[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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