[ntp:questions] Re: ntpd, boot time, and hot plugging

Kenneth Porter shiva.blacklist at sewingwitch.com
Fri Feb 4 13:52:03 UTC 2005


Brad Knowles <brad at stop.mail-abuse.org> wrote in 
news:mailman.36.1107479858.583.questions at lists.ntp.isc.org:

> At 2:20 PM -0600 2005-02-03, Kenneth Porter wrote: 
> 
>>>  If you want to make that delay shorter, I guess you could package 
>>>  Stratum 1 refclocks with every machine. 
>>
>>  Given the cheap price of a consumer GPS receiver, this doesn't sound 
>>  that crazy. How fast can ntpd lock on to the NMEA messages? 
>
> Cheap GPS receivers don't do NMEA.  Those that do NMEA don't give 
> you a PPS signal, so they're pretty much useless in this role.  You 
> have to look very carefully at GPS devices before you can be sure 
> that you've found one that will be able to give you a good time 
> reference.

Point taken, but I have a "looser" definition of "cheap", so a receiver 
that costs a couple hundred dollars would be acceptable for a system that 
was so critical.

How "bad" is the time from a GPS receiver with NMEA but no PPS? Is it 
sufficient to use for an initial setting (replacing ntpdate and ntpd -g)? 
IIRC these things typically issue an update once a second.

> 	For example, the Motorola Oncore 12 might be a piece of crap, 
> while the Motorola Oncore 12+ might be good.  Or the Garmin 18 LCS 
> might be good, but all the other Garmin 18 models might be garbage. 
> You've got to know what you're looking for.
> 
> 
> 	Please note that I don't have any specific knowledge of which 
> models are good or bad, I just pulled these examples out of the air.
> 
> 	You need to do your own research to ensure that you've got a GPS 
> device that will be useful as input to a proposed Stratum 1 time 
> source.

I got an eTrex Vista for xmas and will have to see what it's capable 
of....



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