J.-Y. Dauvignac

Step-frequency radar applied on thin road layers

Abstract In the field of road construction and maintenance, the need for information on the thickness of very thin road layers is not satisfied by means of commercial pulse GPR, due to the inability of such devices to operate over ranges of several gigahertz. As a result, research has focused on the design of a step-frequency radar technique, able to work with very high-frequency synthetic pulses. An ultrawide band antenna, belonging to the family of Vivaldi antennas, has been developed for road applications. It has been created using stripline technology and yields a band width greater than one decade. During an initial step, this antenna was tested on various bituminous concrete samples with a network analyzer. Different parameters were studied, including band width, offset between antennas, and height and shape of the frequency-dependent pulse. A second step involved GPR dynamic measurements. A customized software program enabled recording data from the network analyzer. Several radar profiles were developed from selected road construction and maintenance test sites (e.g. the Circular Pavement Fatigue Test Track, composed of a number of known structures). Results show improved resolution when compared to a commercial impulse GPR system.

Inversion algorithm and measurement system for microwave tomography of buried object

The detection and identification of buried inhomogeneities using electromagnetic waves are of crucial importance for many applications. The paper deals with reconstructed images from measurements using an inversion qualitative algorithm for microwave tomography. The algorithm is based on diffraction tomography for detecting and locating buried objects. The backscattered field is measured at different frequencies over a probing line above or in contact with the soil at different receiver locations and for different positions of a transmitting antenna (multistatic configuration). The reconstruction algorithm processes the backscattered field generated by the buried inhomogeneities and incorporates the incident near-field distribution transmitted in the soil by the broadband antenna. Tomographie reconstructions of buried objects are presented for a situation of practical interest using bow tie antennas in the frequency band [0.3–1.3] GHz.

Reflectarray using an offset prolate feed at 94 GHz

The design of an offset reflectarray illuminated by an original primary source at 94 GHz was tested successfully. The antenna shows very low side lobe and cross-polarization levels. These results are of interest for multi-beam applications, when the system is fed by several sources. Simulations were made with a computer-model that gives, in less than 10 minutes on a standard PC, a good approximation of the radiation pattern in the H-plane. The problem of the specular reflection has still to be overcome as well in simulations as in the design. In order to solve it partially, the next step will be the realization of an offset feed with a main radiation in the specular direction.

Inversion algorithms and measurement systems for microwave tomography of buried objects

The detection and identification of buried inhomogeneities using electromagnetic waves are of crucial importance for many nondestructive testing applications. The paper deals with two different inversion algorithms for microwave tomography. First, a qualitative algorithm based on diffraction tomography for detecting the buried object, and second, a quantitative algorithm for reconstructing the complex permittivity profile of the object. The backscattered field is measured at different frequencies over a probing line above or in contact with the soil at different receiver locations and for difference positions of a transmitting antenna (multistatic configuration). The two reconstruction algorithms process the backscattered field generated by the buried inhomogeneities and incorporate the incident near-field radiation pattern transmitted in the soil by the broadband antenna. Tomographic reconstructions of buried objects are presented for a situation of practical interest using bow-tie antennas in the frequency band 0.3-1.3 GHz.

Microwave imaging of buried objects for ground radar tomography

The detection and identification of buried inhomogeneities using electromagnetic waves are of great importance for nondestructive testing in geophysical, civil engineering, military, or humanitarian applications. The approach described here uses a linear inversion algorithm based on diffraction tomography to process the backscattered field signal generated by the buried inhomogeneities, measured over a probing area, for computation of a tomographic image of the ground. The technique has been extended to cater for time pulse (synthetic pulse or real pulse) illumination and to incorporate the near-field radiation pattern transmitted by a broadband antenna. The backscattered field is measured at various locations with a receiving antenna for different positions of the transmitting antenna. The main advantages of such a method are the use of explicit formulas for solving the imaging problem and the possibility of rapid image display thanks to the use of Fast Fourier transform to implement the reconstruction algorithms on PCs. First, a number of tomographic reconstructions of buried objects are given for situations of practical interest. Results of numerical simulations with a Rayleigh incident pulse in the frequency band [0.3,3.0] GHz and the use of a bow tie antenna in the frequency band [0.3,1.3] GHz are presented. Second, tomographic reconstructions of buried objects are presented and discussed, to demonstrate the influence of object dielectric properties versus soil electromagnetic parameters.

Experimental study of near-surface radar imaging of buried objects with adaptive focused synthetic aperture processing

This paper deals with the application of stepped frequency radar to subsurface imaging of buried targets with a final aim of object imaging and identification. The applications are mainly mines or UXO detection but also buried pipes. The depths considered here are a few cm to 15 cm under the surface (from the top of the object). It is necessary to use a UWB radar in order to separate the soil interface from the top of the object. A versatile system has been built that can be brought outdoor. It is used to find the best parameters for a future optimal radar. Special antennas have been realized that cover the 500 MHz to 8 GHz frequency range. The antenna pair (T/R) moves at a given height over the soil surface along a rail. Radar returns are then processed on a PC in order to deliver in a few seconds a 2D vertical profile of the soil. A special algorithm for near field synthetic focusing aperture has been developed for this task. It takes into account the wave propagation in the soil. Tomographic images are presented for different objects in different soils (.5 to 5 GHz and 2 to 8 GHz bandwidths) that show the quality of the results delivered by this improved technique. Conclusion are drawn on the potentialities and the limitations of the method and future perspectives like 3D imaging.

Comparative study using an UWB measurement system and a RAMAC GPR system for subsurface imaging of the vaucluse karst aquifer

We have presented comparative study with a realistic field-test in the tunnel of the Vaucluse aquifer, of two different microwave imaging systems - the 250 MHz RAMAC pulsed system and the ultra-wideband ETSA system. These initial results are very promising especially regarding the improved resolution of the ETSA image and the concomitant greater depth of penetration. Future work will focus on establishing a quantitative norm for comparing image quality, both in amplitude and frequency.

Investigation of low cost compact base W-band

The purpose of this paper is to present a compact base using a dielectric lens of 500 mm diameter for large antenna measurements. The compact base has been simulated using an electromagnetic solver based on electric and magnetic field integral equations from France Telecom R&D. Studies were carried out at 94 and 77 GHz with different antenna configurations.

Reflectarrays for mm-Wave radar applications

This paper gives an overview of mm-Wave reflectarrays used for civil radar applications such as collision avoidance system for rescue helicopters at 94 GHz or FOD detection on airport runways at 77 GHz. Both antenna designs and examples of radar performances are presented and discussed.

Millimeter Wave Circularly Polarized Fresnel Reflector for On-Board Radar on Rescue Helicopters

The increasing use of millimeter waves for civil radar application, e.g., for automotive or helicopter obstacle detection- requires the development of high gain and low cost antennas in compact form. With this aim, a Fresnel reflector with circular polarization over 5 GHz bandwidth (76-81 GHz) has been designed, fabricated and measured. The gain remains higher than 32 dBi with a peak value of 35 dBi at 79 GHz. For obtaining this performance, specific patches have been designed. They are based on circular rings, rectangular and offset patches. Each of them converts an incident linear electric field into a circular polarized one.