| The medium-short wave radio navigation system is a spread spectrum system, which uses the transmitting delay of straight signal to measure and position. The system works on the band of medium-short wave, the great atmosphere noise, the strong near-far effect, sky-wave interference and the multi-path interference make the channel model complex. The realization of receiver is difficulty. It is a new project to develop spread spectrum system for navigation on the mid-short wave band. Especially, it is the key point that to realize the synchronization of the spread spectrum signal and the extraction of observation quantities for the proper working of the whole system in such a complex environment. This paper concern with the relative theory and method of the synchronization of spread spectrum signal, make a comprehensive and deep research on the pivotal technologies such as the synchronization and extraction of observation quantities of the received signal, propose a precept for the acquisition and tracking of the received signal, extracting the observation quantities and methods to solve the near-far effect, sky-wave interference and multi-path interference.The paper is organized as follow:(1) Operation principle of the system and the character of the channelOperation principle of the system and the character of the channel are analyzed, the modulate mod of the emit signal and the choice of the spread spectrum code are introduced, the general blue print of the receiver and the hardware platform to perform the synchronization algorithm is given, the character of the propagation of the sky and ground wave and environment noise are analyzed, furthermore, the interference scenario of the synchronization about the ground wave is considered.(2)The elimination of the strong sky waveThe application of the iterative information transfer algorithm that presented in compile code of the Turbo code in fast acquisition and real time estimation of sky wave is researched; the performance of the application is analyzed in detail and compared with some other acquisition methods.Serial interference elimination method is investigated to eradicate sky wave interference, the method use the eradicate algorithm to remove the strong sky wave from the received signal, and then the performance of the algorithm is researched, where after simulated.(3)The fast acquisition of ground wave in low signal to noise ratioPerformance of the digital DS/BPSK acquisition system is analyzed in detail and the acquisition performance under Doppler frequency shift and code phase error is investigated. A large step code fast acquisition algorithm is put forward, the realization frame and analyzed the acquisition performance of this algorithm are given out. The results of experimentation on the sea show that this method can make the acquisition time decreased and the performance of acquisition improved.(4) Research on ground wave signal trackingThe model of early-late non-correlation DDLL is established, the loop phase discriminator character, steady character, noise and dynamic performance are analyzed, the effect to the tracking of pseudo-code and carrier wave of the. straight signal that produced by the multi-path signal in multi-path environment is taken into account, a method which make use of multi-correlated value fixed length fitting phase discrimination to improve the performance of signal tracking and precision of pseudo-code tracking in multi-path and interferential environment.The influence which engendered by pre-detection integral time, loop bandwidth and signal to noise ratio of the performance of carrier wave tracking is analyzed, the conclusion that frequency and phase tracking are complementary each other is presented, on the basis of the that, the scheme that frequency and phase tracking loop work alternately is brought forward, the scheme is suited to carrier tracking under high dynamic. The effect of the measurement error produced by error source thermal noise and high dynamic of the receiver carrier is analyzed; the best bandwidth of each loop in design is given. (5) Extraction of pseudo-code observation quantities associated with the carrier waveAn algorithm of extraction observation quantities in pseudo-code associated with carrier wave is presented, the algorithm use rough observation quantities extracted from the pseudo-code signal tracking loop to eliminate integral cycle ambiguity, acquire precise observation quantities which small than a carrier wave period extracted from carrier wave tracking loop, average the differences of pseudo-code distant observation in dynamic environment according to the principle that the pseudo-code and carrier wave combined observation quantities could dynamically counteract, consequently, improve the precision of the pseudo-code distant observation quantities used to fix on the complete carrier wave cycle.(6) The realization of the signal synchronization algorithm and the analyze of experimental dataThis paper presents a realization of the synchronization arithmetic, which make use of a complete digital FPGA+DSP mode, gives each algorithm module relative FPGA circuit design and DSP software realization of the synchronization arithmetic, analyzes the signal synchronization experimental data.At present, the synchronization algorithm that been investigated had already been used in practice. Several united experimentations had been carried out on the sea, the results of experimentations show that the receiver can acquisition and tracking the ground wave signal credibly, sky-wave interference are solved effectively; carrier phase measurement work credible, integral cycle ambiguity and cycle skip are cancelled, the validity of theory analysis is validated and design scheme can arrived at the demand of system. |
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