Research On Time Synchronization Technology Of Satellite Network

Satellite networks not only provide high-quality communication services,but also achieve full coverage of the entire surface,and are largely immune to natural disasters.These advantages can effectively overcome the limitations and shortcomings of terrestrial communication networks.Due to the shortcomings of the single satellite in the satellite network,such as high cost and poor network robustness,satellite formation has emerged.The satellite formations cooperate with multiple small satellites to make up for the shortcomings of a single satellite network.Inter-satellite coordinated control,data transmission,and autonomous flight are the basis for the smooth operation of the entire satellite formation.To overcome these technical problems,the time synchronization of the satellite network must be realized first.Therefore,this paper studies the time synchronization problem of satellite networks.In view of the complexity of the time synchronization process of satellite networks,this paper presents a way of time synchronization of satellite networks by using stratification and synchronization ideas.Layering mainly completes the selection of the network synchronization reference center and classifies the entire network according to the hierarchical structure;synchronization mainly completes the time synchronization of the entire network,but the problem of time synchronization between layers needs to be solved.For the inter-layer synchronization problem of satellite networks,firstly,a broadcast-based single-recovery inter-layer time synchronization method is proposed for satellite formation scenarios that can ignore the link length difference.By comparing the time information between the satellites to be synchronized,the satellites to be synchronized indirectly obtains the clock difference from the reference center.Because of the reduced information interaction between the reference center and the satellite to be synchronized,the time synchronization can be achieved using fewer on-board resources.Then,for the shortcomings of the uncertainty of the receiving end(such as the queuing process of receiving information)in the receiving process,the disturbance error elimination of the broadcast-based singlerecovery inter-layer time synchronization method is proposed.Compared with the basic inter-layer synchronization method,this improved method does not need to completely repeat the time synchronization process,and can achieve the goal of obtaining the clock difference multiple times with fewer information interaction times,and the clock difference can be obtained by multiple measurements.Effectively overcome the accuracy error caused by the uncertainty of the receiving end.Finally,the performance of the two basic inter-layer time synchronization methods and the broadcast-based single-recovery inter-layer time synchronization method in the time synchronization process,such as the amount of packet transmission and the time required for completion time synchronization,are compared.The broadcast-based single-recovery inter-layer time synchronization method all have good network performance.For the selection of time reference center in the stratification process of satellite networks,this paper analyzes the two scenarios of ignoring the difference of propagation delay and considering the difference of link propagation delay.For the scenario of ignoring the propagation delay difference,the two indexes of synchronization time and synchronization error are directly related to the number of layers.Therefore,with the goal of minimizing the minimum number of synchronization layers and the minimum number of average synchronization layers,the selection of time reference center method with the minimum number of synchronization layers in the whole network and the selection of time reference center method with the smallest average synchronization layer are proposed.The simulation results show that the above two methods effectively reduce the synchronization layer of the network compared to the synchronization method of the fixed time reference center.Thus they reduce the synchronization time and synchronization error of the network.In view of the scenario of link propagation delay difference,since the synchronization time has a closer relationship with the link delay of the network,the number of layers should not be considered as the only consideration.Therefore,in this paper,with the goal of the shortest synchronization time and the shortest average synchronization time,the selection of time reference center method with the shortest synchronization time and the selection of time reference center method with the shortest average synchronization are proposed.The simulation results show that the two methods can effectively reduce the synchronization completion time of the network compared to the synchronization method of the fixed time synchronization center.