How do ALCMS devices keep the same time?
A GPS-fed time server passes UTC to every device over NTP, so events from the server, screens and CCRs land on one timeline.
Time synchronization keeps the clocks of all control-system devices in agreement. A GPS/GNSS receiver gets accurate time from satellites and converts it to UTC. An NTP server passes that time over the network to the ALCMS server, workstations, CCR controllers and other devices. A server with the receiver attached directly is called stratum 1.
This matters because the event log is only useful if entries from different devices can be put in the right order. Where a loss of the GPS signal occurs, the time server runs on its internal clock (holdover) and slowly drifts, so the loss should raise an alarm, and many designs use a second source. Logs are often stored in UTC and shown in local time, to avoid confusion from time-zone or daylight-saving settings.
Frequently Asked Questions
Q1Why does an ALCMS need synchronised time?
Events come from many devices: the server, workstations, CCRs and field controllers. If their clocks differ, the event log shows them in the wrong order, which makes fault tracing and incident review unreliable. A shared time source puts every entry on one timeline.
Q2What is the difference between GPS time and NTP?
GPS (or another GNSS) is a time source: a receiver with an antenna gets accurate time from satellites. NTP is a protocol that passes that time across a network. A common arrangement is a GNSS-fed NTP server that the other devices query.
Q3What accuracy does NTP give?
On a well-built local network, NTP typically keeps clocks within a few milliseconds, which is enough for ordering most control-system events. Where much tighter alignment is needed, PTP (IEEE 1588) is used instead. The accuracy actually needed comes from the system specification.
Q4What is stratum?
A measure of how many steps a clock is from the reference. A server with a GNSS receiver attached directly is stratum 1. A device that takes time from it is stratum 2, and so on. Lower numbers are closer to the source.
Q5What happens if the GPS signal is lost?
A good time server keeps running on its internal oscillator (holdover) and raises an alarm, but it slowly drifts. Backup measures include a second time source or server. GNSS signals can also be blocked, jammed or spoofed, so the system should flag a loss or a sudden time jump.
Q6Should logs use local time or UTC?
Many systems store events in UTC and show local time on screen, so that changes such as daylight saving or a wrong time-zone setting do not scramble the order. Confirm the convention used by your system and your airport.
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