Inter-satellite link assignment for the laser/radio hybrid network in navigation satellite systems

The Ka inter-satellite link (ISL) has been applied in the new generation of the BeiDou Navigation Satellite System. The next move is to supplement laser ISLs to improve the communication rate and ranging accuracy. A key challenge of the hybrid inter-satellite link network is how to properly schedule the two kinds of inter-satellite links to achieve better system performance, such as robustness, short transmission delay, and the position dilution of precision (PDOP). To solve this problem, we propose a compatible scheme of the laser/radio inter-satellite link network. The topology of the laser network is optimized based on maximum connectivity and minimum time delay through a multi-objective simulated annealing algorithm. Thereafter, a two-step algorithm optimizing the downlink delay and the PDOP is presented to arrange radio inter-satellite links. The proposed method was tested in a simulated case of the BeiDou Navigation Satellite System. The optimized topologies of the laser network reached the upper limit of connectivity, which was 2 in the simulated case, and downlink delay less than 0.3 s. Compared to the radio inter-satellite link network, the hybrid one enabled higher throughput of laser satellites (more than 125 Mbps), less downlink delay (less than 10 s), and smaller average weighted position dilution of precision (WPDOP) of laser satellites (less than 1.5 for IGSOs and less than 1 for MEOs).