| The skywave Over-the-Horizon Radar (OTHR) is a special type of radar which operates in high frequency band (3-30MHz) and employs the ionosphere to reflect signal to detect target from the top down. OTHR has some native advantages, such as the capability of detecting low-flying target and stealth target, avoiding the attack of anti-radiation missile, as well as providing wide area surveillance with longer early warning time than some other common radar. Therefore, OTHR play a more and more important role in modern air defense system and has potential significant contribution to military applications. However, OTHR's operating modes (high frequency operation band, ionospheric propagation medium and looking-down mode) bring some great challenges to the signal processing. The success in resolving these problems will be crucial to enhance OTHR's performance and extend its advantages.This dissertation is based on the project "Study on signal processing of skywave over-the-horizon radar", which is funded by and cooperated with No.14 research institute of Ministry of Information Industry. In this dissertation, the interference suppression, decontamination of ionospheric distortion, and ship detection in the OTHR are studied in detail.The summary of our research work is as follows:(1) The mathematical derivation of the range-azimuth-Doppler joint signal processing of OTHR is presented in detail. And the criterions in radar parameters selection are also analyzed. Then, based on the ocean scattering intensity formula proposed by Barrick, the ocean clutter spectrum is simulated, with which the characteristics of clutter distribution and its dependence on sea-state are deduced.(2) The suppression of the interference and noise in OTHR is studied in detail. First, two adaptive beam forming schemes are compared based on the fact that OTHR has a very large surveillance area and nonstationary interference may exist. Then according to the specific radar parameters, we analyze the problems that the second adaptive beam forming scheme may encounter in practice. After that, to suppress the nonstationary interference, the method of increasing the update rate of adaptive antenna weight is discussed with theoretical analysis and testifying by simulated data. Finally, the transient interference suppression is studied in detail. The adaptive tune-frequency analysis technique-AGR algorithm is proposed by us to suppress the transient interference,In this new method, both the time-domain variance and time-frequency center are adaptive to match the local characteristic of each signal component. Moreover, the transient suppression and signal reconstruction can be completed simultaneously. In addition, the new technique only needs to discard the signal components that have small time-domain variance and then recombine the remaining signal component. Therefore, it has nolimitation for the quantity of transient interference. Even the transient interference has multiple components, the new technique can still achieve a good performance in interference suppression and signal reconstruction.(3) The decontamination of ionospheric distortion is studied in detail. First, several decontamination methods of ionospheric phase disturbance are introduced and compared. Then, based on the processing of real and simulated data, the effects of intensity of phase disturbance and second-order ocean clutter on decontamination performance are discussed. In the part of multi-mode propagation decontamination, the traditional method based on the conversion from multi-mode to single-mode is analyzed and its problems are pointed out. Then we propose that it should not convert the multi-mode to single-mode, but cancel the multiple Bragg peaks directly by using the ocean clutter iterative cancellation technique, and therefore, detect the target in the multi-mode spectrum. In the case that the phase distortion and multi-mode propagation simultaneously exist, our theoretical analysis shows that the presence of multi-mode will cau...
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