Research On Skywave Over-the-horizon Radar Coordinate Registration And Multipath Data Process

The Skywave Over-the-horizon radar explores and detects distant targets by taking advantage of reflective nature of ionosphere. But the return echos from targets is a false distance, which is the product of time delay and velocity of light. In order to get the real location, it’s necessary to convert the the group delays and azimuths (i.e., slant coordinates) to an estimate of its location (i.e., ground coordinates), the process is known as Coordinate Registration. The skywave over-the-horizon radar, which works at the high frequency band, has a strong relationship with the skywave path or the ionospheric propagation path. The process of coordinate registration is complicated.The traditional coordinate registration technique use ionospheric modeling describing the ionosphere in the radar working area, then utilise ray tracing technique calculating the group path and azimuth of the radar radio wave. Consequently ionospheric changes have a great influence on the over-the-horizon radar coodinate registration. The disturbance of ionosphere have distinct types and different levels. The analysis of the influence of ionospheric disturbance is a crucial technique and kernel content of the over-the-horizon radar coodinate registration. Besides the multipath propagation in ionosphere yields multipath target returns from one target in certain frequencies and circumstance. It’s hard to distinguish and determine the propagation mode, which brings in difficulties in the radar operation, brings down the accuracy of coordinate registration, brings about the errors in target location. In view of the above questions, this thesis presents the basic theory of ionospheric radio propagation and ionosphere modeling, then the problem of skywave over-the-horizon radar coordinate registration and multipath propagation is deeply discussed. The influence of ionospheric diturbances in different circumstance on coordinate registration are studied respectively. Generally speaking, the conclusions in this paper can be mainly outlined as the following:1. A new statistical ionospheric model is investigated using bootstrap method based on the real data from ionospheric sounders in this paper. The effects of random disturbance in the ionosphere on OTH radar coordinate registration are discussed using raytracing based on the proposed model. Additionally, maximum likelihood estimation (MLE) is employed to reduce the random disturbance of ionosphere, and furthermore the result from minimum variance estimate (MVE) method is compared. As the result shows, the influence of ionospheric random disturbance on high-angle rays is more than the low-angle rays. The errors introduced by ionospheric random disturbance vary from 5km to 30km according to different ground distance, which generate deviations to the real PD conversion curve in the PD conversion figure. In the 80% ground range of the simulation, the average absolute error of MLE is smaller than MVE, which indicate that the MLE has a better effect on correcting the ionospheric random disturbance compare to the MVE.2. Aiming at the different circumstance in ionospheric tilt, the influence of ionospheric tilt on the coordinate registration is analysed in detail based on the ionospheric modeling and numerical ray tracing. As the results indicate, when the smaller tilt exists, the effect of ionospheric tilt on the PD conversion is small, great error will not be introduced. For example, when the tilt angel is 1°, accordingly the groud distance is from 600km-1300km, the average error is 5km. Only when the positive tilt exists, the high-angel rays have about 10km error. When it comes to the larger ionospheric tilt scenario, the result is different and the error becoms larger. The results of simulation show, when the tilt angel is 5°and groud distance is from 600km-1300km, the average error is about 50km. For low-angel rays, the effect of the negative ionospheric tilt is greater than the positive; for the high-angel rays, the effect is on the contrary.3. Based on the Travelling Ionospheric Disturbance model and ray tracing technique, the influence of Travelling Ionospheric Disturbance on the coordinate registration is fully investigated. The results turn out, the effects of small scale TID on coodinate registration is insignificant. There is only 0.1km changes compared to the original coordinate registration. So it can be ignored. The medium scale TID has a larger impact on high-angel rays than low-angel ones, the high-angel rays deviate roughly 10km to 20km in coordinate registration. The large scale TID has a great influence on coordinate registration. In some ground distance range the original coordinate registration become invalid. Furthermore the high-angel rays screening, the radio waves focusing and the radar dysfunction will happen. The intricate propagation mode is hard to distinguish. If the direction of radio waves is the same with the TID, the impact of TID on the azimuth conversion is little. When the frequency is 13MHz, the average error is 0.07°; when the frequency is 17MHz, the average error is 0.05°. So the influence of TID on the azimuth conversion is egligible.4. The group path of O-wave and X-wave is calculated based on the ionospheric Quasi-Parabolic Segments model and analytic ray tracing technique. The difference of O-wave and X-wave time delay is studied from the theorical point of view. The potential impact of the ionospheric reciprocity and geomagnetic split on the coordinate registration is fully analysed.5. Focus on the multimode propagation of the over-the-horizon radar target detection, the Viterbi algorithm based on the hidden markov model is advanced. The propagation mode is set as the state sequence, while the slant range is the observation sequence. Mode recognition is carried out at each revisit of radar. A Bayes linear model is used for estimation of target positioning at each revisit based on it. Through the Monte Carlo simulation, the method is of valid at different radar operation frequency and under the circumstance of ionospheric disturbance. The accuracy of Bayes estimation is about 10km improvement than the MVE.6. The current multipath probabilistic data association algorithm is improved based on International Reference Ionospheric Model 2001 and three dimension ray tracing technique. A new skywave over-the-horzion radar data processing and coordinate registration algorithm is provided which is more near to the real ionosphere. The new algorithm is evaluated by the over-the-horizon radar multipath simulation data. The results turn out that, when the frequency is 12MHz and the ground range is from 1500km to 2000km, the average error is 10.46km; when the frequency is 22MHz and the ground range is from 2500km to 3400km, the average error is 2.75km. Therefore the new coordinate registration and multipath probabilistic data association algorithm, which is based on the real ionosphere has a good precision in the over-the-horizon radar target location.7. Based on the theortical analysis and numerical simulation of the above content, an engineering-oriented proposal for over-the-horzion radar coordinate registration is forwarded. This proposal is established on the Single-site Oblique, Backscattering Sounding System. The real data from the Single-site Oblique Backscattering Sounding System combine with the ionospheric data assimilation and modeling technique, which integrate the coordinate registration and radar multipath data processing, will greatly improve the accuracy of target localization and tracking.This paper indicates that ionospheric disturbance, no matter what kind of disturbance, real-time one (ionospheric random disturbance) or systematic one (ionospheric tilt and TID), have a significant impact on the skywave over-the-horizon radar coordinate registration. The combination of coordinate registration and radar tracking system forward here can either be part of the over-the-horizon radar frequency management, or be part of the radar data processing and integrate with radar localization and tracking system, which could improve the accuracy of over-the-horizon radar data processing and offer new thoughts for radar multipath data handling.