Re: Synchronising PC clock on the GPS time using GPS receiver
From: John Navas (spamfilter0_at_navasgroup.com)
Date: 08/27/04
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Date: Fri, 27 Aug 2004 22:16:08 GMT
[POSTED TO sci.geo.satellite-nav - REPLY ON USENET PLEASE]
In <f0701dba.0408270700.21ea6aea@posting.google.com> on 27 Aug 2004 08:00:10
-0700, jon@parmetpc.volpe.dot.gov (Jon Parmet) wrote:
>John Navas <spamfilter0@navasgroup.com> wrote in message news:<OYtXc.9906$54.142640@typhoon.sonic.net>...
>> You don't, but that's not NTP -- that's NMEA.
>
>My understanding may be wrong, but I thought NTP came with a plethora
>of device driver options for different GPS devices and different
>interfaces. So you'd configure it for Garmin whatever on Serial port
>wherever using NMEA.
NTP per se is the standard time protocol used on the Internet. What's
confusing is that ntp.org chose to call their reference software "NTP", which
made about as much sense as calling a browser or webserver "HTTP". It happens
that NTP-the-software can use NMEA to obtain GPS time information, but that
has nothing to do with NTP-the-standard. It's also massive overkill if all
you want to do is set the PC clock using the NMEA output from a GPS.
>Please correct any mistakes I'm making, because I'm interested in how
>the flow actually works here. NMEA is the protocol used in delivering
>the data from (in this case) the serial port.
>From the GPS over a serial connection to the PC (by means of a serial port).
>But after that, NTP
>actually takes care of serving out the time.
If serving is what you want. FWIW, I personally run my own NTP network
server, but I simply sync it from a public Stratum 2 server, rather than using
a local GPS. I can't say for sure, but I'm guessing that NTP from a Stratum 2
server might well be more accurate -- there's a fair bit of uncertainty and
delay in an NMEA data stream.
>Ok, so in the case of a
>standalone box, the NTP Client would be on the same machine as the NTP
>Server.
Neither NTP client nor server would actually be used, just clock setting from
NMEA input. In other words, most of the code would be unused, which is what I
mean by massive overkill.
>I've never worked with a Windows' solution.... nor NMEA for that
>matter in this type of application ;)
>
>We were running under Unix, used a Trimble SVeeSix, TSIP protocol to
>acquire GPS Time, then simply called settimeofday(2) to effect the
>change in system clock time. The accuracy requirements we had for time
>were no where near what NTP provides, so the code was very
>simple-minded (it didn't account for any latencies).
Which is why I personally use NTP from a Stratum 2 server.
-- Best regards, John Navas <http://navasgrp.home.att.net/>
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