L.P. Ligthart

UWB radar for human being detection

UWB radar for detection and positioning of human beings in complex environment has been developed and manufactured. Novelty of the radar lies in its large operational bandwidth (11.7 GHz at -10 dB level) combined with high time stability. Detection of respiratory movement of a person in laboratory conditions has been demonstrated. Based on experimental results human being radar return has been analysed in the frequency band from 1 GHz until 12 GHz. Novel principle of human being detection is considered and verified experimentally

UWB array-based radar imaging using modified Kirchhoff migration

This paper presents a new modification of Kirchhoff migration algorithm for ultra-wideband (UWB) array-based radar imaging. The developed algorithm is evolved from traditional Kirchhoff migration which is based on the classical integral theorem of Helmholtz and Kirchhoff. The new algorithm is designed for array-based radar imaging with arbitrary multiple input multiple output (MIMO) configuration. The developed algorithm is compared with conventional diffraction stack migration using both synthetic data from numerical simulation and measurement data from landmine detection. The results have shown promising improvements in the aspects of beamwidth, side-lobe rejection ratio and the ability to reconstruct shapes of distributed targets.

Coherent Scattering of Microwaves by Particles: Evidence from Clouds and Smoke

Many radar measurements of the atmosphere can be explained in terms of two scattering mechanisms: incoherent scattering from particles, and coherent scattering from variations in the refractive index of the air, commonly called clear-air or Bragg scattering. Spatial variations in the liquid water content of clouds may also give a coherent contribution to the radar return, but it is commonly believed that this coherent scattering from the droplets is insignificant because variations in humidity have a much larger influence on the refractive index than equal variations in liquid water content. It is argued that the fluctuations in water vapor mixing ratio in clouds can be much smaller than those in liquid water mixing ratio. In this article an expression for the strength of the coherent scattering from particles will be derived for fluctuations caused by turbulent mixing with clean (i.e., particle-free) air, where it will be assumed that the particles follow the flow, that is, their inertia is neglected. It will be shown that the coherent contribution adds to the incoherent contribution, the latter always being present. The coherent particle scattering can be stronger than the incoherent scattering, especially at longer wavelengths and high particle concentrations. Recently published dual-frequency measurements of developing cumulus clouds and smoke show a correlation for which no explanation has been found in terms of incoherent particle scattering and coherent air scattering. Scatterplots of the reflectivity factors at both frequencies show a clustering of points in between the values that correspond to pure clear-air and pure incoherent scattering. Those differences in the radar reflectivity factors could be due to a mixture of Bragg scattering and incoherent particle scattering, but then no correlation is expected, because the origin of the scattering mechanism that dominates at each wavelength is different. However, coherent scattering from the particles can cause the radar reflectivities of dual-wavelength radar measurements to become correlated with each other. It may explain the slopes and the differences seen in the scatterplots of the radar reflectivities of cloud and smoke measurements, with reasonable values of the parameters involved. However, the correlation between the radar reflectivities is very tight near the cloud top and seems to be present in adiabatic cores as well. This is an indication that, apart from mixing with environmental air, the inertia of the droplets could also be important for the creation of small-scale fluctuations in droplet concentration.

Scattering properties of a statistically rough interface inside a multilayered medium

In this paper, electromagnetic wave scattering from a slightly rough interface inside a stratified medium is considered. A three-layer model for the medium is chosen, which allows us to investigate both possible cases of the mediums stratification: above and below the rough interface. It also is a simple but realistic model for the ground, in which the upper layer is thought to be vegetation or snow, while the middle layer and the homogeneous half-space below it represent the ground itself. The vegetation-ground interface is considered to be rough with statistically homogeneous corrugation, while the two other interfaces are flat. The backscatter coefficients for vertically and horizontally polarized waves are found in the framework of the small perturbation method combined with the Green function formalism. Such an approach allows one to consider arbitrary layers and, in this respect, go far beyond the radiative transfer theory. The influence of the stratification of the medium above and below the rough interface on the scattered field is analyzed numerically. Qualitatively new angular and frequency dependencies of backscattering are found. It is shown that in the case of small ohmic losses stratification of a medium can cause the deviation of backscattering up to 20 dB.

Comparison of UWB technologies for human being detection with radar

UWB radar for detection and positioning of human beings in complex environment can be developed based on different technologies such as video impulse, quasi-random noise, stepped-frequency continuous wave and frequency-modulated continuous wave. These technologies are compared on the basis of meeting functional requirements to such a radar. Relative advantages and disadvantages of these technologies for such an application as human being detection and positioning are pointed out. Recommendations for selection of an optimal technology are given.

Effective permittivity of and scattering from wet snow and ice droplets at weather radar wavelengths

In this parametric study, wet snow and ice droplets are modeled as sparse collections of Rayleigh scatterers (size small compared to wavelength) consisting either of ice or of composite mixtures of air and ice in water. An effective permittivity is calculated using various extended Maxwell-Garnett-type models to account for variations in shape and orientation of the constituents. The backscatter radar cross section is calculated as an incoherent sum of individual particle cross sections, and for various distributions of shape, size, and orientation. The results indicate a dependence of the radar cross section on the polarizations of the incident and reflected fields. This dependence is shown in the differential reflectivity, defined in terms of the ratio of the backscatter cross sections due to two mutually orthogonal linearly polarized incident electric fields. >

Interleaved Array Antennas for FMCW Radar Applications

An effective and robust strategy for concurrently designing the transmit and receive antennas of a frequency-modulated, continuos-wave radar is discussed. The aperture architecture is based on the use of non-periodic, interleaved sub-arrays. Deterministic element placement is employed for ensuring design efficiency. The procedure yields controllable sub-array radiation patterns and two-way side-lobe levels below - 30 dB, that are also stable over a wide frequency range.

Design considerations in sparse array antennas

The design aspects concerning the sparse array antennas are discussed. Commonly, the beamwidth, the side-lobes level and the gain are the relevant features in antenna design procedures. These parameters are evaluated for different sparse topologies and generalised design relations are provided. The capabilities of sparse architectures are evaluated by comparison with uniform arrays for a similar level of performance

Potentials of ultra-short-pulse time-domain scattering measurements

This article illustrates the potentials of ultra-short-pulse time-domain scattering measurements, and describes a facility to perform such measurements. The main advantages of measuring in the time domain are the high range resolution and the relatively simple measurement setup. A time-domain radar cross section measurement of a flat plate is performed, to illustrate the advantages of such methods over a conventional frequency-domain setup. The measurement was performed with a sampling oscilloscope, a pulse generator, and two 2-12 GHz ridged-horn antennas. Because the horns were not designed for transmitting transient signals, an additional system-response measurement, in combination with a software deconvolution algorithm, restored the impulse response of the object under test. Further processing separated the response of the object from clutter. A comparison of the time-domain data with calculated and measured frequency-domain radar cross sections shows good agreement. The high range resolution (100 ps) enabled the separation of scattering mechanisms (i.e., reflection, single and multiple diffraction). It is concluded that ultra-short-pulse time-domain measurements could be very beneficial.

First measurements with TARA; An S-Band transportable atmospheric radar

Abstract In this paper the S-band T ransportable A tmospheric R adar, TARA, is presented. The system came into operation in April 2000 and combines high temporal and spatial resolution with high flexibility and high sensitivity. Resolution and sensitivity are exemplified in measurements of a rain event above the Delft area in the Netherlands. From the co and cross-polar measurements, an antenna limit of approximately -30 dB is deduced. The measurements also show a typical Linear Depolarization ratio of -15 dB in the bright band.