[ntp:questions] Reasons of NTP not to use GPS source

William Unruh unruh at invalid.ca
Tue Apr 8 22:01:36 UTC 2014


On 2014-04-08, Steve Kostecke <kostecke at ntp.org> wrote:
> On 2014-04-08, a.everett.000 at gmail.com <a.everett.000 at gmail.com> wrote:
>
> According to http://doc.ntp.org/4.2.6p5/drivers/driver20.html the NMEA
> driver uses the last processed sentence received during each cycle.
>
> So the simple solution is to have only one sentence enabled.
>
>>This has the effect of a 1 second offset occasionally being added to
>>time stamps supplied to the NTP daemon. Sometimes, increasing the baud
>>rate from the standard 4800 bps to 9600 or even 19200 bps can help by
>>allowing more characters (data) to be transmitted between each 1PPS
>>output.
>
> 4800bps (8N1) transfers 480 bytes (8-bit characters) per second.
>
> NMEA 0183 sentences are limited to 79 characters. So a single NMEA
> sentence is fits easily into the 480cps limit.

While certainly true, it is not quite so simple because almost all gps
do not immediately send out the nmea sentence at the interrupt. They sit
there cogitating for a while (eg for .2 sec or more) before starting to
send out the sentences. This means the the max number of sentences is
less than a naive calculation would suggest. On the otherhand even .5
sec should be more than enough time to send out one sentence. 

>



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