[ntp:questions] ESR looking for good GPS clocks

Ron Frazier (NTP) timekeepingntplist at c3energy.com
Tue Mar 6 15:22:39 UTC 2012


On 3/6/2012 4:45 AM, Rob wrote:
> unruh<unruh at invalid.ca>  wrote:
>    
>> He is suggesting that the on board gps chip have ntp on board, so that
>> your computer timestamps the request, the gps chip then timestamps when
>> that trequest was received and returns it. No gps chip that I know of
>> has an onboard ntp server. So if you want to get a chip fab to make you
>> a special purpose chip which contains a gps receiver and ntp onboard,
>> you could probably do it-- but it might cost a bit more than $100 ieach
>> in
>> lots of 100.
>>      
> Of course not.  NTP is software, not hardware.
> The only thing you need is special firmware for any existing GPS chip
> that includes the NTP functionality you want, and maybe drops some other
> functionality to make room in the flash.
>
> Some manufacturers provide a software development kit that allows the
> user to write his own code, link that with the object modules written
> by the manufacturer, and create a new flash file for the GPS device.
>
> At least SiRF provides such a development kit to its commercial customers.
>
>
>    

I haven't been following this thread extremely closely, but I did read 
ESR's blog post and exchanged a few emails with him.  He says SIRF GPS's 
exhibit a "wobble" of variance of outputting the NMEA time of 170 ms or 
so.  I've personally observed with my USB BU-353 this effect, where the 
Meinberg Server Monitor shows my computer locked into the GPS time 
within a few ms and a number of internet servers that I have programmed 
will show offsets of say 50 ms.  However, when I originally set the 
fudge factor, I had almost all the internet servers showing single digit 
offsets when the PC is locked to the GPS time.  Sometimes, it shows a 
positive offset for the internet servers and sometimes it shows 
negative.  Anyway, ESR says SIRF GPS's aren't suitable for timekeeping.  
I have not done anything to corroborate his statement, other than 
observing this strange behavior.  I'm going to start a new thread 
talking about what I've seen in more detail, since it's really a 
different topic.  I just mentioned it since you brought up SIRF.

Sincerely,

Ron


-- 

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Ron Frazier
timekeepingdude AT c3energy.com



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