[time-nuts] GPS position survey

Chris Albertson albertson.chris at gmail.com
Sun May 5 23:40:41 EDT 2013

So this is different from the Thunderbolt, even if they both use the
same serial protocol or can the t-bolt also have it's flash rom
programmed from a PC?

The bottle neck in the system in the uncertainty in the interrupt
latency on the PC where NTP runs.  After all this I doubt you can
captures the PPS to better than 1 uS.

For a long time I've been wnting to build an external counter for NTP.
  But it would have to use some very fast logic family.

On Sun, May 5, 2013 at 6:26 PM, Miguel Barbosa Gonçalves <m at mbg.pt> wrote:
> Hi Chris!
> On 06/05/2013, at 01:21, Chris Albertson <albertson.chris at gmail.com> wrote:
>> 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.
> What I am doing at the moment is connecting Trimble's port A (timing
> port) to NTP and configuring the receiver using a Windows laptop
> through port B. I can connect to port B occasionally to check the
> receivers health. The receiver has its configuration stored in an
> EPROM so it will just work anytime unless the configuration gets
> corrupted.
> After the survey ends the position can be optionally stored to the
> EPROM. Also, if the antenna changes its position more than 1000 meters
> a new self-survey will run automatically.
>> 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
> It does. It is available on the Trmble's FTP site.
>> 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.
> The Trimble software is very nice and allows one thing the Motorola
> NTP driver doesn't allow if I remember correctly. On the Trimble I can
> choose the number of fixes for the survey and can watch it run while
> NTP is receiving time. I can't do this on an Oncore because it only
> has one port.
>> 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.
> When surveying accuracy of the PPS will be around 1 us. The accuracy
> will obviously increase when the survey ends.
>> But if yo really so want nanosecond level timing, then you need to
>> care about the survey
> Agree!
> Cheers,
> Miguel
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Chris Albertson
Redondo Beach, California

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