[time-nuts] GPS position survey

[Miguel Barbosa Gonçalves]

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