Lucio Bellomo

Time Reversal Experiments in the Microwave Range: Description of the Radar and Results

We present a new RADAR system able to perform Phase Conjugation experiments over the ultrawideband (2{4)GHz. The system is equipped with a transmit/receive linear array made of eight antennas connected to a 2-port Vector Network Analyzer through eight independent couples of digitally-controlled RF attenuators and phase shifters. Thus, each channel can selectively transmit or receive and can as well attenuate and phase shift the RF signal. For each frequency, either the Phase Conjugation or the Decomposition of the Time Reversal Operator (DORT) is applied to the received signal and the appropriate amplitude and phase law is coded into the prototype; the focusing wave is then experimentally re-emitted by the array. The quality of the achieved backpropagation is evaluated both in frequency and time domain: In this sense we can speak of Time Reversal. The excellent agreement between measured and theoretical results validates the potential of our system.

Strong discrepancies between local temperature mapping and interpolated climatic grids in tropical mountainous agricultural landscapes

Bridging the gap between the predictions of coarse-scale climate models and the fine-scale climatic reality of species is a key issue of climate change biology research. While it is now well known that most organisms do not experience the climatic conditions recorded at weather stations, there is little information on the discrepancies between microclimates and global interpolated temperatures used in species distribution models, and their consequences for organisms’ performance. To address this issue, we examined the fine-scale spatiotemporal heterogeneity in air, crop canopy and soil temperatures of agricultural landscapes in the Ecuadorian Andes and compared them to predictions of global interpolated climatic grids. Temperature time-series were measured in air, canopy and soil for 108 localities at three altitudes and analysed using Fourier transform. Discrepancies between local temperatures vs. global interpolated grids and their implications for pest performance were then mapped and analysed using GIS statistical toolbox. Our results showed that global interpolated predictions over-estimate by 77.5±10% and under-estimate by 82.1±12% local minimum and maximum air temperatures recorded in the studied grid. Additional modifications of local air temperatures were due to the thermal buffering of plant canopies (from −2.7°K during daytime to 1.3°K during night-time) and soils (from −4.9°K during daytime to 6.7°K during night-time) with a significant effect of crop phenology on the buffer effect. This discrepancies between interpolated and local temperatures strongly affected predictions of the performance of an ectothermic crop pest as interpolated temperatures predicted pest growth rates 2.3–4.3 times lower than those predicted by local temperatures. This study provides quantitative information on the limitation of coarse-scale climate data to capture the reality of the climatic environment experienced by living organisms. In highly heterogeneous region such as tropical mountains, caution should therefore be taken when using global models to infer local-scale biological processes.

Empirical Forecasting of HF-Radar Velocity Using Genetic Algorithms

We present a coastal ocean current forecasting system using exclusively past observations of a high-frequency radar (HF-Radar). The forecast is made by developing a new approach based on physical and mathematical results of the nonlinear dynamical systems theory that allows to obtain a predictive equation for the currents. Using radial velocities from two HF-Radar stations, the spatiotemporal variability of the fields is first decomposed using the empirical orthogonal functions. The amplitudes of the most relevant modes representing their temporal evolution are then approximated with functions obtained through a genetic algorithm. These functions will be then combined to obtain the hourly currents at the area for the next 36 h. The results indicate that after 4 h and for a horizon of 24 h, the computed predictions provide more accurate current fields than the latest available field (i.e., persistent field).

An Improved Antenna Calibration Methodology for Microwave Diffraction Tomography in Limited-Aspect Configurations

This work underlines the importance of properly modeling the directivity of the antennas and proposes an improved calibration strategy in the framework of microwave diffraction tomography. Experiments to prove the effectiveness of the approach are carried out with a system comprising two ultrawideband linear antenna arrays. In the configurations presented herein, targets are in the near-field of the arrays and small aperture angles are considered ( ~ 40 deg). Multi-frequency multi-view multi-static quantitative inversions are performed using the modified 2 gradient method. The calibration approach reconstructs the directivity of the antennas using measured incident fields. The novelty consists in taking the reconstructed antenna pattern into account within the inversion algorithm both at transmission and at reception through a multipolar expansion of, respectively, the incident field and the data equation Green function. The results show that a fine modeling of the directivity of the antennas is paramount to image and separate scatterers, particularly when placed closer than the resolution limit of the system and beyond the Born approximation regime.

Waveform design based on phase conjugation and time reversal

The recent development of the full digital array technology paves the way to the design of multistatic RADAR systems relying on agile waveforms at emission. This new paradigm fits perfectly with the concept of phase conjugation (or time reversal if applied to large-band data). This technique allows indeed to adaptively build a wave focusing onto a target, leading to an improvement in detection range or in search time as compared to classical approaches. The DORT method, issued from phase conjugation, even permits to detect multiple targets from the knowledge of the multistatic matrix of the antenna array. Based on both a theoretical analysis and an experimental proof provided here, it appears that these methods appear as very promising, especially for cueing and ultra-fast reacquisition modes where the phase conjugation/DORT SNR requirements are more easily fulfilled.