[time-nuts] Help in Locking a Windows Computer to GPS Time

Chris Albertson albertson.chris at gmail.com
Thu Aug 22 07:42:24 UTC 2013


Hit "send" a bit to soon.

NTP will use the "best" reference clocks it finds.  If that is GPS it will
use that.  It can also use other NTP servers.   Typically people use those
rather then getting their own GPS receiver.  If the PC has a network
connection you can likely get time to within a few milliseconds using NTP
and a few of the pool time servers.

I would defiantly recommend getting NTP to work using the pool servers
first.  Then add GPS.

Be warned that the NMEA spec says that messages apply to the current
second.  This means the NMEA data from the serial port can be up to one
second "off".  It is used only to tell you the number of the second, not
for accurate timing.    For that NTP uses the PPS reference clock.  On some
GPS receivers (not your Garmin unit) the PPS is good for a few tens of
nanoseconds.

I looked up NMEATime.  It uses "SNTP" protocol.  It will never be very
accurate.  Think about a mechanical clock that you want to set.  First to
set it then you wait a day or so and see if it gains or looses time.  Then
you adust the rate, faster or slower.  Eventually the clock keeps good time
after a few more cycles of adjust and wait and check.   NTP works like
this.  SNTP simply sets the clock once then quits and never even looks at
the rate.

This is the place to get NTP
http://www.ntp.org
However many Windows users like to get third party versions of NTP.  These
are packages with installers and are good for people who can't build and
install the source distribution.  Google should find one.

About your Garmin GPS.   You can buy a real "timing receiver" for under $20
on eBay.  If you need nanoseconds that is the way to go.   A timing
reciever will have at least two features (1) position hold, where the
receiver is told it is NOT moving and (2) self survey, where the reciever
can take about 30 minutes or longer to deterim it's position to typically
less than a meter.  Position uncertainty creates time uncertainty (by the
speed of light) so not knowing you location by a meter means you don't know
the time within about 3 nanoseconds.






On Wed, Aug 21, 2013 at 11:48 PM, Chris Albertson <albertson.chris at gmail.com
> wrote:

> It's simple, just install and run "NTP"
> http://www.ntp.org/downloads.html
>
>
> On Wed, Aug 21, 2013 at 9:02 PM, Rex Moncur <rmoncur at bigpond.net.au>wrote:
>
>> I am trying to lock a Windows XP computer to GPS time taking advantage of
>> both the NEMA sentence and the 1PPS with the hope of getting to within a
>> few
>> ms.
>>
>> I am using a Garmin GPS 18PC and the NMEATime program.  When I tick the
>> box
>> to implement the 1PPS feature on NMEATime the program locks up each time
>> it
>> attempts to correct the PC time.  Perhaps there is something I need to do
>> to
>> configure the GPS 18PC to fix this.
>>
>> I would be grateful for advice as to whether and how one can use NMEATime
>> for this purpose with a Garmin GPS 18 PC or advice on other programs to
>> achieve accurate locking of the PC.
>>
>> Rex VK7MO
>>
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>
>
>
> --
>
> Chris Albertson
> Redondo Beach, California
>



-- 

Chris Albertson
Redondo Beach, California



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