## 2 System Design  
The proposed system is a Master/Slave architecture, with the aim of the system to synchronize the VC-OCXO with a remote reference clock. GNSS-like technology is adopted as a basic element to establish the communication network. The master clock is considered as the reference clock and is assumed absolutely accurate and stable. It could be situated either on another satellite or at the ground station. For the sake of simplification, we call it the master satellite in this paper. Figure 1: An example of connectivity with three slave satellites and a master satellite. The slave satellite carries an on-board VC-OCXO as its frequency source. More than one slave satellite may be employed in this system, which means it has a centralized topology. As the STDD & FDD configuration is employed, each slave satellite and the master satellite simultaneously intercommunicates with each other in defined time slots in terms of the CDMA & frequency division multiple access (FDMA) channel, which is different from the PTDD and broadcasting scheme adopted in GPS crosslink. An example of this system is illustrated in Figure 1, which also includes a table showing the connectivity between each of the elements. There are two fundamental scenarios for this system: ground to satellite mode (GSM) and satellite to satellite mode (SSM). Each of the two scenarios needs to be discussed separately due to the differences in parameters such as baseline length, velocity, communication frequency, visibility and space environment. Figure 2 shows the simplified representation of this system. The receiving device receives the GNSS like signal and after down conversion, the intermediate frequency (IF) is quantized using an analogue to digital converter (ADC). In the Measurement Device, signal acquisition, tracking and decoding processes are included for data recovery, code and carrier phase extraction, while measurement of pseudoranges and pseudorange-rates is also performed. The phase shift is measured by the comparison of the pseudo-noise (PN) code of the master satellite and the slave satellite. The transmitting device transmits the GNSS-like signal that incorporates clock error information by a series of processes that include a digital signal generator, digital to analogue converter (DAC), up-converter and amplifier. Different from the master satellite, the slave satellite carries not only a transmitting device, a receiving device and a measurement device, but also a control device. It is used to process the range measurement information from two satellites. The phase shift that is caused by the clock error is extracted by means of delay correction, after which the voltage control signal is generated to adjust the voltage of VC-OCXO. Figure 2: Simplified representation of this autonomous time synchronization system. Note that only one master is present, although this may be space or ground based. In addition, it is noted that this physical time synchronization method could be considered as a closed loop. The clock error of the slave satellite relative to the master satellite is extracted from pseudo-range measurement. It is translated into the control signal, which is used to adjust the VC-OCXO of the slave satellite. Meanwhile, the corrected VC-OCXO generates the timing signal for ranging measurement activities. Therefore, the scheme could be divided into two parts: clock error extraction and clock adjustment, which are the two fundamental issues we consider. In order to solve these two problems, a specific software simulator based on dual one-way ranging measurements and PLL [17] has been developed. The following is the principle of this new method, as shown in Figure 3.  
Based on the discussions above, the differences between the proposed system and the existing similar systems can be presented, as shown in Table 1.