[time-nuts] Precise positions for GPSDOs

Chris Albertson albertson.chris at gmail.com
Thu May 2 11:19:17 EDT 2013

Google maps is NOT that good, it can be off by a lot, tens of meters.

I had to have my property line surveyed some years ago to get a city
building permit. So now I have two brass markers at know position.
The survey crew used traditional transits from a brass benchmark.
Google Earth thinks these brass markers are a few meters from here the
survey crew said. (Yes I know about WGS84, we are all working in that

I think the problem is that the lland is not flat here.   If I lived
in Kanas the Google system might work.   But I don't think Google
warps the images to account for hills and even slopes.  I don't know
the source of Google's error.  The 1 Sigma on the self survey is about
.5 meters more or less.

I think the best why to measure is to let the self survey run for a
full 24  hours so you get two full orbital periods of each satellite.
And also to  make sure you have 360 degree view of the sky.    I think
a view in only one direction might be biased.

But yu can check Google.  Find a few brass government benchmarks near
your house and have Google locate them and if you got a match go with

On Thu, May 2, 2013 at 2:29 AM, Stewart Cobb <stewart.cobb at gmail.com> wrote:
> A GPSDO typically makes the assumption that the position of its antenna is
> fixed and well-known. That removes position uncertainty from the navigation
> equations, and allows all the "information" from the satellite measurements
> to be used to improve the time estimate. Errors in this position create
> errors in timing, with a magnitude scaled by the speed of light (one ns per
> foot, three ns per meter).
> Most GPSDOs do some sort of position averaging when they are first turned
> on, to come up with a good-enough estimate of antenna position. For a true
> time-nut, that might not be good enough.
> GPS surveying equipment can easily determine the position of your antenna
> to within a few centimeters (~20 ps). Unfortunately, such equipment is
> expensive and difficult to borrow.
> A high-end GPSDO designed today should have the ability to record phase
> data into RINEX files, which could be sent to a service like OPUS to find
> the antenna position.
> <http://www.ngs.noaa.gov/opus/>
> But few do, so far.
> The next best idea is to locate your antenna on Google Maps. Type in the
> self-surveyed position to the Google search box, either as decimal degrees
> or as DMS, formatted like this but without the quote marks:
> "37.384542, -122.005526"
> "37 23 4.35, -122 0 19.89"
> Click on the map and zoom in. Click on the "Map" box in the upper right and
> uncheck the "45 degree view" icon. Then right-click on the spot on the
> picture where your antenna is actually located, and select "What's here?"
> from the pop-up menu. A green arrow marker will appear, pointing to your
> antenna. Left-click on the arrow, and read your latitude and longitude in
> both formats. Enter one of them into your GPSDO, replacing the self-survey,
> and enjoy increased accuracy.
> A true time-nut will take one more step to improve accuracy. (Sorry, but
> the rest of this is specific to North America. Similar details apply to
> other parts of the world, but I only know the recipe for the place I live.)
> Google Maps photos are registered (quite accurately) to the North American
> Datum "NAD83". Unfortunately, your GPSDO operates in a different datum
> known variously as WGS84, ITRF, or IGS (these are all essentially the
> same). The difference between these two datums can be a couple of meters,
> easily visible on the map photos and worth 5 ns or more of time error.
> Fortunately, you can convert NAD83 to ITRF2008 at this website:
> <http://www.geod.nrcan.gc.ca/apps/tmobs/tmobs_e.php>
> For "ITRF epoch", just enter today's date. For "ellipsoidal height", use
> the value from your self-survey if you don't have a better one. You might
> be able to get a better one from Google Earth, or by finding a nearby
> benchmark from this site (US only) and extrapolating to your antenna
> location.
> <http://www.ngs.noaa.gov/cgi-bin/ds_radius.prl>
> Note that the WGS84 ellipsoid is tens of meters higher than sea level
> through most of North America, so if you live near the ocean, your
> "ellipsoidal height" will probably be negative.
> Hope someone find this useful.
> Cheers!
> --Stu
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Chris Albertson
Redondo Beach, California

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