T801-4 Installation and Operation Manual
System Configuration
57
© Tait Electronics Limited September 2007
4.4
T801-4 with Internal and External GPS Receivers
This configuration uses a standard product, the T801-4x-8100, which is
supplied with the Trimble Bullet III active antenna (also available separately
as Tait part number 007-00021-00). The external Dalman 40742 GPS
antenna/receiver is available separately as Tait part number T801-40-0000,
and comes complete with a 30m connection cable.
Using the T801-4 with two GPS receivers provides a level of redundancy.
Either the internal or the external GPS receiver can be set as the primary
unit, with the other then becoming the standby unit.
By setting the appropriate DIP switches, the system will “changeover” to
the standby receiver when an alarm event occurs. The changeover can be
triggered by both GPS and/or oscillator alarms, or by oscillator alarms only.
Important
Do not
trigger a changeover by “oscillator alarms only” if
your system depends on the T801-4 1PPS outputs. As
described in
“Invalid GPS Reference” on page 30
, the loss
of the GPS fix could mean the Satsync 7 VCO is “frozen”
and the oscillator alarms are delayed for 24 hours. The
1PPS outputs would therefore be disabled or invalid for this
24 hour period.
The changeover process triggered by a GPS alarm is described below:
■
The primary GPS receiver becomes faulty.
■
The GPS alarms are activated after the programmed fault output delay
period has elapsed.
■
The 1PPS outputs are disabled and the changeover is triggered
1
.
■
The standby GPS receiver supplies the 1PPS signal to the T801-4.
■
The Satsync 7 module checks the 1PPS signal is valid (GPS fix).
■
Once the GPS fix is acquired, the alarms are deactivated and the 1PPS
outputs are restored.
Once a changeover has been triggered, the 1PPS outputs are restored in less
than a second. This is because the standby GPS receiver is always powered-
up and already has a GPS fix (assuming it is operating correctly).
The changeover to a functional and properly positioned standby receiver
ensures that the accuracy of the T801-4 reference frequency outputs is
maintained and the 1PPS outputs are quickly restored.
1. Changeover may be delayed a further 24 hours if set to trigger by “oscilla-
tor alarms only” (refer to
“Invalid GPS Reference” on page 30
).
