Precise Onboard Time Synchronization for LEO Satellites

Flowchart of the orbit and clock determination algorithm; the receiver time t_rcv is predicted based on the time delta between the IMT beats b of two consecutive epochs as well as the last known fractional frequency offset . After the measurement update, the predicted time is corrected with the residual clock offset to form the best estimate of GPST at epoch i

Real-time orbit estimation error compared to the POD solution in radial, along-track, and cross-track axes

Difference of the GPST-aligned or GST-aligned receiver timescale with respect to the timescale of the CODE clock products

Timescale estimation error with respect to CODE clock products using GPS and Galileo measurements (left) and corresponding ISB curves (right) for different ISB process noise settings; the negative broadcast GGTO is plotted as reference

Time deviation of the estimated receiver time t_rcv relative to the reference clock product t_CODE; the receiver time was estimated using an ISB process noise of σ_ISB = 6 mm over 30 s as shown in the second row of Figure 5

Carrier-phase residuals (post-measurement update) during an SAA pass at around 2 am on January 22 for a free clock estimation (top) and an over-constrained clock model