[ntp:questions] Standalone NTP PPS Configuration
ryan eustice
rme at jhu.edu
Fri Dec 2 19:05:42 UTC 2005
Hello,
I have a question about configuring NTP for standalone PPS support. I
would like to use a local free-running PPS source to disciple the local
clock on an embedded PC. This PC resides in a Unmanned Underwater
Vehicle (UUV).
When on deck of the research vessel, the UUV can be connected via the
local LAN to a Meinberg LANTIME/GPS time server box. Additionally, when
at the surface, the UUV can receive GPS fixes from a Garmin receiver;
note that GPS signals do not penetrate subsea, however. In addition to
the above time sources, the UUV also has a free-running TXCO that
outputs a stable PPS signal. I would like to use this PPS reference to
discipline the PC clock while subsea.
My question is, what is the best way to configure NTP to take advantage
of this? Specifically, I'm uncertain of how to use the "prefer" keyword
modifier in this context as both the network and GPS time sources will
be unavailable subsea (they are available at the surface, though, for
initial NTP synchronization).
I have patched a 2.4.29 kernel with PPSkit and have installed ntp-4.2.0.
Right now my ntp.conf file looks like:
-----------------------------------------------------------------------
# ntp.conf for use with PPSBOARD
#
# Date Who What
# -------- ------------ ----------------------------
# 20051201 Ryan Eustice Created and written.
enable auth stats monitor pps
statsdir /var/log/
statistics loopstats
logfile /var/log/ntp.log
driftfile /etc/ntp.drift
# Network time servers
server meinberg.lantime.edu iburst prefer # network LANTIME/GPS
# Generic NMEA GPS receiver (4800 baud)
# The driver expects a $GPRMC msg every second (others silently ignored).
server 127.127.20.0 mode 1
# PPS clock discipline (requires PPSAPI interface)
# flag2 sets PPS on-time edge: 0 assert, 1 clear
# flag3 enables kernel PPSAPI: 0 disable, 1 enable
server 127.127.22.0
fudge 127.127.22.0 flag2 0 flag3 1
# Undisciplined local clock
server 127.127.1.0
fudge 127.127.1.0 stratum 12
-----------------------------------------------------------------------
Should I instead make the local clock driver (127.127.1.0) the
"preferred" peer so that the PPS reference will be used while subsea?
Very Best,
ryan eustice
--
Ryan M Eustice, Ph.D.
Postdoctoral Scholar
Department of Mechanical Engineering, Johns Hopkins University
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
email: rme at jhu.edu ph: 410.516.8021 fax: 410.516.7254
http://www.whoi.edu/people/reustice
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