Analyse et developpement de radar a diversite spatiale: Applications a l'evitement de collisions de vehicules et au positionnement local

The most frequently used method in local positioning systems is to make use of three base stations at different places and to measure the range of the tag by each base station. Then the exact location of the tag is calculated by triangulation. In practice, a fourth base station is added for more reliability and time synchronization. In some situations like the surveillance of a beach or a building on fire, installing the third base station would be a difficult or time consuming task. Our idea is to elevate the third base station at a reasonable height. This will provide a better signal quality and more information about the target can be obtained. It is a new type of local positioning system that we call VLPS (Vertical Local Positioning System). We will examine the constraints of VLPS in the second part of this thesis.A part of this thesis is dedicated to the analysis and the modeling of radar facilitating the comprehension of the scintillation phenomenon, as well as the proposition of a practical solution mainly useful in the context of avoidance/warning radar. This technique, named "spatial diversity radar", is inspired from MIMO solution (Multi-Input-Multi-Output). Initially intended for the reduction of error rate and improvement of channel capacity in wireless communications, the MIMO technique aims at providing a solution to this problem by introducing a type of redundancy in the propagation of information using waveforms orthogonality in spectral or time domain. The essential idea of this solution is to locate multiple antennas at the emission and reception, then using a coding technique making possible the reconstruction of the original signal from its different replicas. Many types of diversity (spatial, polarization, pattern and frequency) can be found in the literature. In the context of a fading or scintillation, there is a good probability that at least one of the receiving antennas provides the power beyond the threshold level of the receiver.In the first part of this thesis, we have modeled the spatial diversity radar. We then provide the mathematical model which allows calculating the scintillation mitigation of the received power. Similar to the definition of the antenna beamwidth, we introduce the notion of angular range of a radar system. This corresponds to half-power angular width with respect to the maximal received power. In this thesis, we also demonstrate the improvement of the angular range due to spatial diversity solution.The last part of this thesis is about the implementation of a VLPS base station. The RF front-end has been fabricated in two versions: open and shielded. The shielded version has better isolation between the different parts of the circuit. At the end, we have designed a DSP board which provides the frequency of the beat signal and determines the distance of the tag by calculating the FFT (Fast Fourier Transform) of the signal. The integration of an entire VLPS is left as future work. (Abstract shortened by UMI.)Moreover, it is well known that the radars, as well as all wireless telecommunication systems, are confronted with the problem of fading signals. Generally, this problem is due to multi-path effects of signal propagations. In other words, the multiple signal reflections by the surrounding stationary and mobile objects are randomly neutralized at the arriving point of the receiving antenna. In a different context and for apparently unlike motives, the radars are subject to the same issue. Even when the target is in the line-up site of transmitting and receiving antennas (radars), they face the same type of scintillations due to the variation of the radar cross section (RCS) of a target. Indeed, the radar cross section of the majority of targets strongly depends on the aspect angles of the receiving and transmitting antennas. This phenomenon, commonly known as RCS scintillation, becomes visible in case of a lateral motion of rotation of the target relative to the radar antenna.