[ntp:questions] GPS/PPS and "enable calibrate"
Mark C. Stephens
marks at non-stop.com.au
Sat Sep 7 21:01:59 UTC 2013
Thanks Steve, some really good material there.
-----Original Message-----
From: questions-bounces+marks=non-stop.com.au at lists.ntp.org [mailto:questions-bounces+marks=non-stop.com.au at lists.ntp.org] On Behalf Of Steve Kostecke
Sent: Sunday, 8 September 2013 2:46 AM
To: questions at lists.ntp.org
Subject: Re: [ntp:questions] GPS/PPS and "enable calibrate"
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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