Improved two-way satellite time and frequency transfer with Multi-GEO in BeiDou navigation system

Due to the limitation of Geostationary Earth Orbit (GEO) satellite resources, only very few TWSTFT links with parallel counterpart operating via different GEO satellites between two ground stations are available. There are 5 GEO satellites in BeiDou Navigation System, which enables an improved TWSTFT with Multi-GEO. Two algorithms are put forward to take the advantage of Multi-GEO links to improve the performance of time and frequency transfer respectively. The dynamic weight average algorithm based on a newly proposed dynamic time variance aiming at improving the performance of time transfer is shown to be able to reduce the dynamic time deviation of the two-way measurement result to 1/$\sqrt N $ dynamic time deviation of the single link using N-GEO links, if each link has similar level of link noises. The algorithm of cancelling out the link noise Allan variances with 2-GEO links is expected to obtain the Allan variance of clock difference between two ground stations. The experiment results using 2-GEO links in BeiDou Navigation System show that the dynamic weight average algorithm can improve the dynamic time deviation of the two-way measurement to around 0.6 of that of the single link in an average concept. And the estimated Allan deviation of the clock difference using the algorithm for cancelling the noise Allan variances of both links can reach 7×10−12(τ−0.65) for the averaging time 1–105 s which is within the same order of magnitude as the estimated Allan deviation based on carrier Doppler measurement for the averaging time 10 s–104 s, despite the noisy links with the Allan deviation follows 3 × 10−10(τ−1).