Re: Synchronising PC clock on the GPS time using GPS receiver
From: Jon Parmet (jon_at_parmetpc.volpe.dot.gov)
Date: 08/27/04
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Date: 27 Aug 2004 08:00:10 -0700
John Navas <spamfilter0@navasgroup.com> wrote in message news:<OYtXc.9906$54.142640@typhoon.sonic.net>...
> [POSTED TO sci.geo.satellite-nav - REPLY ON USENET PLEASE]
>
> In <f0701dba.0408261033.7a64e43a@posting.google.com> on 26 Aug 2004 11:33:40
> -0700, jon@parmetpc.volpe.dot.gov (Jon Parmet) wrote:
>
> >> Works fine here, with the server run by my ISP (Sonic.net), as well as open
> >> public servers.
> >
> >the Original poster asks:
> >
> >"with a (Garmin) GPS receiver connected to serial port of the PC, is
> >it
> >possible to synchronise PC clock to GPS time?"
> >
> >Why would you need to connect to another server when you've got a GPS
> >Receiver hooked onto the serial port?
>
> 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.
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. But after that, NTP
actually takes care of serving out the time. Ok, so in the case of a
standalone box, the NTP Client would be on the same machine as the NTP
Server.
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).
Still, I suppose we could have served out reasonably accurate enough
time from that box using NTP even though NTP wasn't controlling the
device itself. They'd all be offset from USNO, but they'd all be
synced close to each other ;)
> >If you've got problems connecting to the serial port, that's a
> >different thread drift ;)
>
> Indeed. This thread drift was about NTP the protocol versus NTP the software.
> ;-)
>
> My basic point is that the OP could instead use NMEA software that can set the
> PC clock with possible other GPS benefits.
>
> One such option is GPS Pac 2.2 for Windows, available for free download at
> <http://vancouver-webpages.com/peter/idx_automap.html>.
Sounds like it's analagous to the solution we implemented. Hopefully,
the newer versions of Windows have some security built into who can
call the equivalent to
settimeofday() :P
It's fun dealing with time, given there's the GPS Epoch which is Jan
6, 1980, the C Time Epoch is Jan 1, 1970, the GPS Almanac reference
times (Week#s and TOAs), and then the time you are actually concerned
with which may or may not be wall-clock time.... whew!
Regards,
Jon
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