[ntp:questions] Estimating time1 and time2 for nmea+pps

Doug Calvert dfc-list at douglasfcalvert.net
Sun Jan 6 21:38:19 UTC 2013


Hello,
 I have read through the docs on the wiki for estimating time2 with a
nmea referecne clock. I have a Sure GPS after a week or so the offset
seemed to stabilize round -450. Everything was making sense and then I
came to:

"And now the truth about the PPS locking: actually only the sub-second
part of the difference between PPS time stamp and (receive time stamp -
fudge time2) is evaluated and checked. If this difference is less than
400ms or bigger than 600ms (which is equivalent to -400ms) the receive
time stamp will be substituted with the properly adjusted PPS time
stamp. This might sound a bit complicated, but if you have a device that
sends the data before the associated PPS pulse, you can use the proper
negative value for fudge time2 and compensate for that behaviour."

I am a little confused about the absolute value of the offset but I
think setting time2 to 0.450 is the correct approach. Am I right? Does
the following chart accurately summarize when to use the a positive or
negative time2?

Offset w/PPS disabled   |   Value to use for time2 (offset/1000)
================================================================
-999 through -600       |   Use negative offset
-599 through -400       |   Use absolute value for offset
-399 through 399        |   Use negative offset
 400 through 600        |   Use absolute value for offset
 600 through 999        |   Use negative offset


I was not able to find a lot of information about how to identify what
to use for an estimation of time1. The majority of what I have read
seems to indicate that unless something is screwy ntpd will do okay with
no value supplied for time1. Even if ntpd will do the right thing I was
curious if there is any benefit at all to identifying a value for time1?
How do I go about figuring this out? Part of me thinks that the answer
may be using one of the utilities in pps-tools.

Thanks in advance.


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