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

Steve Kostecke kostecke at ntp.org
Sat Sep 7 16:46:10 UTC 2013


On 2013-09-07, Charles Elliott <elliott.ch at verizon.net> wrote:

> Unruh wrote:
>
>> Why? Why not just do it the average offset?
>
> Many inexpensive GPS devices exhibit a saw tooth pattern of their
> offsets relative to a known good time source, where the period is
> less than constant, and the amplitude often is plus or minus 60 ms
> relative to 0 offset. According to Dave Hart, this saw tooth pattern
> is inherent in the GPS device itself (the time between fix and time
> output "wanders") and not caused by NTPD processing.

There is a nice overview of GPS available at
http://web.njit.edu/~rlopes/6.2.0%20-%20GPS-overview.pdf

The discussion of GPS receivers begins on slide #23.

GPS uses the concept of time of arrival (TOA) of signals to determine
user position (slide #42).

Signal travel time measurement is discussed in slides #44 and #45.

Clock synchronization is discussed in slides #44 through #50. The
receiver's quartz clock is continually reset to be in sync with the
satellites' atomic clocks.

There is a discussion of how the USNO monitors the timing of the GPS
satellites at http://tycho.usno.navy.mil/gpstt.html

> Regardless of its cause, until one understands this saw tooth pattern
> it appears like no time2 adjustment works for any length of time,
> hence the attempt to sneak up on it.

I'm not sure where you're going with this attempt to calculate the
perfect time2 value.

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



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