[ntp:questions] SIRF time output wobble, GPS or NTP fault?

Ron Frazier (NTP) timekeepingntplist at c3energy.com
Tue Mar 6 20:33:20 UTC 2012


On 3/6/2012 12:00 PM, David J Taylor wrote:
>> Hi all,
>>
>> I've been seeing some very strange behavior from my USB BU-353 GPS 
>> based on readings from the Meinberg Server Monitor.  The unit is 
>> based on the SIRF Star III e/LP chipset.  At first, I thought it was 
>> the GPS at fault.  But, now, I'm not so sure that it's not either NTP 
>> or the Meinberg monitor program.  Let me explain.
> []
>> Sincerely (confused),
>>
>> Ron
>
> Ron,
>
> Sorry to be blunt, but please stop using serial NMEA output and 
> expecting good results!  If you see similar problems with PPS to the 
> DCD pin of an RS-232 port, please report them.
>
> Cheers,
> David
>

David and others,

I do appreciate the help and advice you've given me to this point.  I 
read all the replies up to this point on the ESR thread and this 
thread.  That includes David's mention of NMEA variances on the Garmin 
and Chris's mention that NMEA "within the second" is within spec.  I 
still think something is wrong with the NTP subsystem in my opinion.  
I'm not going to lose a lot of sleep over it.  I report it here just in 
case someone who knows how wants to investigate it.  Just because PPS 
and RS-232 work better, doesn't mean the NMEA driver and / or the 
Meinberg monitor shouldn't work properly.  I will NEVER be able to use 
RS-232 on this computer without a USB - serial adapter (which I intend 
to try).  It doesn't exist.  The capabilities of a USB GPS are perfectly 
adequate for my application.  + / - 6 ms is just fine.  There are, by 
the way, many other applications where 6 ms would be acceptable.  For 
comparison, the best I can get on Windows polling exclusively from the 
internet, with any kind of reasonable polling interval and not pounding 
the internet servers all the time, is about + / - 80 ms.  So, using the 
USB GPS is about a 12 fold improvement.  Also, there are many many 
computers where there is no RS-232 port.  ESR's application is an 
example.  So, the question is not is NMEA great, the question is - is it 
good enough for me?  I will be pursuing PPS with the Sure board for 
intellectual reasons, but there is no reason that I have to do that.

There is no way in the world I should have an offset from internet 
servers of 950 ms one minute and then show an offset from internet 
servers of 50 ms thirty seconds later after I restart NTPD, when I'm 
locked to my GPS time in both cases.  I can tell when my GPS is working 
consistently and when it's not.  Something is wonky here and I don't 
think it's the GPS.  I'm trying to figure out if the GPS is "drifting" 
or if NTP is reporting internet server offsets wrong or if there is a 
problem with the Meinberg monitor.  If everything is running properly, I 
should be able to maintain that + / - 6 ms all day, every day, and the 
variance between my clock and at least one NIST clock should be minimal 
and consistent.  I don't expect great results from NMEA.  But I do 
expect acceptable results, and consistent results.  My GPS should be 
more than capable of outputting NMEA sentences consistently.  I've seen 
it maintain that + / - 6 ms performance for days.  It's only outputting 
the GPZDA sentence, and that only has time in it.  The unit has nothing 
better to do than to tell me the time.  It should not be "drifting".  
But, SOMETHING is drifting, and I'd just like to know what it is.  I 
REALLY think there is a problem with NTP or Meinberg, but I could be 
wrong.  It would be nice to know if I have to avoid all SIRF GPS's for 
this purpose.  I think NTP should be able to work just fine, within the 
limits of the equipment, on NMEA only.

No offense is intended in any way by anything I've said.  As I said 
before, I do appreciate all the help.  8-)

Sincerely,

Ron


-- 

(PS - If you email me and don't get a quick response, don't be concerned.
I get about 300 emails per day from alternate energy mailing lists and
such.  I don't always see new messages very quickly.  If you need a
reply and have not heard from me in 1 - 2 weeks, send your message again.)

Ron Frazier
timekeepingdude AT c3energy.com



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