Time synchronization between multiple sensors on multiple Raspberry Pi's
I have ROS kinetic running on two Raspberry Pi's (RPi's). Each RPi has is connected to a set of sensors. The RPi's are not connected with each other as they will be out in the field, but will be connected at least once via WLAN to a master computer to start (and eventually stop) the recording (rosbridge service to start/stop a rosbag file). Once data is collected, I would like to combine and analyze the data from both RPi's and therefore synchronize the data over time.
How can I ensure that they both record at the same clock time? I can add a real-time clock (RTC) to each of them and synchronize each RTC with the internet time once before the recording. But is that precise? I have seen in other threads here that chrony is recommended, which I have no experience with, yet. Another idea could be to record GPS reference time on both RPi's and synchronize the data to that one, but it would be recorded at low frequency (about 10 Hz) and I am not sure either how precise it could be.
Ideally, I would like to have a time sync error below say 1 ms.
Do you have any spontaneous suggestions how to go after it? I hope I provided enough details.