[ntp:questions] Standalone NTP PPS Configuration

ryan eustice rme at jhu.edu
Fri Dec 2 19:05:42 UTC 2005


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 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
fudge flag2 0 flag3 1

# Undisciplined local clock
fudge stratum 12

Should I instead make the local clock driver ( 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

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