Two-way satellite time and frequency transfer (TWSTFT) is general used in high-precision time and frequency transfer

the key equipment of TWSTT system is the modem.Nowadays

a new time transfer modem for TWSTFT has been developed at BIRMM.The direct sequence spread spectrum(DSSS)and binary phase shift keying (BPSK) modulation techniques are used to generate a pseudo-random noise (PRN) signal.A fast Fourier transform (FFT) parallel algorithm is applied to achieve fast acquisition of the PRN modulated receiving signal.A 2

nd

order frequency lock loop (FLL) assisted 3

rd

order phase lock loop (PLL) is designed to keep both of the performance of loop dynamic stress and carrier

phase tracking accuracy

and a 2

nd

order delay lock loop DLL is used to track and measure the code phase.A short baseline TWSTFT experiment was done with two 1.2m very small aperture terminal (VSAT) earth stations and a commercial geosynchronous orbit communication satellite to evaluate the modem's performance.The result shows very low noise with the standard deviation (1) equal to 0.13ns at a 2.5MChip/s code rate.