[time-nuts] Some results of PRS10 and Trimble Resolution

bg at lysator.liu.se bg at lysator.liu.se
Fri Jun 30 16:10:42 EDT 2006

On Fri, June 30, 2006 20:49, Brooke Clarke said:
> Hi Björn:
> I don't think that's that case.  I live in a small town and hired a
> local surveyor chosen because he's the guy that uses GPS.

Maybe I was wrong with extrapolating the local situation to a global level.
With the present dense network of "CORS"-like stations [1]. Surveyors are
moving away from running their own RTK-base stations. With close to full
coverage RTK-corrections is distributed via the cellphone network, which
means the surveyor only need the rover side of the traditional equipment.

> antenna.  To do this he needed to go back to his office and post process
> the data.

That makes his GPS recievers cheaper, than the ones where the software
options for RTK is enabled. He also does not need a real time radio
connection. Precision is about the same or better, since he can download
better ephemeris, and can tinker more with parameters in the software.

> It's my understanding the the military has the crypto key needed to use
> the L2 channel to full effect and that's not available to surveyors.

That is true in theory. In reality Ashtech (and others) early found that
the crypto code rate is much lower then the chipping rate. Details was
posted here in the last 6 months? IIRC the known P-code will flip sign
every 511 bits depending on the crypto code. This makes the highend
receivers do much more than intended on L2.

> What surveyors do is record the carrier phase of both the L1 and L2
> signals and resolve the ambiguity in post processing.  There are a
> couple of flavors of post processing.

And there is real time flavors as well, with the best going down below 5cm

> I think what you may be referring to are the differential correction
> methods, either Low Frequency or satellite broadcasts that improve the
> position accuracy of GPS receivers.  But I don't think these can be used
> to improve timing accuracy.

These are the SBAS (WAAS, EGNOS, MSAT), commercial satellite based, "coast
guard" DGPS, etc.  Where the good do sub 1m and SBAS a little worse.

Why would these not make absolute accuracy better? They do improve



[1]   http://swepos.lmv.lm.se/natverksrtk/nat_postj_031012.gif

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