30
Operation
T801-4 Installation and Operation Manual
© Tait Electronics Limited September 2007
2.5
Invalid GPS Reference
When the 1PPS signal is considered invalid, the following two actions
occur:
■
The
GPS
LED goes red (after the fault output delay period). The GPS
LED will flash (green) during the programmed delay period.
■
The GPS status relay (internal) is de-energized, activating the GPS alarm
output on the T801-4 rear panel.
If the previous state of the correction process was that the accuracy was
better than ±1 part in 10
8
:
■
The VCO is “frozen”. It will remain in this state for 24 hours (as long
as the GPS fix remains invalid).
■
The
MASTER OSCILLATOR
LED goes amber (flashing), indicating that
the oscillator frequency is locked, and the alarm is delayed.
If the previous state of the correction process was that the accuracy was
worse than ±1 part in 10
8
, or after the oscillator has been locked for 24
hours:
■
The
MASTER OSCILLATOR
LED goes red, indicating that it is not safe to
assume that the master oscillator frequency is correct.
■
The ageing correction process (almanac) is applied (refer to
“Ageing
Correction Process”
below).
■
The master oscillator relay (internal) will be de-energised, activating the
master oscillator alarm output on the T801-4 rear panel.
Note
You can delay the response time of the alarms by a variable time
period (refer to
“Fault Output Delay” on page 33
).
2.6
Ageing Correction Process
When operating with a valid 1PPS signal and an accurately referenced
oscillator, the Satsync 7 module keeps a record of the long-term movement
of the oscillator control voltage. This record is then used to track the control
voltage along the likely ageing path if the Satsync 7 module has to operate
without a valid 1PPS signal,
The Satsync 7 ageing correction almanac gathers error count values every
time a successful error count occurs (i.e. when the system has a GPS fix and
the test period is 30 seconds). These values are averaged to generate a daily
figure, and these figures are stored in a seven day rolling table.
When the ageing correction process needs to be applied to the master
oscillator, the seven day rolling table is used. If the GPS validity has not been
restored after seven consecutive days, the almanac process will generate a
DAC correction value based on the contents of the seven day rolling table.
It will continue to do this every seven days, or until GPS validity is restored.
