DOC 6501_Release V
ATS-6501 Users Guide
97
At a system level, this allows the ATS-6501 to use the 10 MHz external reference as the phase
reference for the system. The LNS makes phase measurements at a higher rate and with lower
noise against the 10 MHz reference input than it would otherwise be capable of doing with the
4393A Card (PPS TIC).
4.4 Output Signals
The PPS signal from the GPS receiver/reference source is used to align both the IRIG-B signal
and the CPU clock to UTC(USNO). At startup this PPS is used to align the internally generated 1
PPS and IRIG-B signal with the correct 10 MHz clock cycle. From that point on the 1 PPS and
IRIG-B signals are kept on time by using the OCXO 10MHz signal as its reference. The 1 PPS
signal from the GPS receiver generates an interrupt to the ARM 9 processor which serves as an
indicator of the top-of-second. This is used by the CPU along with messages received from the
GPS receiver to set the CPU time to UTC(USNO). In doing this, the CPU can serve as a
stratum-1 network time protocol (NTP) server and provide time to other computers and systems
on the network. The OCXO control voltage is polled every LNS time constant in order to weed
out transients due to steering allowing the system to accurately determine if the OCXO control
voltage is at a minimum or maximum value. The averaging of this control voltage provides more
stable steering over time.
The fiber optic outputs from the 4372A module, when plugged into an ATS 6501, contain a
proprietary bit stream with 1 PPS marker and time code information. The ATS 6511A (Dual
Fiber Input Timing Distribution) decodes this bit stream, extracts the time code, and steers its
internal OCXO to the frequency and phase of the fiber signal. When shipped from the factory, the
ATS 6511A will automatically switch to input B upon loss of input A. If input A becomes good
again, the unit will automatically switch back to input A after input A is detected as good for 60
seconds. The user can select manual switching or disable the automatic return to input A as a
reference after input A has failed.
The output cards for the ATS 6511A are the same as those in the ATS 6501, with the exception of
the 4399A L-Band card. A 4340A fiber optic distribution chassis may be connected to an output
of the 4372A module to provide up to 8 outputs which can be connected to ATS 6511As. Future
releases of software will include status monitoring in the ATS 6501 for all ATS 6511As
connected to the 4372A in the ATS 6501. See the ATS 6511A manual for more detail.
The 4393A Time Interval Counter (TIC) provides the capability to measure up to four external
1PPS sources against the internal 1PPS. The measurement data is collected by the unit and
provided on telnet ports 2100 and 2101. Port 2100 is the averaged measurement, where the
default is 10 seconds. The averaging time can be changed to 1, 10, 20 60, or 300 seconds using
the
ptdavg
command. Port 2101 provides one second measurement data. The 4393A can be used
in any of the slots in the ATS 6501 or ATS 6511. Multiple 4393A cards can be installed in the
unit when more than four 1PPS measurements are required. Data collected can be used by an
external system to create a paper clock based on the measurements.
4.5 GPS Data Collection (Not available in SAASM Type 1)
GPS measurement data is available to users in a variety of log formats. The ATS-6501 uses a
Novatel OEMV (dual-frequency and WAAS capable) as its GPS receiver and thus any logs
generated by the receiver are available to users. For a complete description of logs and their
formats refer to the OEMV manual at
http://www.novatel.com/support/docupdates.htm#oemv
.