Target Information Acquisition In Combined Skywave And Surface Wave Shore-based High Frequency Radar

High Frequency(HF)radar is usually used to detect ships or low-altitude aircrafts beyond the horizon.Traditional single-mode HF radars include High Frequency Surface Wave Radar(HFSWR)and Over the Horizon Radar(OT HR).Among them,HFSWR uses a single ground wave propagation mode and its detection range is about 370 km,which is limited by ground wave attenuation and cannot detect longer-distance targets;OTHR uses a single skywave propagation mode and its detection range is about 800km?3500km with a close-range detection blind zone,which is affected by the ionospheric state and free space attenuation.It can be seen that neither of them can meet the needs of simultaneous monitoring targets from near shore to mid-to-far sea area.In order to achieve this demand in shore-based HF radar systems,this thesis proposes a new type of shore-based HF radar that uses multiple combined propgation modes of ground wave and skywave to detect targets,called Skywave and Surface Wave Combined Shore-based High Frequency(CSSS-HF)Radar,hereinafter referred to as multi-mode high-frequency(HF)radar and focuses on the method of target information acquisition in the radar system.Multi-mode HF Radar adopts the mode that the transmitting station and the receiving station are at the same position,and simultaneously radiates HF vertically polarized electromagnetic waves to the ionosphere and sea surface,and uses multiple combined propagation modes of sky wave and ground wave for target detection.At this time,regardless of the ionospheric startification,there are four combined propagation modes of skywave and surface wave in CSSS-HF Radar: the mode with ground wave transmitting path and ground wave receiving path(can be used to detect Near-shore targets within about 370km);the mode with ground wave/ sky wave transmitting path and sky wave / ground wave receiving path(can detect targets in near and middle sea areas within about 600km);the mode with sky wave transmitting path and sky wave receiving path(can detect father targets within a range of 300?1000km).Under appropriate system parameters and ionospheric conditions,CSSS-HF Radar can completely cover 0?1000km that great farther than HFSWR and without close-range blind zone in OTHR system,which makes up for the shortcoming of traditional single-mode HF radar.Compared with the traditional mono-static shore-based HF radar,CSSS-HF Radar system using multiple propagation modes to detect targets and has two main charateristics: first,the detection range of the new radar system is large,and the detection range is related to the system frequency and ionospheric state;the second is that the coverage of different propagation modes overlaps,which will cause multiple propagation mode echoes on the same target,or the same echo may come from multiple targets.Combining these two characteristics,in view of the related problems of target information acquisition in the CSSS-HF Radar system,the thesis has carried out the following four main contents research:First,Multi-mode combined system detection model establishment.When obtaining the ground distance information of the target,it is necessary to convert the target echo distance(echo delay multiplied by the velocity of radio wave)into the target ground distance according to the detection model of the corresponding propagation mode of the target echo.In the thesis,a multi-mode detection model is established for sea-surface targets and air targets,and the conversion formula between the echo distance and the ground distance of each propagation mode is derived.In order to preliminarily verify the established multi-mode detection model,this paper proposes a method to verify the detection model,which is to use the multi-mode echo of the known information target to estimate the ionospheric height,and then use the measured data from the ionospheric detection station to verify the ionospheric reflection height estimate.Second,detection capabilities analysis.The detection range of CSSS-HF Radar system is researched,and the detection range of each mode under different frequencies is analyzed to provide a priori information for the frequency selection of the system and the subsequent multi-mode target positioning method.In order to obtain a more accurate system detection range,this thesis further studies the influence of the ionosphere on the detection range of CSSS-HF Radar system.Third,the distance information acquisition of multi-mode target echo.CSSS-HF Radar uses the sky/ground estimating-ground/sky receiving and the sky estimating-sky receiving mode to detect targets in the middle and far sea areas.The echo distance of these two modes can reach about 1000 km,the method of obtaining an unambiguity detection range of about 1000 km by expanding the signal pulse period will seriously reduce the duty cycle of the signal,thereby affecting the performance of the system.Therefore,the single station system needs to design the detection signal without reducing the signal duty cycle,wh ich will cause range ambiguity problems of the long-distance target.Then,the acquisition of the correct echo distance needs to first solve the problem of range ambiguity.The thesis proposes two methods to solve the range ambiguity: one is to use the cross-correlation characteristics of quadrature phase coding to construct a cyclic delay echo-mode judgment filter to process the echo,the output of different time delay filters is the echo of different distance ranges.The other is the Alpha-phase modulation method,which phase-modulates the original transmitted signal,then the range ambiguity target will be moved to other frequency bands.This method increases the unambiguity distance range by sacrificing the unambiguity Doppler range.Fourth,target motion information estimation.Because there are multiple propagation modes in this new radar system,estimating the motion information of the target requires judging the mode of the target echo and then fusing the multi-mode echo of the target.The thesis proposes a multi-mode target tracking method,which directly estimates the target motion information from the target's multi-mode echo,and completes the echo mode judgment and multi-mode echo fusion processing at the same time.The multi-mode target tracking method establishes a multi-mode measurement model,calculates the correlation hypothesis of echo and propagation mode and target,and finally obtains the unique track of the target in the ground coordinate system,so as to correctly estimate the distance,velocity,azimuth and other movement information of the target.The thesis first proposes a multi-mode tracking algorithm based on the Bayesian framework.Then,to further improve the algorithm's computational efficiency and tracking performance,an improved multi-mode target tracking algorithm with the detection range as a priori information is proposed on the basis of this algorithm.In summary,this thesis establishes the detection model for CSSS-HF Radar system,analyzes the detection range of each mode and the influence factors of the ionosphere and system frequency,solves the range ambiguity problem to obtain target echo distance information,and proposes multi-mode target tracking method to obtain the target motion information.Finally,the target information acquisition without considering the actual ionospheric conditions and clutter interference has been preliminarily completed,laying a foundation for the follow-up research of multi-mode high frequency radar.