[time-nuts] GPS position survey

Chris Albertson albertson.chris at gmail.com
Sun May 5 20:20:30 EDT 2013

If you are talking about using this with NTP.  I don't know if you
have a choice.  The Trimble receiver is going to do whatever it is
going to do when you power it up.    Software running on a PC can
perform a 24 hour survey and report the location but the "Type 29"
driver in NTP has no way to tell the Trimble receiver what to do on
power up.  And there is no way to specify a location you have surveyed
by some other means.

Perhaps the Trimble GPS has some way to program it's FLASH Rom with
changed parameters but NTP only reads the packets.  It does not send
any.  Can LH download a surveyed location r change the length of the
survey?   NTP can't do any of that

Other NTP drivers such as the Type 30 Motorola driver are more
flexible.  Those alow you to specify a lat, long that was surveyed or
have the receiver do a survey.  The type-30 Motorola driver is a lot
more configurable.

But as was said, light travels about 30cm/nanosecond and NTP works in
microseconds.  So your location can be "off" by 1000 times 30cm before
NTP will care.  So 3 or 4 meters of location error will not matter and
the 2,000 point self survey will be good enough.

But if yo really so want nanosecond level timing, then you need to
care about the survey

On Sun, May 5, 2013 at 4:28 AM, Sarah White <kuzetsa at gmail.com> wrote:
> On 5/5/2013 6:35 AM, Miguel Barbosa Gonçalves wrote:
>> Hi fellow time nuts!
>> I've recently bought a Trimble Acutime gold that will be used as a
>> reference clock for a NTP server.
>> This receiver has the possibility of averaging it's position before
>> entering what Trimble calls the overdetermined clock state. The
>> default is to average the position with 2000 fixes.
>> What do you believe is a good number of fixes for the survey in this case?
>> Cheers,
>> Miguel
> Accurate position might be important. Light travels a distance of
> something like 29.9792 centimeters per nanosecond
> You might find the results of this study to be helpful:
> http://www.syz.com/gps/gpsaveraging.html
> I myself did a sampling over a period of 1 million samples
> (actually, it was a value of 2^20, not exactly 1 "million")
> quickest it could have completed such a survey is 12+ days
> ... However, I had "masks" set for elevation, signal, etc.
> (resulted in occasional periods when there was no "lock")
> I'm quite satisfied / happy with the results :)
> hope this helps?
> --Sarah
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Chris Albertson
Redondo Beach, California

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