Clock Synchronization Research For Distributed Micro-Spacecraft System

Supported by the Major State Basic Research Development Program of China (973 Program)—"Study of new concept, new mechanism of micro-spacecraft", inter-satellite onboard clock synchronization methods for Distributed Micro Spacecraft System (DMSS) are deeply studied in this paper.As for DMSS, inter-satellite onboard timing and clock synchronization are critical. DMSS requires many micro-satellites to constitute formation flying system or satellite network to perform a common space task, like coordinated observation or information exchange etc., there inter-satellite timing and synchronization information play an essential role for ensuring the correctness of these space tasks. Up to now, few papers are concerned with timing and synchronization for DMSS. This paper proposes a framework of solution for DMSS clock synchronization problem systematically, which includes research of micro inter-satellite relative timing and synchronization tool, Global Positioning System (GPS) timing method, inter-satellite communication protocol and inter-satellite clock synchronization method. This paper analyzes various factors which influence timing precision, and proposes solutions of improving the precision, thus it provides both practical and theoretical foundation for high precision of inter-satellite timing and clock synchronization. Based on the analysis of current abroad inter-satellite relative state measurement equipment, this paper proposes micro inter-satellite relative timing and clock synchronization equipment, General Cell Synchronization System.Based on principle of inter-satellite time information exchange, binding inter-satellite radio frequency (RF) communication technology and onboard micro electronic system technology, this paper proposes the architecture of inter-satellite synchronization system, which involves signal processing unit, RF transceiver and GPS receiver, and realizes software/hardware co-design and validation for this synchronization system. This paper develops a demonstration system with Commercial off-the-shelf (COTS) technology, which has compact circuit architecture instead of multiple modules architecture used by traditional RF communication tools. By building testbed, this paper confirms validity and feasibility of this demonstration system's functional modules.Based on analysis of GPS time transfer technology, this paper proposes a set of reasonable high precision inter-satellite timing scheme for DMSS. To improve inter-satellite timing precision, this paper analyzes GPS pesudorange difference method and GPS satellites selection optimization method, and influence of receiver oscillator's frequency stability on inter-satellite timing and synchronization precision. By analyzing error characteristics and measurement method of oscillator, this paper chooses temperature compensate crystal oscillator (TCXO) as clock source of inter-satellite relative clock synchronization system, and proposes to build accurate clock bias model for the clock source. Based on the clock bias model, this paper applies Kalman filter to process observed GPS timing data to validate the improvement of relative timing precision among satellites.Based on research of present inter-satellite communication protocols, inter-satellite link topologies and multiple access methods, this paper proposes appropriate inter-satellite link protocol to support timing and synchronization service among satellites. This paper analyzes criterions of data link layer and physical layer of CCSDS Proximity-1, and discusses its timing correlation method. By investigating master/slave topology, this paper proposes automatic master satellite seletion method. On investigation of multiple access technologies, this paper analyzes MAC protocol of IEEE 802.11, especially Point Coordination Function (PCF), and proposes methods to decrease average access waiting time, considering inter-satellite clock synchronization service.By comparing traditional two-way time comparison method with clock synchronization method in distributed computer system, this paper proposes to introduce probabilistic synchronization idea to inter-satellite synchronization field, so as to solve the inter-satellite onboard clock synchronization problem when DMSS is devoid of external time reference signal, like GPS signal. This paper analyzes inherent meaning of probabilistic clock synchronization with geometry method, and then it proposes an improved inter-satellite probabilistic onboard clock synchronization method. The method starts with difference between uplink and downlink signal delay to analyze how to increase timing precision, and applies Bayesian approach to compute satellite clock bias so as to calibrate it when DMSS is on orbit flying. Based on experimental results obtained from measuring signal transmission delay distribution with the demonstration system of General Cell Synchronization System, this paper systematically deduces arithmetical expression of clock reading error, probability of success synchronization and timing precision of the method.