Francois Le Chevalier

An analytical model for the description of the full-polarimetric sea surface Doppler signature

This paper describes an analytical model of the full-polarimetric sea surface scattering and Doppler signature. The model combines the small-slope-approximation theory (at the second order) with a weak nonlinear sea surface representation. Such a model is used to examine the variation of the Doppler central frequency/bandwidth and of the normalized radar cross section as function of wind speed and direction. The results suggest that the model can be a valuable tool for the accurate observation of sea surface currents.

Future Ocean Scatterometry: On the Use of Cross-Polar Scattering to Observe Very High Winds

This paper investigates the potential of cross polarization (VH) to extend the upper dynamic range of the wind measurements from ocean scatterometry. An analytical model for the VH polar scattering of the microwave radiation from ocean is proposed. The model combines the second-order small-slope approximation theory with the vector radiative transfer theory to obtain a statistical expression of the ocean scattering in presence of foam. Cross-polarized backscatter signals from RADARSAT-2 C-band synthetic aperture radar imagery, which were acquired during severe weather events, and collocated/time-coincident stepped-frequency microwave radiometer wind measurements by the National Oceanic and Atmospheric Administrations hurricane-hunter aircraft are used to verify the model. The validity of the model has been also proven against European Centre for Medium-Range Weather Forecasts forecasted winds. The results suggest that the present scattering model can be a valuable tool for understanding VH at very high wind speeds and for interpreting the data collected by the future dual polarimetric wind scatterometer (SCA), which will be flown on the Second Generation Meteorological Operational satellite program (MetOp-SG) as an evolution of the Advanced Scatterometer instrument on board MetOp.

Antenna Coupling Effects for Space-Time Radar Waveforms: Analysis and Calibration

In this paper, we investigate the impact of mutual coupling between the antenna array elements on the performance of a multiple-input multiple-output (MIMO) radar with colored waveform transmission (collocated coherent MIMO system). Simulation results confirm the decrease in system performance when antenna mutual coupling is taken into account. A dedicated scan-dependent calibration technique is proposed for elimination of the coupling effect. The experiments with a real X-band antenna array validate the coupling analysis and the calibration procedure presented in this paper.

Bayesian sparse estimation of migrating targets for wideband radar

Wideband radar systems are highly resolved in range, which is a desirable feature for mitigating clutter. However, due to a smaller range resolution cell, moving targets are prone to migrate along the range during the coherent processing interval (CPI). This range walk, if ignored, can lead to huge performance degradation in detection. Even if compensated, conventional processing may lead to high sidelobes preventing from a proper detection in case of a multitarget scenario. Turning to a compressed sensing framework, we present a Bayesian algorithm that gives a sparse representation of migrating targets in case of a wideband waveform. Particularly, it is shown that the target signature is the sub-Nyquist version of a virtually well-sampled two-dimensional (2D)-cisoid. A sparse-promoting prior allows then this cisoid to be reconstructed and represented by a single peak without sidelobes. Performance of the proposed algorithm is finally assessed by numerical simulations on synthetic and semiexperimental data. Results obtained are very encouraging and show that a nonambiguous detection mode may be obtained with a single pulse repetition frequency (PRF).

Bayesian sparse Fourier representation of off-grid targets with application to experimental radar data

We consider the problem of estimating a finite sum of cisoids via the use of a sparsifying Fourier dictionary (problem that may be of use in many radar applications). Numerous signal sparse representation (SSR) techniques can be found in the literature regarding this problem. However, they are usually very sensitive to grid mismatch. In this paper, we present a new Bayesian model robust towards grid mismatch. Synthetic and experimental radar data are used to assess the ability of the proposed approach to robustify the SSR towards grid mismatch.

DopSCAT: A mission concept for simultaneous measurements of marine winds and surface currents

A radar scatterometer operates by transmitting a pulse of microwave energy toward the ocean’s surface and measuring the normalized (per-unit-surface) radar backscatter coefficient (r8). The primary application of scatterometry is the measurement of near-surface ocean winds. By combining r 8 measurements from different azimuth angles, the 10 m vector wind can be determined through a Geophys- ical Model Function (GMF), which relates wind and backscatter. This paper proposes a mission concept for the measurement of both oceanic winds and surface currents, which makes full use of earlier C-band radar remote sensing experience. For the determination of ocean currents, in particular, the novel idea of using two chirps of opposite slope is introduced. The fundamental processing steps required to retrieve surface currents are given together with their associated accuracies. A detailed description of the mission proposal and comparisons between real and retrieved surface currents are presented. The proposed ocean Doppler scatterometer can be used to generate global surface ocean current maps with accuracies better than 0.2 m/s at a spatial resolution better than 25 km (i.e., 12.5 km spatial sampling) on a daily basis. These maps will allow gaining some insights on the upper ocean mesoscale dynamics. The work lies at a frontier, given that the present inability to measure ocean currents from space in a consistent and synoptic manner repre- sents one of the greatest weaknesses in ocean remote sensing.

Modified Capon and APES for spectral estimation of range migrating targets in wideband radar

In many radar and sonar applications, the narrowband signal assumption is not respected. To preserve good performance detection and/or parameter estimation for wideband signal, it is necessary to develop appropriate signal models and processing algorithms. More specifically, the range migration of the target during the processing interval induces a coupling between range and Doppler that is not taken into account by standard algorithms such as the Capon and APES methods. In this paper, we present a modified and enhanced version of the Capon and APES algorithms for a wideband signal model. These two algorithms are adaptive filters designed for spectral estimation. Performance of the wideband-Capon and the wideband-APES algorithms are studied via numerical simulations.

A Pulse Compression Waveform for Weather Radars With Solid-State Transmitters

Due to the contradiction between the high sensitivity requirement and low transmission power of weather radars with solid-state transmitters, a pulse compression technique is necessary. For the purpose of range sidelobe suppression, methods based on amplitude modulation and a mismatched filter are commonly used for target detection radars, which are not applicable for weather observations because of its drawbacks such as main lobe expansion and power loss. This letter presents a nonlinear frequency modulation pulse compression waveform to achieve a very low range sidelobe level. A mathematical model for waveform design is established, in which the time-frequency relation is expressed as the combination of a linear function and a sine series. Thus, a set of parameters can fully characterize the time-frequency curve. By using a simulated annealing algorithm, the optimal parameter set can be obtained, which shows that a peak sidelobe level under -60 dB is achievable. Finally, this kind of waveform is implemented on hardware, and its performance is verified.

Range-Doppler processing for indoor human tracking by multistatic ultra-wideband radar

Indoor measurements were performed to examine the feasibility of using range-Doppler processing for human motion detection in a highly cluttered environment by the multistatic ultra-wideband (UWB) radar. A novel hypothesis-testing based range migration compensation algorithm, which is especially suited to UWB radars, is proposed to increase the coherent processing interval for range-Doppler processing. For each receiver, the multistatic UWB radar provides a video sequence constructed by high-resolution range-Doppler images of a human target, which is promising for the analysis of human target characteristics.

A Migrating Target Indicator for wideband radar

The standard way to suppress clutter in narrowband radar is to use Moving Target Indicator (MTI) cancellation techniques. High Range Resolution (HRR) radars are becoming more and more important because they can detect and track targets more accurately. As for such radars the bandwidth is increased, the resolution is decreased and leads to target range migration over the coherent pulse interval (CPI). Due to this range walk, standard low resolution MTI processing is not adapted anymore to HRR MTI radar data. We propose here to extend the principle of the MTI processing to the wideband case. We refer to this method as the Migrating Target Indicator (MiTI), since it eliminates the non-migrating targets from the received signals.