[time-nuts] Adafruit Ultimate GPS timing message arrival times

Paul tic-toc at bodosom.net
Mon Aug 1 09:52:18 EDT 2016


On Mon, Aug 1, 2016 at 3:01 AM, Hal Murray <hmurray at megapathdsl.net> wrote:

> Depends on your internet connection and/or the specific GPS module you are
> using.
>
> I don't know of any good GPS modules that use NMEA.  I do know of really
> crappy internet connections.  Bufferbloat is the buzzword.
>

Bufferbloat is an issue but the NTP problem is unknown assymetric delays.

If you're a bit flexible regarding "module" you can do quite well using
NMEA.  Both points are shown in the output below (sent using a constant
width font).  FURY is a Fury, offset and jitter in milliseconds.

$ ntpq -p
     remote           refid      st t when poll reach   delay   offset
jitter
==============================================================================
oPPS(0)          .GPPS.           0 l    3    8  377    0.000   -0.002
0.003
*GPS_NMEA(0)     .FURY.           0 l    2    8  377    0.000   -0.010
0.018
 time-d.nist.gov .ACTS.           1 u  264  512  353  105.770   29.742
0.348
+nub             .GPPS.           1 s    4   16  377    0.151   -0.025
0.022


Compared to the MTK3xxx in the Adafruit UGPS circa 2-3 years ago.

     remote           refid      st t when poll reach   delay   offset
jitter
==============================================================================
oPPS(0)          .GPPS.           0 l    4    8  377    0.000    0.002
0.004
*GPS_NMEA(0)     .ADAU.           0 l    3    8  377    0.000  -78.472
25.623


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