Key Techniques Of Skywave Positioning For Enhanced Loran System

Navigation system has been providing convenient navigation services for human beings since its birth.Loran navigation system,as a ground-based wireless system is independent of Global Navigation Satellite System(GNSS),has already met the needs of modern navigation after upgrading to the Enhanced Loran system.Limited by the distribution of stations,Loran groundwave signal can only cover the coastal surrounding areas of stations,while the inland provinces cannot receive it.On the contrary,Loran skywave signal,with the performance of long-distance spread and small amplitude attenuation,can greatly enhance the application range and flexibility of Loran system.Therefore,this dissertation will research the key techniques of how to use the Enhanced Loran skywave signal to position.The main contents and contributions are summarized as follows.1.A high-precision separation algorithm of Loran sky-ground-wave signal is proposed.Aiming at the problem of low resolution and large error of earlier algorithms,a new algorithm based on weighted Fourier transform and relaxation is proposed.The algorithm transforms the problem of signal separation into the optimization of signal amplitude and delay estimation,and obtains the results by loop iteration and direct calculation.Experimental results show that the algorithm proposed in this dissertation has an error of only 0.1μs.Under the same conditions the existing algorithms have an average error of 3-5μs.The new algorithm increases the separation accuracy of Loran sky-ground-wave signal.At the same time,the dissertation also gives the results of the algorithm used in the measured signal.It is proved that the algorithm is not only effective for simulation but also measured signals.2.A simple algorithm for estimating the ionospheric time delay of Loran skywave signal is proposed.The existing algorithms for estimating the time delay of skywave passing through the ionosphere are mostly based on wave-hop or waveguide theory.The calculation process is complicated.To solve this problem,this dissertation presents a cosine model which is easy to calculate by using a semi-measured model.The model combines ionospheric characteristics and change regulation of skywave ionospheric delay.The experimental results show that the delay error of model calculation in a single day is less than 1μs in the stable period of day and night.It can correct 90% of the ionospheric delay of skywave.The model error increases with the use of the same set of parameters.But different seasons have no effect on model error.In addition,this dissertation also gives suggestions for broadcasting the ionospheric parameters required for the model.The ionospheric parameters can be broadcast by modifying the data channel message structure without changing the hardware facilities of the Enhanced Loran system.3.The algorithm of neural network is also applied to the Loran system to enhance the separation of skywave signals in low SNR and predict the time delay of skywave passing through ionosphere.This dissertation proposes an algorithm on improved BP neural network to solve problems caused by the limited precision of mathematical model.The algorithm improves multi-layer BP neural network through data enhancement,regularization and error function optimization.Therefore,problems such as slow learning and over-fitting are avoided.Experiments on separation of sky-ground-wave signal demonstrate that the presented algorithm still has the accuracy of 1μs in the receiving environment of 0d B or-5d B SNR.For the time delay estimation of skywave passing through the ionosphere,the improved BP neural network algorithm also has a prediction accuracy of less than 1μs in a day.4.An algorithm for joint positioning using Loran skywave and groundwave signals is proposed.The Loran system is based on the hyperbolic positioning principle.It needs to receive the groundwave signal of station chain to position.This dissertation proposes a joint positioning algorithm for Enhanced Loran system based on time of arrival.When the groundwave signal cannot be received,the skywave signal is used for positioning.The algorithm does not require complete Loran station chain signal.It only needs to receive signal from any three or four stations to position.The experimental results show that the algorithm uses skywave signal to solve the problem of being unable to position far away from the station.At the same time,increasing the available skywave signal can also improve the positioning accuracy.This dissertation also receives the measured signal to position,which shows the effectiveness of the algorithm.