The Theory And The Technique Of Engineering On Low-frequency Time-code In Time Service System

Several theoretical and practical problems of engineering on low frequency time-code, including the prediction of the summarized range of coverage of low frequency sky-ground wave, the optimizing and upgrading of system configuration parameter and structure, and the design of digital low frequency time-code timing terminal with high precision are researched. The main works and contributions can be summarized as follows:(1) Low frequency timing signal interference of ground waves and sky waves are studied. Based on the classical theory and 3262 project, the reliable calculation of ground wave field-strength and phase delay is presented. By the analysis of the nonhomogeneous of earth magnetic field, low ionosphere electron density and collision frequency with altitude, the nonhomogeneous of electrical characteristic piecewise, and the physical influence of atmospheric refraction, the "full wave theory" can solve the problems on sky-wave prediction. Upon the mathematical model of interference between ground waves and sky waves, the software package for calculation of low frequency radio systems field-strength and phase as well as the timing signal sky-ground wave summarized coverage are achieved. In order to estimate the signal decline caused by the interference of sky-ground waves, a method of the adaptive equalizationis presented.(2) The low frequency sky-wave propagation geometry parameters are deeply analyzed. The model of atmospheric refraction divided into three-segment is introduced into the calculation of geometry parameters, and the mathematical model of atmospheric refraction is extended from 50 km to 90 km in altitude. As well, the boundary among the angle of 'elevation of antenna cutback factor in lighting zone, penumbra zone and shadow zone is calculated, and the dividing value between the big argument and small argument of Airy function is acquired.(3) Based on the analytical theory of communication baseband signal, the spectrum of low frequency time-code signal is deeply studied, and the analytical expression of practical signal is provided.(4) In order to realize the high precision timing in low frequency tune-code timing service system, the combine spread spectrum modulation based on ASK modulation and further the feasible combine modulation scheme are given. The design of correlation reception low frequency time-code receiver shows that using additional spread spectrum can realize the high precision timing up to microsecond.(5) Method of producing pseudo noise code is one of the key techniques of spreadspectrum. In this thesis, the author gives a deep study on the theoretical calculating,parameter designing and implement methods from the point of view of project practice.(6) The orthogonal sampling demodulating technique for low frequency time-code signal is introduced. Based on the theoretical analysis about its principle and implement method, The simulation is tested. Based on the principle of orthogonal demodulating, the digitalization of low frequency tune-code receiver can be realized by the use of DSP, which laid a base for high precise low frequency time-code receiver.(7) The key factor of using DSP to realize the low frequency time-code signal demodulation and process is the design of software algorithm. In the thesis, the synchronizing and demodulating algorithm is adopted, the synchronization as well as demodulation are introduce. It can greatly save DSP resource, and leave space for equalization, filtering and other signal process.(8) To measure the coverage of low frequency time-code signal, a special field-strength meter is developed by ourselves. Several practical tests on reliability andstability have been testified.