A FFT-based parallel acquisition algorithm for Direct Sequence/Spread Spectrum radio navigation system is studied to solve long PN code acquisition using serial method. Frequency domain analysis of FFT to acquire signal is applied in this paper and state transfer function is used to derive the expression of average acquisition time. Theoretical analysis shows that FFTbased acquisition algorithm can efficiently solve the correlation peak fade caused by the Doppler shift in high dynamic receiving and transform two-dimension search of the code phase and frequency in one acquisition period, thus resolving the fast acquisition for PN code in high dynamic situation. The results shows that the acquisition time is shortened up to one order in high SNR compared with serial acquisition method.

Precise time synchronization is the precondition of precise navigation in radio navigation system, high frequency stability cesium atomic clock is used in this system as frequency reference and GPS time receiver is used. To solve the problem of system synchronize error caused by low quality of GPS time receiver in short time, shift average method and Kalman filter is introduced in this paper. The results of simulation indicate that Kalman filter can decrease the error caused by GPS time receiver jitter effectively.

Time service is the precondition of precise navigation in radio navigation system, the cesium atomic clock of high stability is used in this system as frequency reference and GPS time receiver is used. To solve the problem of system synchronize error caused by satellite movement , Doppler frequency shift, ionosphere refraction, etc. sliding window method and discrete wavelet method are introduced in this paper. The results of calculation indicate that discrete wavelet transform can decrease the error caused by GPS time receiver j itter effectively.The main factor to influence synchronization between time transmission receiver and ground atomic clock is: Doppler frequency excursion, ionosphere, troposphere and the receiver itself [2][3][4] . So it has time transmit error in the receiver. Discrete wavelet is used to deal with the time difference measured by the time counter in this paper, using signal decompose and destruction treat the difference between atomic clock and time transmission receiver to decrease system position error caused by GPS time receiver. II. SYST EM CONSTITUTION Ind ustria l Computer IOMllz Cesi um atomic clock Sig na l ge nerator l PPS IPPS Cou nter PM668 1 Hig h speed compara tor GPS time receiver , --_ _-----, IPI'S , -----, Fig. I Structure of time synchronization partTime synchronization trigger is a tim e synchronization equipment in the navigation system, the equipment is connected with the GPS time receiver and atomic clock, the main function of the equ ipment is to choose time synchronization standard, the trigger generate two channel signals of the same frequency and steep rising edge pulse through trigger circuit according to the PPS from time synchronization standard. One of the channel is connect to atomic clock for synchronization, the other is connected to the high speed counter in order to measure the synchronize difference between the atomic and the time standard.Because each synchronized atomic clock and GPS receivers are at the different location and work in the different environment, after the GPS receiver time transferring, the The synchronization part structure in radio navigation system shows in Fig.l. The GPS receiver PPS signal is used as time standard in the synchronization part and the GPS pulse signal is disposed to a steep rising edge pulse through the synchronization trigger, then connected with atomic clock synchronization port, providing precise time standard to the system. Sate llite /Ĩ . INTRODUCTIONThe position of receiver is derived by its distances to different transmitting stations and the distances are measured by the delays between the transmitting stations and the receiver of the radio signals in radio navigation system. In reality the delay times are derived by the signal phase difference after propagation from different transmitting stations. Radio wave transmits at the speed of light that means one nanosecond synchronization error in transmitting station will equivalent to 0.3 meter distance measuring error in the receiver. Therefore time synchroniz...

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