Time reversal of radar waves. New method, which can increase performance?

The report is concerned with time reversed waves. A time reversed wave is obtained from an original one by reversion of all movements, so that it propagates back towards its source like in a film of the original wave, run backwards. With proper system properties of the antenna, receiver and transmitter, an automatic focusing towards the source is obtained, without closer knowledge of the medium, inhomogeneities, etc. The whole system is assumed to be time invariant and linear. The report has been given a compendium-like character, by compiling material in journals, conference publications and Interet documents. The work has been generated from application needs in FOI projects for short-range observation with radar sensors, chiefly in urban scenarios. A main difficulty here is the influence of the background, against which targets are to be detected, localized and identified. Time reversion of radar returns is judged to have an important potential in this case. In some respects it has properties that almost seem to be created for just this application with wave propagation in a strongly inomogeneous mdium. The subject area has received attention only in the last few years in the case of radar, while inacoustics, experimental and theoretical studies have been conducted since the beginning of the 1990s. The report gives an account of the physical and mathematical basis. It describes the implementation of time reversal in the radar case by an array antennna and how self-focusing may be obtained. It is also described how information about the scenario can be extracted computationally from measured transfer functions between the antenna elements, so that the antenna system can be made to focus separately on single, dominating objects (point targets). In some applications this will suffice - in others, especially with radar, a situation overview is desirable, e.g. via a radar image. This requires knowledge of the transfer properites of the medium, its so-called Green function, which represents how the wave propagates from one point in space and time to another. For a complicated scenario, this is a major problem. If the function is known, inhomogeneities in the medium, normally seen as "clutter" (e.g. walls), may be used to get "super resolution" beyond the nominal angular resolution of the antenna. This can be seen as including these inhomogeneous scatterers into the antenna system. One then gets an illumination of a target across a broader aspect interval than that offered by the antenna size, and hence an increased angular resolution, as in the case of SAR/ISAR.