Inter-satellite communication and ranging link assignment for navigation satellite systems

Inter-satellite links improve the performance of global navigation satellite systems (GNSSs) via communication and ranging. When there are limited facilities and links, a key challenge involves assigning links for the downlink of telemetry data in time and effective ranging. We describe this problem and propose a corresponding link scheduling method consisting of three steps. A genetic algorithm was used to identify optimal downlink routes for all non-visible satellites in the first step. The optimization of minimizing timeslot delays usually has multiple optimal routes. The optimal routes were selected to expand to a superframe's length, which constituted a downlink route scheme for the superframe. The position dilution of precision (PDOP) of ranging links was limited through satellite selection in the second step. Four visible satellites with minimal PDOP were selected and scheduled with idle timeslots. The PDOP decreased with increasing links, and its upper limit was therefore determined by the selected satellites. The final step was to schedule idle timeslots for visible satellites unless the requested link number was met. To test the feasibility of the proposed method, the link assignment was implemented for 10,080 superframes of a typical GNSS constellation. The final link assignment enabled all satellites to transmit telemetry data back to the facility with a delay of no more than 4 timeslots (with one ground facility tracking), and more than 10 links were obtained with PDOPs approaching the minimum. These scheduling results confirm the utility of the method.