[time-nuts] Common sky pps errors for any GPSDOs?

Björn Gabrielsson bg at lysator.liu.se
Tue Jan 6 08:11:00 EST 2009

Hi Matt,

On Mon, 2009-01-05 at 22:03 -0800, Matt Ettus wrote:
> On Mon, Jan 5, 2009 at 9:27 PM,  <SAIDJACK at aol.com> wrote:
> > Hi Matt,
> >
> > having 140ps matching of the 1PPS between units is the equivalent of  knowing
> > your antenna position to within ~0.14 feet total error max.
> >
> > Thats less than one inch error per antenna!
> That makes it sound a lot more difficult than it really is.  The vast
> majority of the error in GPS is systematic, such that two GPS systems
> with antennas near each other should have highly correlated error.
> This is the basis of differential GPS.  It doesn't matter if the
> absolute error is hundreds of feet, as long as  both devices have the
> same error.

One idea with a "traditional" GPSDO... I assume the sites have a real
time communication link, Internet or Radio.

Make sure the sites use exactly the same GPS SVs. This can be forced by
telling the GPS to exclude SVs you do not want to use. Either you go
with 1-SV timing mode and manually tell the receivers which SV to use.
Or you juggle 4-6 SVs that are high on the sky on all sites.

This will help the GPSDO to measure against the same satellites which
gives you kind of a differential effect.

> I spent a couple of years nearly a decade ago doing differential GPS
> for steering heavy equipment.  You can get sub-centimeter errors over
> baselines in the tens of km.  Again, this is relative error.

Sub 10cm, and even sub 5cm over distances up to 40km is possible with
dual freq GPS receivers in real time. 

There are some reasonably priced dual freq GPS that can be driven by
your external 5 or 10 MHz OCXO. The Novatel OEMV-2, beeing one


Make one site the "base" and the others "rovers". Pass RTK-corrections
from your base to the rovers. You should get a very reasonable position
errors, especially since your sites are stationary and you can let the
RTK algorithms get plenty of time to converge. Then look at the
receivers estimate of local oscillator drift for your frequency
measurement. This is a fairly simple integration of COTS modules. Since
the rovers are not taking advantage of beeing at a surveyed stationary
position this is not the optimal solution. 

Someone on the list with the equipment and time to set this system up?
Tvb, have you done this?

> Matt



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