[Talk-GB] accurate GPS

Russ Garrett russ at garrett.co.uk
Wed Oct 9 11:43:14 UTC 2019


On Wed, 9 Oct 2019 at 12:31, Simon Ritchie <simonritchie.uk at gmail.com> wrote:
> > You're not going to find a (publically-accessible) physical location which has better location error than 1m or so.
> That was the kind of conclusion that I was coming to.  There's the meridian line at Greenwich, but that only gives one coordinate.  It's a pity that they don't have a crosshair with a published position.

Sadly I have to be the bearer of bad news there as well: the marked
meridian line at Greenwich no longer corresponds to a fixed coordinate
in any modern coordinate system! The WGS84 zero degree meridian is
approximately 100m to the east but that too will move due to
continental drift and other esoteric gravitational factors.

> I think that the only way forward may be to get hold of another accurate GPS device and compare results.  Unfortunately, the others tend to be quite expensive.  Trimble have a cheap deice called the Catalyst, but you still have to buy their correction service at £300 per month.

Getting to 2cm accuracy will be tricky by any mechanism, but there are
cheaper solutions if you're willing to do some work. Broadly:

You'll need a GPS receiver with the capability of outputting carrier
phase data (u-blox receivers will do this) and ideally a
well-characterised external antenna (these are quite expensive).
You can then fix this antenna somewhere and record several days worth of data.
This data can be post-processed with RTKLIB (http://www.rtklib.com/)
using the RINEX atmospheric correction data from OS
(https://www.ordnancesurvey.co.uk/gps/os-net-rinex-data/) - this data
is free for historical use, it's the realtime atmospheric corrections
which cost the money.

The combination of averaging, phase measurement, and atmospheric
correction should at least get you sub-20cm.

-- 
Russ Garrett
russ at garrett.co.uk



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