[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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