[time-nuts] More Z3801/Tbolt comparisons

Mark Sims holrum at hotmail.com
Thu Sep 3 19:40:50 UTC 2009

The standard self-surveys in these units leaves MUCH to be desired.  You need to do surveys for 24 hours to help mitigate satellite constellation and atmospheric effects.  Note that due to periods of poor satellite geometry,  you can't just survey for 24*60*60 fixes.  You need the survey to be based upon absolute time.

Altitude errors are typically at least three times as large as lat/lon errors,  but can be very large.  Luckily,  altitude errors seem to have a much smaller effect on GPSDO performance than lat/lon errors.

Be careful getting your altitude from topo maps.  You need to make sure your map reference geoid matches the GPS WGS84 coordinate system.  Geoid errors can be tens of meters.  For instance the NGS OPUS system reports altitude as both ellipsoid height and orthometric height.  The difference here is 25 meters.

The next version of Lady Heather will have a very precise self-survey routine.  It takes data for 48 hours (which has 24 overlapping 24 hour intervals) and statistically processes it to get a very precise location.  Typically the error is around 1 foot lat/lon,  1 meter altitude (there he goes, mixing measurement systems again).  With good antennas in good locations it can get to under 4 inches of error.  Poor antennas in bad locations might be 2 feet off.
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