Difference Between Standard LANTIME And MRS

The standard configuration is an embedded Linux system running ntpd, and a GPS receiver module connected to it to provide accurate time. The GPS receiver has the high quality oscillator on board, which is disciplined by the GPS signal only. During normal operation the time on the Linux system is adjusted by the accurate time from the GPS receiver, and the disciplined time is sent to the NTP clients.

ntpd doesn't accept a time source that isn't synchronized. So if GPS reception fails in this scenario the receiver becomes unsynchronized. Anyway, if GPS reception has been possible before then the time from the GPS receiver is still accepted for a certain trust time interval, but if the GPS receiver was not synchronized before it is discarded immediately, so the time derived from the high quality oscillator isn't always used.

If an external server has been configured then ntpd can still query that NTP server, and adjust the Linux system time from it, if the server replies and claims to be synchronized, but this takes no advantage of the high quality oscillator on the GPS receiver module..

If you have a LANTIME/MRS (Multi Reference Source) then you have a kind of extended GPS receiver with an oscillator that can be disciplined from several input signal sources. If GPS reception isn't possible, but another signal source is available then the high quality oscillator is from that source. In this configuration the NTP service running on the embedded Linux system gets the time from the GPS/Oscillator module continuously, as long as any input signal is available.

In this scenario you must not configure another NTP server as external server like in the example above, but as a possible MRS source, i.e. an NTP source beside the GPS source. So the remote NTP server isn't polled directly by ntpd, but by the GPS/Oscillator module, which disciplines the oscillator with it, and the time derived from the oscillator is always and solely fed to ntpd.

This is a totally different approach which really benefits from the high quality oscillator in holdover mode.

This depends on what you need. If there are several reliable NTP servers on the network it is sufficient to use a NON-MRS LANTIME with external servers configured since no holdover is required as long as another reliable NTP server is available.

If you need or want a the device to provide high accuracy even in case of holdover, i.e. when no other time source is available anymore, then a LANTIME/MRS is the better choice.

— Martin Burnicki martin.burnicki@meinberg.de 2018-11-12