One fundamental limit for synchronizing a packet trace is that the two measurement systems that can be used to timestamp the packets, the operating system and the network interface, have mutually exclusive properties. The OS clock can be stabilized over the long run but the OS timestamps are not accurate; the PHY timestamps are accurate but they show a drift. To leverage the best features of both systems, we propose to stabilize the OS clock with a GPS and to remap the PHY timestamps over the OS/GPS time. This method is well suited for challenged scenarios like VANETs as no signaling is needed between nodes. We apply our approach to VANET monitoring and evaluate it on packet traces collected from our testbed. The results show that our solution reduces the mean synchronization error to 3 μs.