[time-nuts] Precise positions for GPSDOs

EB4APL eb4apl at cembreros.jazztel.es
Thu May 2 20:36:00 EDT 2013

I fully agree with Chris, do not trust Google Earth for any serious 
technical use, I found errors in 100-200 m range.  You only need to 
check where two images are stitched.
Google Earth images are not produced by Google, they get them from other 
companies or government bodies involved in making geographical 
information, I can't speak about it in a whole but I actually know cases 
in what Google tried to get the info for free. The metric quality (or 
QUALITY) is not controlled by Google as far as I know. Think of Google 
Earth as a means of providing geographical information for the layman, 
for finding places, advertising  and so but you don't know how accurate 
it is, even the date of the images can be erroneous, you can verify this 
I had professionally advised many customers to not rely on this info for 
any serious use, giving them actual examples.  It is a very good and 
amazing product but its goal is not to make any precise measurement, and 
the GPS antenna position determination is in fact a surveying task.

Ignacio EB4APL

On 02/05/2013 17:19, Chris Albertson wrote:
> 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
> system)
> 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
> Google
> 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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