[time-nuts] time-nuts Digest, Vol 62, Issue 25

[Mark Sims]

I have a geodetic grade Ashtech Z12 GPS receiver and antenna.  You collect data with it (both L1 and L2 carrier phase) and email the data to the National Geodetic Survey OPUS processing system.  They crunch the numbers and come back with a location (it's free).

The location is calculated by crunching the numbers against the data collected simultaneously at  3-9 CORS reference stations.  The CORS network is a network of several hundred fixed high-precision GPS stations that continuously collect data and monitor and cross-check each other.

Included in the OPUS results is an error estimate.  I have done several runs and the error estimates are usually under 4mm.  However if you compare the spread in the actual locations generated,  they are usually within 400 microns.  

I am pretty darn sure I know where I am...  except that I am drifting across the planet at about 10 mm / year.  (The location info for the CORS reference stations includes a velocity vector).  And let's not get into things like thermal expansion of my front deck and tidal deformation of the earth's crust...  

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How do you KNOW your position to such accuracy? Is it in fact possible to survey your position to that accuracy by any independent means?
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