Model 5601MSC
Model 5601MSC Master SPG/Master Clock System
Page - 40 Revision 2.2 OPERATION
2.3.3.1. GPS Time Reference
When GPS is selected as the time reference, time and date are obtained from the GPS receiver that is
connected to the 5601MSC. The GPS time provided by the receiver is then converted to UTC time by
adding leap seconds. This time reference is extremely accurate and once the system clock has been
jammed to the GPS receiver, it should remain in sync for a very long time without requiring further jams,
provided the selected frequency reference is also accurate.
Of all the time reference sources, GPS is the most accurate and reliable. When a difference of greater
than 2 milliseconds is detected between the system clock and the time from the GPS receiver, the
5601MSC will generate a jam event or jam warning, depending on the Lock Type setting. Such a
difference can be caused by accumulated stability errors or by the introduction of a leap second. For
information on installing the GPS receiver and a detailed description of its operation see section 2.5.2.
When the frequency reference has been set to GPS, the time reference is also
forced to GPS.
2.3.3.2. Modem Time Reference
If the 5601MSC has been equipped with a modem module, it can receive time and date by dialing into a
high-level time standards service. The two protocols supported at this time are NIST Automated
Computer Time Service, and NRC Computer Time & Date. The modem is configured using the Modem
menu in the GENERAL root menu as described in section 4.5.6.
After a time service has been selected, the phone number is entered and the 5601MSC can be
configured to automatically dial at a specific time once a day or once a week. The dial time should be
selected so that any correction to the system clock will cause minimum disruption. The modem can
also be dialed manually.
When the modem dials successfully into a time standards service, it subtracts the round-trip delay and
jams the system clock and all timecode clocks to within a few milliseconds of UTC. If the call fails, it
can be configured to retry a number of times until a successful negotiation is achieved.
Upon acquiring valid time, a countdown timer is started and the system clock is considered valid for a
period of 10 days. If the timer expires before another successful dial-up then the system clock will be
considered unlocked. This will generate a time reference fault and also affect NTP packets being sent
out by the NTP server.
See section 4.5.6 for information on configuring the modem. Section 4.1.2 describes the modem status
screen. In NRC mode, the modem can also be configured to answer calls and act as a time service
host no matter which time reference is selected.
NIST ACTS:
http://www.nist.gov/physlab/div847/grp40/acts.cfm
NRC:
http://www.nrc-cnrc.gc.ca/eng/services/inms/time-services/time-date.html
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