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

Steve Kostecke kostecke at ntp.org
Tue Apr 8 12:29:51 UTC 2014

On 2014-04-08, a.everett.000 at gmail.com <a.everett.000 at gmail.com> wrote:

> On Monday, 16 September 2013 08:00:09 UTC+1, Igor Pavlov wrote:
> [---=| Quote block shrinked by t-prot: 45 lines snipped |=---]
>>  64.279
>We find that the problem with many (not all) NMEA GPS receivers is that
>often too much data is transmitted between each PPS output. This can
>have the effect that the time output (ZDA) sentence can occasionally
>shift either side of its corresponding pulse output.

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

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.

Steve Kostecke <kostecke at ntp.org>
NTP Public Services Project - http://support.ntp.org/

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