The older GPS wiki suggested this:
Note: The time is not stratum 1 as RouterBOARD devices do not have PPS implemented as of RouterOS v6. This might change with new NTP server in v7.
Do the LTAP and KNOT devices have PPS wired in? Can the DCD pin on the serial port be used for PPS from external GPS units? If so when can the ROS NTP server use the built in GPS & PPS to act as a stratum 1 time reference?