based RADAR experiment aimed at assessing the long-term clutter statistics for different scenarios to complement the Internal Clutter Motion model, mainly derived for windblown trees. Finally, we discuss the expected performances of two GEOSAR systems with different acquisition geometries.

The paper discusses the implementation of a quasi-geostationary MIMO Synthetic Aperture Radar swarm, also defined as ARGOS in [1]. Such system combines the all-time-all-weather image capability of a SAR, with the resolution enhance, robustness and scalability of a swarm and the continuous observation provide by the geostationary orbit. Two C-band concepts are analyzed in details that differ for the image time and azimuth resolution.

This paper presents the results of a RADAR experiment aimed to assess and eventually complement the Internal Clutter Motion model (ICM) provided by Billingsley, for medium and long integration time SAR imaging. Employing a real-aperture rotating antenna ground-based RADAR, rural areas have been observed in different periods of the year according to two different modes, properly designed to obtain short-term and long-term clutter de-correlation analysis. The results showed interesting aspects of the phenomenon. In particular a non-stationary behavior with time, say from minutes to hours, and a day/night characteristic. Furthermore, the experiment showed how, for a long integration time, the model parameters are different from the ones foreseen by Billingsley.

This paper shows the necessity to include the impact of instrument-driven constrains in the physical modeling of polarimetric data from spaceborne SAR missions, specifically for TerraSAR-X (TSX) case. The intrinsic acquisition of fully (quad-pol) polarimetric data, exploiting the dual-receive antenna (DRA) mode, may induce two additional sources of decorrelation when observing challenging dynamic scenarios, such as the ocean or sea: limited signal-to-noise ratio (SNR) as the receive antenna is halved and temporal decor-relation induced over the along-track configuration (spatial baseline between the polarimetic channels) due to internal clutter motion. Experimental quad-pol TSX data over the ocean has been used to study the applicability of the original X-Bragg model as well as its extension accounting for these system/scenario dependent limitations, which shows a much better fitting.

This letter analyzes the problems of imaging small and fast boats with synthetic aperture radar (SAR), which may experience severe defocusing and/or smearing. This situation is encountered, inter alia, in ocean traffic monitoring and surveillance applications where the detection of small and fast vessels is of great interest. The target modeling and impact of moving vessels in SAR images are presented and discussed using simulated results. A matched filter bank is used before applying moving target indication techniques over multichannel SAR images, refocusing the target signal in the azimuth domain and enhancing the detectability of these types of vessels.

The Advanced Radar Geosynchronous Observation System is proposed to be a multiple-input-multiple-output synthetic aperture radar (SAR) system hosted on a swarm of minisatellites in quasi-geostationary orbits. The system is made of N iso-frequency sensors, each of them transmitting and receiving the signals. The system would combine the continuous imaging capabilities of a geostationary SAR, gaining a factor N2in signalto-noise ratio (SNR). The real aperture would be achievable in ~40 min, enabling applications so far unseen, such as monitoring fast deformations, landslides, and other applications for emergency and security. Still, the SNR of the long acquisition time would be conserved. The optimal design of the swarm is addressed, in order to trade resolution, coverage, and revisit time.

This paper proposes a new optimized multichannel synthetic aperture radar (SAR) configuration, based on receiving antennas with nonuniformly displaced phase centers, intended for ground moving target indication (GMTI) applications over maritime scenarios. This system is compared with current SAR missions, such as TerraSAR-X (TSX) or TanDEM-X (TDX). The GMTI capabilities of the different configurations are analyzed in a two-level performance approach. First, an intensive numerical simulation evaluation, based on Monte Carlo trials, is carried out in order to characterize the probabilities of detection under different system parameters and scenario conditions. Different GMTI techniques, i.e., displaced phase center antenna, along-track interferometry, and extended displaced phase center antenna, are assessed. In a second step, synthetic simulated SAR data, obtained in a study case scenario, is used to demonstrate the potential improvement of the proposed multichannel configuration compared with current SAR missions, providing subclutter visibility for maritime surveillance of small and slowly moving boats.

The paper evaluates the effects of Atmospheric Phase Screen on focused Geosynchronous SAR data. We derive theoretical models for the resolution, the interferometric de-correlation and the induced clutter in presence of an APS, whose joint time and space power spectrum is modeled as an extension of the Kolmogorov power law. Finally we provide some statistics gathered from a GB RADAR experiment.

In this paper, the impact of the internal clutter motion in GEOSAR acquisitions is analyzed. A nearly zero inclination with low eccentricity geosynchronous orbit is considered and, therefore, long integration, in order of 20-30 min (for 1-2 Km resolution) or even hours (for tenths of meters resolution), is required in order to reach the desired synthetic aperture length and the required along-track resolution. Moderate transmitted powers and antenna sizes are considered in the analysis. The power link budget of a long integration GEOSAR acquisition is presented and the limitations imposed by the clutter decorrelation are studied. The minimum target RCS to assure the correct performance of the system under clutter interference scenarios is evaluated. Finally, experimental data from Ground Based SAR X-band acquisitions will be used to validate the Billingsley model in long integration acquisitions.

The paper presents a preliminary statistical evaluation of the sea clutter returns imaged by a spaceborne synthetic aperture radar (SAR) mission as TerraSAR-X. Two polarimetric data takes, Dual-polarized (Dual-Pol) and Quad-polarized (Quad-Pol), covering the range of incidence angles from 30 to 32 degrees, are considered for the stochastic characterization. The results show that the K-distribution, extendedly assumed statistics for high resolution radar sea clutter imaging, fits well the data's amplitude. This information jointly with the inverted sea spectrum are used to simulate SAR images of realistic scenarios that can be exploited for proper performance evaluation of ground moving target indication (GMTI) techniques and missions for maritime surveillance.

In this paper, the performance of a nearly zero inclination and low eccentricity geosynchronous synthetic aperture radar (GEOSAR) mission for midlatitude (30°-60°) Earth observation is analyzed. The slow motion of such satellites with respect to the Earth's surface makes it necessary to consider long coherent combination of pulses during hours to reach the desired along-track resolution. A system based on moderate transmitted powers and antenna sizes is considered. The necessary sensitivity in such GEOSAR system is obtained from the accumulated energy of the raw data using a pulse repetition frequency above the Doppler bandwidth and a long integration time. Several issues as a result of the long acquisition, such as target and atmospheric phase screen decorrelation, speckle noise impact on the received signal, and satellite station-keeping requirements, are analyzed. The feasibility of such systems to be placed on a broadcasting communication satellite makes nearly zero inclination GEOSAR a low-cost alternative of current SAR missions.

We propose an efficient method for the azimuth compression and Atmospheric Phase Screen estimation of Geosynchronous SAR data. The method is based on the iterative processing of sub-apertures of increasing size, allowing to gradually refine the quality of the focused data and of the estimated APS. The whole processing can be easily parallelized. Results over simulated data are shown.

This paper presents a comparative study of the radiometric sensitivity and spatial resolution of three near-field (NF) passive screener systems: real aperture, 1-D synthetic aperture (SA), and 2-D SA radiometers are compared. The analytical expressions for the radiometric resolution, the number of required antennas, and the number of pixels in the image are derived taking into account the distortion produced by the NF geometry at nonboresight directions where the distortion is dominant. Based on the theoretical results, a performance comparison among the studied systems is carried out to show the advantages and drawbacks when using the radiometers in a close-range screening application. Moreover, the screener performance in a close-range environment is discussed from the results obtained in the aforementioned comparison.

In this paper, a geosynchronous synthetic aperture radar (SAR) (GEOSAR) mission for atmospheric phase screen (APS) retrieval using a long coherent integration of pulses is analyzed. The nearly fixed position of the geosynchronous platforms makes GEOSAR systems suitable for continuous monitoring applications. However, using moderate transmitted powers and antenna sizes, very long integration times up to hours are required. In GEOSAR, the two-way propagation of radar signals can decorrelate significantly due to atmospheric changes during the long data take, resulting in an APS which can cause image defocusing and artifacts. In this paper, the APS effects are analyzed, and an APS correction algorithm from short-term periodic acquisitions (subapertures) of the whole long-term GEOSAR synthetic aperture is described. The results obtained from the APS retrieval algorithm in a simulated GEOSAR acquisition affected by atmospheric decorrelation are presented. Finally, an experimental test of the APS algorithm performance with a long-integration ground-based SAR acquisition is shown.

In this paper we discuss a preliminary design for a constellation of geosynchronous (GEO) Synthetic Aperture Radars (SAR). The key design concept is to employ one or more pairs of closely-spaced twin receivers flown onboard GEO minisatellites moving with a velocity of few meters per second with respect to the Earth's surface, so as to form a synthetic aperture on the order of few tenths of kilometers twice a day. The employement of closely-spaced receivers would enable the estimation of the temporal gradient of the tropospheric delay via along track Interferometry, resulting in the possibility to coherently integrate the signal over an aperture time on the order of hours. As a result an area as wide as one thousand kilometers could be imaged while providing: i) continuous temporal coverage at coarse resolution (hundreds of meters); ii) high resolution (few meters) imaging and interferometric capabilities two or more times a day by integrating the signal over few hours, thus ensuring high SNR performance (for stable targets) with a transmitted power comparable to currently operated spaceborne SARs.

This paper presents an evaluation of a post-Doppler space-time adaptive Processing (STAP) technique for non-uniformly displaced phase center receivers synthetic aperture radar (SAR). Both theoretical and simulation analysis are carried out in order to prove the ground moving target indication (GMTI) capabilities for specific non-uniformly spaced multi-channel configurations on-board a single satellite platform. Simulated SAR data in the maritime scenario show improved performance, specially for small and slow targets, compared to current SAR missions, equipped with just two channels.

This paper presents an initial analysis of the possibilities for velocity and acceleration measurement with the Bi-Directional SAR imaging mode (BiDi). It comprises single satellite single path acquisitions as well as a constellation of two satellites. The translational velocity components into azimuth and range directions are simulated. A BiDi approach for measuring the azimuth velocity from one satellite with one receiving channel is proposed. Image examples acquired with the TerraSAR-X (TSX) and TanDEM-X (TDX) satellites show velocity and acceleration effects on ships and the proposed BiDi velocity approach is verified. Interferometric fringes were observed on anchoring ships. A first approach into the understanding of rotational effects was achieved by simulation of rotational fringes.

In this paper, a high integration gain GEOSAR mission for Earth imaging is presented. The spatial-temporal atmospheric decorrelation issues of a long integration time GEOSAR mission must be carefully studied. The feasibility of a GEOSAR mission for Atmospheric Phase Screen retrieval purposes is studied. The raw data received from several hours of acquisition is divided into sub-acquisitions with integration time on the order of the APS decorrelation time. From each sub-matrix, a low-resolution but enough to sample the spatial variations of the atmospheric map is obtained. The simulated results of the APS retrieval from a GEOSAR acquisition are shown at the end of this paper.

In this letter, an optimized scanning in terrain observation by progressive scan synthetic aperture radar (TOPSAR) mode is studied. A nonuniform steering rate of the radar array antenna in the along-track direction is proposed in order to obtain constant radiometric sensitivity and signal-to-noise ratio. This is achieved owing to the longer integration time of the echoes received at both ends of the antenna azimuth sweep. By optimizing iteratively the array discrete steering rate law, the radiometric impact of the array basic element pattern (subarray pattern) can be accurately compensated. First, simulation results are presented to validate the nonuniform steering TOPSAR.

This paper proposes an analytical formulation of the instrumental errors in multilevel hierarchic SAR architecture equipped with active phased array antennas. The basis of the study is the derivation of the so called post-calibration errors, which remain as residual error contributions after the application of a dedicated internal calibration procedure. The limitations of the current internal calibration approaches for the-state-of-the-art space-borne SAR missions are analyzed and alternative internal global calibration strategies are proposed to reduce the impact of the post-calibration error. Numerical simulations of instrumental errors are presented to evaluate the different proposed calibration procedures.

In this paper, the feasibility of a geosynchronous satellite for Synthetic Aperture Radar applications is assessed. The nearly fixed position of geosynchronous satellites offers new features to SAR acquisition such as the continuous monitoring of wide areas with short revisit times. A monostatic dedicated mission, working with moderated antenna sizes and transmitted powers with long integration time (several hours), is firstly presented and the main SAR parameters (PRF selection, power budget, ambiguities) analyzed. The system Point Spread Function is obtained from simulated raw data showing the system spatial resolution in a realistic orbital configuration.

The paper presents the development of a ground based bistatic receiver using TerraSAR-X as a transmitter. The receiver subsystems like antennas, low-noise amplifiers, mixers, filters, synthesizers, etc. have been developed using low-cost monolithic devices in order to allow affordable deployment and at the same time offer final year students a challenging SAR engineering project. First raw data have been acquired on the Barcelona harbor area that has been focused producing geocoded images well matched with existing maps. A preliminary interferogram have been also produced.

Fractal geometry is widely accepted as an efficient theory for the characterization of natural surfaces; the opportunity of describing irregularity of natural surfaces in terms of few fractal parameters makes its use in direct and inverse electromagnetic (EM) scattering theories highly desirable. In this paper, we present an innovative procedure for manufacturing fractal surfaces and for measuring their scattering properties. A cardboard-aluminum fractal surface was built as a representation of a Weiestrass-Mandelbrot fractal process; the EM field scattered from it was measured in an anechoic chamber. A monostatic radarlike configuration was employed. Measurement results were compared to Kirchhoff approximation and small perturbation method closed-form results that were analytically obtained by employing the fractional Brownian motion to model the surface shape. Matching and discrepancies between theories and measurements are then discussed. Finally, fractal and classical surface models are compared as far as their use in the EM scattering is concerned.

We consider a geosynchronous Ku band system, and the defocusing effects due to the temporal and spatial variability of the atmospheric water vapor. Due to the very slow formation of the spatial chirp (minutes or even hours), the temporal change of the local water vapor content could be significant, thus preventing a correct image focusing and the formation of a coherent image. Using GPS and ground based radar data, we estimate and model the spatial temporal correlation function of the water vapor delay. Then, for a given spatial scale, we estimate the maximum time within which the temporal evolution of the water vapor should be carried out, to be able to track its temporal evolution. This requisite constrains both the EIRP and the apparent velocity of the satellite. Then, we evaluate the possibilities of real time ground motion analyses, as a function of the spatial and temporal resolution and the EIRP.

In this paper, the capability of ultra-wide-band (UWB) sensor arrays for tomographic radar of electrically large objects is presented. The major concern when imaging is extended to real objects is to achieve a correct reconstruction of the object shape and its electric properties. A general framework based on a UWB bifocusing operator (UWB-BF) with good tomographic imaging capabilities is presented. This general approach provides a comprehensive understanding of the basic tradeoffs with regard to sensing geometry and image quality parameters. Through numerical simulations and measurements applied to canonical as well as to complex objects, basic design criteria are assessed and the potential of UWB tomographic radar imaging is presented.

Fractal geometry provides reliable models to describe geometrical properties of natural surfaces. Therefore, their use in the electromagnetic scattering methods deserves careful research. In order to have complete insight into the phenomenon, a measurement campaign on a fractal surface in a controlled environment is a key step. In this paper, we propose a technique for building a fractal surface that can be used for electromagnetic scattering evaluation purposes. The surface characteristics are imposed by computer synthesizing a bandlimited Weierstrass-Mandelbrot function, whose actual shape is constructed by means of a cheap innovative technique: the synthesized surface is made from cardboard covered with aluminum foil, which gives a conducting surface and creates the micro-scale conditions, useful to represent manufacturing errors. Statistics of the overall surface shape are then measured, analyzed and compared with the imposed ones, providing and verifying the rationale for a fully controlled surface to be applied in any kind of experiment on natural surfaces.

This paper presents a numerical tool that uses high frequency electromagnetic methods to generate polarimetric interferometric SAR (POLIN-SAR) images of complex targets. It provides high scenario flexibility and a good trade-off between accuracy and computer costs. This simulator is suitable for numerous applications and this paper will assess its usefulness for vessel classification. Preliminary results of the potentialities of height maps for building classification patterns are commented. In this framework, the features of a simple approach based on the Pauli scattering vector that retrieves height information even from different scattering mechanisms within a resolution cell are analyzed with simulated images.

Continuous terrain fast changes monitoring is difficult to implement via airborne/satellite SAR systems, mainly due to the lack of flexibility and low revisiting times. Other SAR approaches based on small and simple ground-based systems, easy to deploy wherever are needed, must be considered. Transportability, low cost, and ruggedized structure are the main constrains, but the required resolution and performances have to be preserved. An experimental, short to medium range, ground-based, with optional polarimetric capability, Synthetic Aperture Radar (SAR) will be presented. First results of an experimental X-band SAR with a 100 MHz bandwidth, with 20 dBm of radiated power in differential interferometry operation will be shown

This paper presents a study performed with high resolution simulated SAR data that outline the limitations of polarimetric coherent decompositions on vessel classification. In these days, these theorems have been set up as the most advisable alternative for this application but an evident lack of real data avoids know their true usefulness. In this way, this study overcomes this limitation opening the doors to other alternatives, which would provide better performances in this research line.

This paper discusses the detection and correction of residual motion errors in airborne SAR interferograms. They usually appear due to the lack of precision in the navigation system. Two techniques existing in the literature to detect such errors are presented, highlighting differences and similarities. The effects of residual motion errors in the final interferogram are mainly two: azimuth phase undulations and azimuth registration errors. A new correction approach is proposed, which corrects both effects in one step, avoiding the use of interpolations. Additionally, the spectral diversity technique, used to estimate registration errors, is critically analyzed. Airborne L-band repeat-pass interferometric data of the German Aerospace Center (DLR) experimental airborne SAR (E-SAR) is used to validate the method.

In this paper we present a procedure devoted to validate innovative theories on the electromagnetic scattering from natural surfaces, using fractal functions for the surface description. A fractal surface has been built as a superposition of cardboard and aluminium layers, according with a computer design based on the Weierstrass-Mandelbrot (WM) fractal function. Its statistical properties have been verified with optical measurements. The electromagnetic field scattered at X band from the built surface has been measured in controlled environment as a function of the incidence angle. Obtained results have been compared with those of the Kirchhoff Approximation (KA) and the Small Perturbation Method (SPM) in conjunction with the fractional Brownian motion (fBm). Congruence and discrepancies of measured data with theoretical results are discussed, providing a validation of the methods

Bistatic radar systems will pay a great role in the coming decade since a large number of radar missions are being foreseen. Using existing transmitters, formations of small passive receivers will enhance our capability to gather backscatter information from Earth. A bistatic SAR system operates with separated transmitting and receiving antenna and both antennas can follow independent trajectories. In this paper, the recently developed imaging algorithm for the case where the transmitting antenna follows a rectilinear trajectory while the receiver remains in a fixed position and orientation will be evaluated. This new imaging algorithm is based on a projecting the bistatic geometry onto the chirp scaling algorithm which results on a scaling factor in the azimuth compression function. This scaling factor is derived from the bistatic configurations and assumes a flat topography. The main purpose of this paper is to determine the accuracy of the algorithm when used with real case simulated scenarios such as the airborne case where the transmitter is onboard an airplane while the receiver will be installed in the top of a high tower

This paper presents a closed formulation to evaluate the quality of terrain deformation maps obtained with multi-image differential interferometry. The analytical formulation, based on the application of the asymptotic covariance matrix, that propagates the interferogram coherences to the linear deformation and DEM error accuracies when using a stack of interferograms, is developed and validated with real data from the ERS satellites.

This paper presents a technique to calibrate interferometric airborne SAR images based on a multisquint processing approach, i.e., by processing the same image pairs with different squint angles we can combine the interferograms to obtain the desired phase correction. Airborne single-pass interferometric data from the DLR's E-SAR is used to validate the method.

In this paper, an advanced technique for the generation of deformation maps using synthetic aperture radar (SAR) data is presented. The algorithm estimates the linear and nonlinear components of the displacement, the error of the digital elevation model (DEM) used to cancel the topographic terms, and the atmospheric artifacts from a reduced set of low spatial resolution interferograms. The pixel candidates are selected from those presenting a good coherence level in the whole set of interferograms and the resulting nonuniform mesh tessellated with the Delauney triangulation to establish connections among them. The linear component of movement and DEM error are estimated adjusting a linear model to the data only on the connections. Later on, this information, once unwrapped to retrieve the absolute values, is used to calculate the nonlinear component of movement and atmospheric artifacts with alternate filtering techniques in both the temporal and spatial domains. The method presents high flexibility with respect to the required number of images and the baselines length. However, better results are obtained with large datasets of short baseline interferograms. The technique has been tested with European Remote Sensing SAR data from an area of Catalonia (Spain) and validated with on-field precise leveling measurements.

A recently developed technique, polarimetric radar interferometry, is applied to tackle the problem of the detection of buried objects embedded in surface clutter. An experiment with a fully polarimetric radar in an anechoic chamber has been carried out using different frequency bands and baselines. The processed results show the ability of this technique to detect buried plastic mines and to measure their depth. This technique enables the detection of plastic mines even if their backscatter response is much lower than that of the surface clutter.

This paper develops an analysis of the SAR impulse response function from the interferometric point of view, with the intention of studying its phase behavior in the presence of high squint angle values. It will be pointed out that in this case, a phase ramp is present in the range direction, which, in combination with a certain degree of misregistration between the two images induces an offset in the generated interferometric phase. This behavior, if not compensated, imposes strong limits on the performance of the interferometric techniques in a squinted case, especially for airborne SAR systems. The article proposes two new techniques, which are appropriate to correct the phase bias coming from this source. The first one is based on a modification of the azimuth compression filter, which cancels the phase ramp of the range impulse response function for one specific squint value. In case the SAR processing is performed with variable squint over range, the authors propose a second method oriented to estimating the expected misregistration and thus, the phase bias by means of an iterative approach. Simulated data as well as real corner reflector responses are used to show that the correct topography can be recovered precisely even in the presence of phase bias coming from the squinted geometry.

A coherence optimization method, which makes use of polarimetry to enhance the quality of SAR interferograms, has been experimentally tested under laboratory conditions in an anechoic chamber. By carefully selecting the polarization in both images, the resulting interferogram exhibits an improved coherence above the standard HH or VV channel. This higher coherence produces a lower phase variance, thus estimating the underlying topography more accurately. The potential improvement that this technique provides in the generation of digital elevation models (DEM) of non-vegetated natural surfaces has been observed for the first time on some artificial surfaces created with gravel. An experiment on a true outdoor DEM has not been accomplished yet, but the first laboratory results show that the height error for an almost planar surface can be drastically reduced within a wide range of baselines by using the optimization algorithm. This algorithm leads to three possible interferograms associated with statistically independent scattering mechanisms. The phase difference between those interferograms has been employed for extracting the height of vegetation samples. This retrieval technique has been tested on three different samples: maize, rice, and young fir trees. The inverted heights are compared with ground truth for different frequency bands. The estimates are quite variable with frequency, but their complete physical justification is still in progress. Finally, an alternative simplified scheme for the optimization is proposed. The new approach (called polarization subspace method) yields suboptimum results but is more intuitive and has been used for illustrating the working principle of the original optimization algorithm.

For newly developed iterative Newton-Kantorovitch reconstruction techniques, the quality of the final image depends on both experimental and model noise. Experimental noise is inherent to any experimental acquisition scheme, while model noise refers to the accuracy of the numerical model, used in the reconstruction process, to reproduce the experimental setup. This paper provides a systematic assessment of the major sources of experimental and model noise on the quality of the final image. This assessment is conducted from experimental data obtained with a microwave circular scanner operating at 2.33 GHz. Targets to be imaged include realistic biological structures, such as a human forearm, as well as calibrated samples for the sake of accuracy evaluation. The results provide a quantitative estimation of the effect of experimental factors, such as temperature of the immersion medium, frequency, signal-to-noise ratio, and various numerical parameters.

A comparative study at 2.45 GHz concerning both measurement and reconstruction parameters for planar and cylindrical configurations is presented. For the sake of comparison, a numerical model consisting of two nonconcentric cylinders is considered and reconstructed using both geometries from simulated experimental data. The scattered fields and reconstructed images permit extraction of very useful information about dynamic range, sensitivity, resolution, and quantitative image accuracy for the choice of the configuration in a particular application. Both geometries can measure forward and backward scattered fields. The backscattering measurement improves the image resolution and reconstruction in lossy mediums, but, on the other hand, has several dynamic range difficulties. This tradeoff between forward only and forward-backward field measurement is analyzed. As differential temperature imaging is a weakly scattering problem, Born approximation algorithms can be used. The simplicity of Born reconstruction algorithms and the use of FFT make them very attractive for real-time biomedical imaging systems.