[ntp:questions] GPS/PPS and "enable calibrate"

David Lord snews at lordynet.org
Sat Sep 7 22:42:28 UTC 2013

Horvath Bob-BHORVAT1 wrote:
> -----Original Message-----
> From: questions-bounces+bob.horvath=motorolasolutions.com at lists.ntp.org [mailto:questions-bounces+bob.horvath=motorolasolutions.com at lists.ntp.org] On Behalf Of unruh
> Sent: Saturday, September 07, 2013 11:20 AM
> To: questions at lists.ntp.org
> Subject: Re: GPS/PPS and "enable calibrate"
> On 2013-09-07, Charles Elliott <elliott.ch at verizon.net> wrote:
>> Or use PPS and be accurate to  a microsecond or so.
> I guess this is the fundamental question I don't understand.  
> I thought with PPS, which I do have using the Adafruit GPS and a Raspberry Pi, I would be fairly accurate off of the GPS time and PPS together.   Then I started seeing fudge parameters of 0.496 which seem to work well, but seem like a large amount of time to be "fudging" for something that is supposed to already be accurate.  That is in seconds, right? So it is about a half a second?  I am confused what that parameter does, what it needs to be set to, and how one figures out what to set it to, or whether I am just really confused. 

The default nmea sentences can be different between different
devices and even firmware versions. These sentences can be of
variable length between polls and at 4800 baud can even be
more than a second making use for ntpd impossible. The gps
devices can usually be configured to send only a single nmea
sentence and/or use a much higher baud rate or use some custom
binary format. One of these options along with s fudge time
should be able to give a suitable output for use by ntpd but
might still have variation between polls of above +/- 50 ms.

If your GPS device has a PPS output, that should probably be
accurate to better than 1000 ns down to about 20-50 ns. A PC
serial interface might be limited to processing an interrupt
within around 1 us. My Sure GPS on VIA EPIA i386 gives in ms:
mean=0.000-0.001, rms=0.004-005, max=0.027-0.037. Those high
values occur when nightly cron jobs are being processed.


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