Study On Altitude Measurement Algorithms And Applications In VHF Radar

VHF radar has unique advantage in anti-stealth, ARMs because of its long wavelength. In recent years, the research in VHF radar has been attached great importance and is weightily situated in the radar detection system.The location of the target is detected by the electromagnetic wave reflection. The range and arrival angle are directly extracted from the target echoes when the target is found. So the target altitude is a calculated value not a measurable one. The problem of the altitude measurement in VHF radar is the low elevation angle estimation.Because of the beam wideness, the beam often reflects to the ground when radar detects the long range targets. The received signal includes the multipath interference refracted from the ground, sea surface, and the electric wave refracted non-uniformly from the low level atmosphere. The multipath interference effects on the measurement accuracy greatly. It makes the direct wave and multipath wave superposition, and causes the received signal electric level glistening. When the electric glisten badly, the radar might lose the target. The key technology to be solved is improving the low-elevation angle accuracy.On the basis of the achievements obtained by now, combined with my research project, further research is given on the frequent engineering methods. The improving algorithms are also presented. The related problems on the ground-base radar, shore-base radar and shipped-base radar are studied. The main work can be summarized as follows:1.The target's direct arrival signal and multipath signal are close complete correlation, so the elevation-angle estimation is a problem to distinguish two correlated sources. Spatial smoothing techniques are effective to estimate the directions of coherent arrivals. The performance of these techniques deteriorates rapidly as the coherent arrivals become closely spaced. In this correspondence, an improved spatial smoothing technique is proposed. The algorithm makes cross-correlation of all auto-correlation matrixes of subarray output, and then takes the spatial spectrum estimation to the equivalent spatial smoothing matrix after the forward and backward correlation matrixes averaged. The formula for the algorithm is derived, and the performance of the algorithm is also analyzed. Simulation results demonstrate that the improved algorithm possesses a better resolving ability and a lower SNR threshold than conventional SS technique and subarray-covariance SS technique for DOA estimation of coherent signals.2.Altitude measurement requires the vertical aperture, but the bad beam-split response of the vertical array is not suitable for low-altitude detection. Thus, the oblique array is common used in the ground-base radar and the shore-base radar. The vertical array is widely assumed in the algorithms, so it is necessary to study the altitude measurement for the oblique array. The temporal-spatial sequential Maximum Likelihood algorithm is extended to estimate the two dimensional angle for the oblique planar array, which is not only avoiding the affect of the multipath, but also decreasing the mutual coupling by two-dimensional scanning and improving the estimation precision. The altitude measurement based on the oblique line array by the method of altitude diversity is presented.3.Based on the background of altitude measurement for the shipboard radar, by compensating the phase difference among the array manifold in the accumulating time, the influence of ship swaying is alleviated. After the compensation processing, the array super resolution algorithm can be used to compute the angle. The effect of ship attitude angle to angular error measurement is analyzed. The quantitative formulas of error model are constructed by the three-dimension geometry relation between the ship attitude and target position. The research on angle error analysis with dynamic oblique array is benefit for analysis of ship swaying influence.4.The shipboard radar is essential for the distant early warning and tracking, so the further study of the multipath model over the sea is necessary. At present, electromagnetic reflecting characteristic of the VHF band over the sea is seldom, but of the microwave band. The paper proposes the multipath model over the sea, and researches the change of the coefficients of mirror reflecting; diffuse reflecting with the sea state. The conclusion is obtained in some condition that the sea surface can be assumed as smooth and the multipath model in VHF band satisfies the mirror reflecting condition.