Abstract:  The autonomous orbit determination and time synchronization algorithm of spacecraft that uses the extended service of BeiDou Navigation Satellite System(BDS)  inter-satellite-link（ISL） was focused on.An orbit determination and time synchronization algorithm based on polynomial fitting was put forward，aiming at avoiding the constraint in traditional algorithm that the initial orbit and clock parameters are necessary for using ISL two-way ranging measurements.The algorithm will expand the application scenarios of BDS ISL effectively.In the proposed algorithm，the spacecraft’ s orbit and clock bias were described by polynomial and its coefficients were estimated based on least squares algorithm and ISL two-way ranging measurements.Thus，the spacecraft’ s orbit and clock parameters could be calculated.The test results based on onboard ISL two-way ranging measurements show that the orbit determination and time synchronization accuracy of the proposed algorithm can achieve less than 0.12m，which proves its feasibility.

Key words:  inter-satellite-link, autonomous orbit determination, time synchronization, least squares algorithm, polynomial fitting