Odetics
Telecom
GPStar
plus
Model 365
365-8006
Specifications 2 - 2
2.2.1.4
Time Locked
In the
Time Locked
mode the unit's major and minor time is synchronized to UTC. The
1 PPS output from the unit's internal GPS receiver is used to discipline the internal
oscillator and the phase of the 1 PPS output reference pulse. The effects of Selective
Availability are averaged out to minimize errors otherwise caused by Selective
Availability. The accuracy of the unit's 1 PPS output is indicated by the Time Figure Of
Merit, TFOM. Normally, the TFOM is 4 while operating in the
Known
position mode,
and 4 or 5 when operating in the
Dynamic
or
Position Averaging
mode. The TFOM
value is displayed on Menu 1 of the front panel display.
2.2.1.5
Coasting
The
Coasting
mode is entered after out of range inconsistencies in the major time (days
through seconds) or 1 PPS have been detected, or if the internal receiver has lost lock
with all satellites. This mode can only be entered from the
Time Locked
mode.
In the first stage of this mode, the internal oscillator is in holdover (i.e. not being
disciplined) to retain the accuracy of the output signals. The timing output accuracy is
indicated by the Time Figure Of Merit (TFOM). In the first stage of this mode, the TFOM
is the same value as that while operating in the
Time Locked
mode.
When operating in the
Coasting
mode for extended periods, the TFOM value changes
according to the estimated time error computed from the following formula: Error = nf;
where n = number of seconds since time lock was lost, and f = frequency default error.
In the last stage of the mode, when the conditions needed to start the disciplining and
synchronization process are met and then started, the unit will automatically change to
the
Recovering
mode if the timing error is greater than 10 µs or, if the timing error is
less than 10 µs, directly to the
Time Locked
mode.
At the moment the last stage of the mode is entered, the TFOM indicates the actual
measured accuracy of the timing output. The following table lists the TFOM values that
could be displayed and provides the corresponding Time Error.
TFOM Value
Time Error
TFOM Value
Time Error
4
to < 1 µs
7
> 100 µs to < 1 ms
5
> 1 µs to < 10 µs
8
> 1 ms to < 10 ms
6
> 10 µs to < 100 µs
9
> 10 ms
Note: During warm up, TFOM is assigned a value of 9.
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