Target Altitude Estimation In Bistatic High Frequency Surface Wave Radar

High frequency surface wave radar (HFSWR) transmits vertically polarized electromagnetic wave to detect surface vessels and low-flying aircraft at ranges far beyond the visible horizon, which is essential for the coast defence and marine traffic control. The operation frequency in high frequency (HF) band makes it difficult to form a narrow beam on elevation. HFSWR traditionally cannot detect the target altitude information and lacks the capability to identify the target flight modes, such as over the horizon or within the line of horizon. The current altitude estimation methods in monostatic HFSWR always yield low accurate and unstable results. Therefore, in this dissertation, the novel approaches are proposed to estimate target altitude and identify the flight mode with bistatic HFSWR. The performance of the proposed approach is evaluated with real trials. The research in this dissertation would promote the application of bistatic HFSWR in target classification and marine traffic control fields. The main contents of the dissertation are summarized as following:1. The propagation attenuation mechanism of the ground wave at different altitude intervals is analyzed with Rotheram model. The propagation attenuation of ground wave indicates distinct characteristics at diffferent altitude intervals, which is the fundamental principle of target altitude estimation and flight mode identification in HFSWR. Besides, target RCS is an crucial parameter for the altitude estimation of low-flying target. The variation property of RCS on different attitudes is also studied.2. An altitude estimation model is proposed for the low flying target with bistatic HFSWR. The incomplete observability of the traditional altitude estimation model leads to poor performance for altitude estimation. A novel model is proposed to estimate the altitude of the low-flying target with multiple model approach. In this new model, the altitude interval of interest is divided into several subintervals. An independent estimation model is constructed on each subinterval. The ultimate altitude estimation result is achieved by weighted summation. Besides, autoregressive model is also incorporated to eliminate the effection of the RCS fluctuation on altitude estimation. 3. An altitude estimation model with the range, azimuth and target echo is presented for the high-flying target. Both target range and azimuth are available information for the target altitude estimation of high-flying target. Target echo amplitude does not benefit the altitude estimation as it is independent on altitude, which is only utilized to estimate target RCS. An altitude and RCS estimation model set is proposed with the target range, azimuth and echo amplitude information received by T/R-R HFSWR. The model set contains three different estimation models. Each model is completely observable to the state.4. Simultaneous target altitude and flight mode identification. The objective of this research is to identify the flight mode as well as estimate the target altitude. With the property that the estimation and identification processes are mutually dependent, an integrated method named simultaneous identification and estimation(SIE) is proposed by applying two level multiple model approach to the flight mode probability mass function(pmf) and state probability density function(pdf) simultaneously. The multiple model approach incorporated in SIE is different from the traditional interacting multiple model (IMM). It is applied at two levels: within each mode-conditioned estimation model set and across all the mode-conditioned estimation model sets. Simulations and real trials demonstrate the performance of the proposed SIE method.5. Flight mode identification algorithm in HFSWR. In practical engineering applications, it is preferable and more promising to identify the flight mode directly than estimate the specific target altitude for threat assessment. Thus a flight mode identification model is proposed with the distinct propagation attenuation characteristics on the low and high altitude intervals. A certainty factor value is also derived to evaluate the accuracy of the flight mode identification, in which cosine similarity is involved to eliminate the impact of the target echo amplitude errors for flight mode identification.