| With the advancement of science and technology,GNSS technology plays a vital role in social life and various scientific fields.However,the measurement signal of GNSS receiver will be affected by various error sources in the process from transmitting to receiving;many of these errors can be eliminated by modeling methods,but there are still some errors that will be added to the signal,which will have a certain impact on the data processing results.Therefore,how to perform noise reduction on GNSS coordinate time series with non-linear and non-stationary characteristics is a hotspot of current research.On the basis of the existing Empirical Mode Decomposition(EMD)method,this paper makes some improvements to the existing problems of the algorithm,and applies the improved empirical mode decomposition algorithm to the noise reduction of the coordinate time series of the base station of the Crustal Movement Observation Network of China.The main research contents of this article are as follows:(1)The basic theory of EMD is introduced systematical y,which lays a solid theoretical foundation for the research and analysis of the whole paper.This paper first discusses three typical time-frequency analysis methods,then introduces the relevant concepts and basic principles of EMD method in detail,comprehensively describes the superior properties of EMD method,and final y summarizes the research status and main problems of EMD method.(2)In order to solve the problem of modal aliasing between signal and noise in the process of EMD noise reduction,this paper deeply studies the characteristics of the components of the Intrinsic Mode Function(IMF),and proposes an Improved EMD noise reduction method to reduce modal aliasing.This method reconstructs the second IMF to the boundary IMF component obtained by EMD decomposition,and then decomposes the reconstructed signal to the next EMD to obtain the "real" signal;repeat this step three times,and final y accumulate all the low-frequency signals to achieve the purpose of noise reduction.Three sets of simulation data are used to verify the feasibility of the method.The experimental results show that the method is better than the traditional EMD method in noise reduction.(3)In order to better eliminate the noise attached to the GNSS signal,this paper proposes an improved EMD noise reduction method by comprehensively analyzing the statistical characteristics of noise.This method uses EMD decomposition to divide the signal into two parts: low-frequency signal and high-frequency noise;then randomly shuffles the high-frequency noise part twice and accumulates with the original high-frequency noise to find the average value;final y,it accumulates with the low-frequency signal A new signal is formed to perform EMD decomposition again to extract a useful signal.Two groups of simulation data with different signal-to-noise ratios were added for verification.The results show that when the signal-to-noise ratio is high,this method will get better noise reduction results.(4)In order to solve the problem that the K value of the boundary Intrinsic Mode Function determined by the correlation coefficient(p)criterion is not accurate,two new methods are proposed to distinguish the K value of the boundary eigenmode function.One method is to automatical y determine the K value of the boundary IMF by using the product index of average period and energy density,reconstruct the IMF component of highfrequency noise,and then subtract the reconstructed noise from the original signal,so as to achieve the purpose of noise reduction.Another method is to use the composite evaluation index(TD)to consider the curve smoothness(r)and Root Mean Square Error(RMSE).When the TD reaches a certain threshold,it is determined as the boundary IMF component and the output K value.Through the verification of simulation data,the experimental results show that the two new methods given in this paper are more accurate than the K value of the boundary IMF function determined by a single correlation coefficient index,and can identify the boundary point between noise and signal more reliably,making the noise reduction effect more reliable.(5)In this paper,the coordinate time series data of of the reference stations of the Crustal Movement Observation Network of China(CMONOC)are reviewed,and the characteristics of GNSS time series signals are summarized.Finally,all the improved EMD algorithms are applied to the noise reduction analysis of the coordinate time series of the reference station of the CMONOC.The experimental results show that the improved methods have better denoising effect,overcome the problems of traditional EMD denoising methods,and can further improve the accuracy of GNSS signal. |
