[ntp:questions] Ultralink 325 WWVB receiver
Dennis Hilberg, Jr.
timekeeper at dennishilberg.com.invalid
Thu Jan 31 07:03:48 UTC 2008
I recently snagged an Ultralink 325 WWVB receiver off Ebay to use as a
refclock with an ntp server, hoping to peer that one and my gps-referenced
ntp server together.
The unit works fine, and I get good signal (R5) most of the time, as shown
in the clockstats file. The behavior is a little odd though, compared to
what I'm used to with my with my gps-referenced ntp server:
After fudging out an initial offset of about 280 ms, I noticed the offset
will travel a 10 ms range towards the negative. For example, if the offset
initially starts at say 0.500, it will gradually, in about 0.300 - 0.400 ms
steps, make it's way to about -0.500. Once it reaches the low end, it will
jump back up to 0.500 and start its decent over. I'm assuming this is
fairly typical, as I observed a different WWVB machine and it shows similar
stepping behavior within a range.
The initial offset changes also. One time it was 280 ms, another time it
was 960 ms. The most recent time I didn't use any correction at all initially.
All that aside, the machine's offset seems to wander around quite a bit
compared to my gps-referenced ntp server. I haven't noticed any specific
pattern to it though.
I've repeatedly tried to fudge out the offset, but it seems to wander right
back to about a 4.5 ms offset compared to my gps-referenced ntp server.
Coincidentally (or not), this is also about what I've calculated my
propagation delay to be based on information Dave Mills posted in another
thread. Do I include the propagation delay in the fudge factor, or is there
a separate step for that?
The docs say the unit is capable of +- 1 ms accuracy, so perhaps I'm missing
something. Are there special steps that need to be taken for WWVB
receivers? The unit's user manual, the driver documentation and source code
didn't reveal any secrets.
Thanks for any help,
Dennis Hilberg, Jr. \ timekeeper(at)dennishilberg(dot)com
NTP Server Information: \ http://saturn.dennishilberg.com/ntp.php
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