| Troposcatter trans-horizon propagation is an important approach to realize the microwave band trans-horizon communication and reconnaissance,and it is also an important reason that causes interference between systems sharing the same frequency bands.The layered structure and atmospheric turbulence presented in the tropospheric may cause the reflection and scattering of the radio wave,and these propagation effects can make the radio wave propagate from hundreds up to one thousand kilometers.This propagation is called troposcatter.The troposatter communication based on this propagation mechanism has been widely used in military and civilian communication fields until now own to its far simple-hop communication distance,high reliability,good confidentiality and strong anti-unclear explosion.Hence,further research on the propagation and the dynamic characteristics of the troposcatter trans-horizon propagation has great significance for the design,performance evaluation and short-term prediction of the trans-horizon radio system.Combined with the global troposcatter data bank and the global radio meteorological data provided by the Radio-communication Sector of International Telecommunication Union(ITU-R),the prediction method of the tropocatter trans-horizon transmission loss and the troposcatter parabolic equation method are studied in this dissertation.And on the basis of the measured data,the dynamic characteristics of the troposcatter trans-horizon propagation have also been researched.The main contents and innovations are as follows:1.Based on the in-depth study and the comparative analysis of the three average annual troposcatter transmission loss prediction methods introduced in the ITU-R P.452,ITU-R P.617 and ITU-R P.2001 respectively released by the ITU,the statistical relativities of troposcatter transmission loss as captured by changing frequency,path length,scatter angle and meteorological condition are each analyzed using the terrestrial trans-horizon propagation loss data banks,the average annual sea level surface refractivity and the radio-refractive index lapse-rate over a 1km layer from the surface of the earth released by the ITU.A novel average annual troposcatter transmission loss prediction model is presented.The model is generated by optimizing the correlation coefficients of propagation path conditions utilizing the genetic algorithm.And then,combined with the tropospheric duct anomalous propagation loss prediction model,a new troposcatter trans-horizon propagation prediction method is presented.The prediction results of this new model are compared with other ITU models and the comparisons show that the new model has a better estimated accuracy.2.Based on the in-depth study and the comparative analysis of the troposcatter average worst-month transmission loss prediction results introduced in Recommendation ITU-R P.617 and the troposcatter time percentages conversion of annual statistics to worst-month statistics introduced in Recommendation ITU-R P.841,a novel conversion model of annual statistics to worst-month statistics is presented.T The prediction results of this new model are compared with the Recommendation ITU-R 841,and the comparisons show that the new model not only has improved the estimated accuracy,but also can reflect the meteorological characteristics of troposcatter propagation accurately.Meanwhile,based on the curve graph of the basic transmission loss difference between the average annual distribution and the average worst-month distribution,a new model for calculating the basic transmission loss difference between the average annual and the average worst-month is presented.This model can predict the basic transmission loss difference between the average annual and the average worst-month as captured by changing the average annual sea level surface refractivity,time percentage and path length.The results of this model have good agreement with the curve graph of ITU-R P.617.Hence,this model can substitute the curve graph of ITU-R P.617 and be used in prediction of the troposcatter average worst-month transmission loss.In the end,the applicability of the two new model mentioned above under different propagation condition are analyzed.3.The parabolic equation(PE)method of the troposcatter trans-horizon propagation is in-depth analyzed and researched.Combined with the height and the scatter angle of the common volume,and considering the computational efficiency and computational accuracy of PE by choosing different maximum calculated height and maximum propagation angle,a method to choose the maximum calculation height and the maximum propagation angle of the troposcatter PE method is presented.Finally,using the refractive-index structure parameters obtained by exponential fitting of the measured data in Beijing,the seasonal variation mechanism of the troposcatter propagation loss is discussed from the seasonal variation of the refractive-index structure parameter.4.Based on the transmission loss data collected in the oversea experiment at 14.1 GHz on the area of Yellow sea and Bohai sea of China and the synchronous meteorological data collected from the meteorology grads tower which founded in the transmitter station,the relativities of the transmission loss and the evaporation duct height are analyzed by changing the wind direction,wind speed and the difference of air temperature and sea temperature,respectively.The usability of the coastal meteorological data for the tropospheric microwave trans-horizon propagation is studied.The method for studying the dynamic characteristics of the rain attenuation is firstly introduced in the study of the dynamic characteristics of the troposcatter trans-horizon propagation.Using the measured data,the conditional probability distribution of the fade slope for different attenuation level is analyzed,and the conditional probability distribution and the standard deviation distribution of the fade slope for each attenuation level under different low-pass filter bandwidth are also calculated.According to the analysis above,the stability of the trans-horizon microwave signal under different meteorological conditions has also been analyzed. |
PDF Full Text Request