Meinberg GPS LANTIME - Handbuch (Seite 12)

Booting the Single Board Computer
The LINUX operating system is loaded from a packed file on the flash disk of the single
board computer to a RAM disk. All files of the flash disk are stored in the RAM disk
after booting. Because of that it is guaranteed that the file system is in a defined
condition after restart. This boot process takes approx. one minute.
After starting up the LINUX system the network function is initiated and the program
for communication with the GPS and the NTPD (NTP daemon) is started. After that
NTPD starts synchronisation with the reference clocks (usual the hardware clock of the
single board computer and the GPS receiver).
For the synchronisation of the NTPD with the GPS it is necessary that the GPS receiver
is synchronous with the GPS time (LOCK LED is turned on).
Because of the internal time of the NTP which is adjusted by a software PLL (phase
locked loop) it takes a certain time to optimise this offset. The NTPD tries to keep the
offset below ±128ms; if the offset becomes to large the system time is set with the GPS
time. Typically values for the offset are +-5ms after the NTPD has already
synchronized. Until the NTPD is not synchronized yet while the GPS receiver is, the
green LOCK-LED is blinking.
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