Research On Applications Of Backscatter Ionospheric Sounding Techniques

The ionosphere which is time varying, working as the medium for the shortwave propagation has serious influences on ionospheric sounding techniques and facilities. Real-time ionospheric parameters which represent the state of the ionosphere are essential for the applications of sounding techniques. Sounding techniques and facilities can be applied adequately only after the ionospheric information is accurately obtained. Therefore, obtaining the ionospheric information is the foundation of the applications of sounding techniques, and how to make full use of the radar sounding system is another key issue. Wuhan ionospheric sounding system (WISS) whose operating frequency is3-30MHz has been developed by the Ionosphere Laboratory of Wuhan University since2001. It has big advantages like small in size, low power with strong mobility, etc. Hardware and the corresponding software are updated continuously to meet various demands. It can be used as a powerful tool to make ionospheric sounding, its own characteristics can be applied for many other uses as well, like target reconnoitering, monitoring electromagnetic environment, short-wave channel management, earthquake prediction, etc. In this thesis, research on the obtaining the ionospheric parameters from the backscatter ionograms has been carried out and the inversion algorithm which is fast and steady is proposed. Based on the ionosphere information obtained in that way, HF channel management, application design of WISS, and target location techniques are investigated. Experiments have also been carried out to validate the methods and the practicability of WISS. The main research results are presented as follows:1. The real-time backscatter ionograms are inverted, and the two-dimensional electron density profile along the propagation path is obtained. One of the applications of WISS is backscatter sounding and acquire real-time ionospheric information from the backscatter ionograms. The reliability and practicability of WISS depend on the accuracy of ionospheric parameters obtained from the ionograms. In this paper, a new inversion algorithm using simulated annealing which is a non-liner geophysical inversion method is proposed to inverse the leading edge of sweep-frequency ionogram. Time-consuming and instability of traditional algorithm are conquered. By combing the simulated annealing algorithm with ionospheric model and characteristics, inversion algorithm has been carried out with efficiency and stabilization to acquire the real-time ionospheric parameters(critical frequency, peak height of electron density, and semi-thickness). The initial guess of the ionospheric parameters are no longer restricted and the leading edge data plays a role of input data. Furthermore, the difference between the input data and real data is considered to avoid the error caused by input data. The ionospheric information along the propagation path can be acquired from one backscatter ionogram by dividing the leading edge into several segments. Inversion results can be obtained within several minutes thus makes the algorithm practicable. This algorithm has been validated on real-time data. The results present that this method can give good results within an acceptable range of error for most daytime and nighttime data.62.97%of the inversion results accord with the vertical data, that is, the error of critical frequency is less than0.5MHz. Up to85.2%is less than1MHz. As the special model which represents the inhomogeneity or irregularity is not added to the ionospheric model, the inversion results during sunset have big errors. The two-dimensional electron density profile presents that the electron density varies with latitude obviously.2. Frequency diagnosis and management system based on WISS has been designed to supervise the HF communication frequency and short-wave propagation channel. This system not only meets the need of daily use, but also contents the wartime. Detailed descriptions of application objective, national composition, system index, and application modes are presented, thus each capability of WISS can be performed adequately. This system has been tested through experiment on the engineering realiability of the optimum frequency selection by short-wave communication. The results present that this system is absolutely reasonable as the two short-wave radio stations have acquired successful communication quality.3. Based on the work of WISS, single-static and multi-static location techniques have been investigated.HF radio-wave propagates in a special way which leads to evident differences between this single station and the traditional ones. The major reason is that signals from the targets are reflected by ionosphere thus trapped by the receiver. In this thesis, working principle of this single location system is presented. A new technique is presented to locate over-the-horizon targets. This technique combines passive mode with active mode in which the arrival angle can be acquired in a passive way and the distance of the target can be obtained in an active way. Compared to traditional systems, it needs to monitoring ionosphere at first, and then trap the signals emitted by targets. Based on this point, experiment has been arranged to test and verify the realizability of the system.The high frequency multi-static location system which uses ionospheric sounding system as its core portion works in an active way to locate over-the-horizon target by ionospheric reflection. Combining sky-wave propagation characteristics with Bowring formula which is an inverse solution of geodetic problem, a location model is advanced to obtain target’s coordinates and localization precision under sky-wave condition. Given the single-path propagation mode, geometric dilutions of precision (GDOP) under different station deployments are obtained with the help of computer simulation. The results present that the model is well realizable in engineering with high precision, and certain deficiencies of traditional models are conquered. High precision still can be obtained even near the baseline which is the disadvantage of traditional stations, and earth curvature is considered. Afterwards, coordinate registration(CR) index which is the key issue has been analyzed with the help of computer simulation. The results presents that critical frequency and electron density peak height have serious influences on CR index, while the influences caused by the semi-thickness can be neglected. Influences from high-angle rays are greater than low-angle rays, and higher frequency causes much more influences on CR error. The error caused by the ionospheric parameters can leads to several tens to a hundred kilometers ground range error. Therefore, lower frequency and low-angle ray mode are prior during practical sounding, and accuracy of ionospheric parameters must be improved.