XLi Time & Frequency System 191
XLi-man, Issue 8, 6/17/2008, Rev. H
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Alarm Operation
The logic resident in the FPGA, U7 implements major and minor alarm generation by monitoring two
bytes broadcast once-per-second over the XLi bus by the host microprocessor residing on the GPS XL
card, and the /LFA and /LFB signals sourced by U8 and U9, which indicate E1 output line faults.
Major and minor alarm status is provided from the clock. The E1 option combines the fault signal inputs
to form a summary major alarm indication, minor alarm information verbatim to control minor alarm
indication. In addition, if position 8 of S1 is in the OFF position, the E1 output transmits the AIS pattern
and will turn off the optional output signals whenever a major alarm occurs.
The FPGA also implements a “watchdog” function by requiring that the broadcast bytes are in fact
broadcast once a second. Should they not arrive on time, then the FPGA would assert a major alarm.
This assures that failure of the main XLi processor would be detected.
Maintenance
This card requires no maintenance.
GPS C/A Receiver (87-8028-2)
Introduction
The optional GPS C/A Receiver acts as a Stratum 0 timing reference source to the XLi. It tracks up to 12
L1 GPS satellites, decodes their signals for time and position, and feeds this data to the XLi through the
internal backplane. When available and enabled, the GPS C/A Receiver card provides superior time and
frequency accuracy on the XLi (See “System Time & Frequency Accuracy” on page 6
). The GPS C/A
Receiver card comes with an L1 GPS antenna, cabling, and mounting hardware unless otherwise
specified at the time of purchase.
The GPS C/A Receiver uses a TRAIM (Time Receiver Autonomous Integrity Monitoring) algorithm to
monitor the integrity of the receiver’s timing solution. Using redundant measurements, TRAIM detects
and quarantines anomalous GPS signals, independent of the GPS health ephemeris data. The
quarantined signal is excluded from the timing solution for 12 hours before it is requalified for inclusion in
the timing solution.
See “Installing the GPS Antenna” on page 19
for information on selecting an antenna site, mounting the
antenna, and signal strength requirements.
The GPS C/A Receiver card can be managed and configured using F53 – GPS Operation
Mode (page 80) and F119 – GPS Receiver Configuration (page 154).
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