[time-nuts] Precise positions for GPSDOs

Thu May 2 14:04:32 EDT 2013

Actually, wouldn't you need a satellite visible mark to use google earth? Not every marker can be seen on google earth.

Then often these markers are in places you can't use safely, such as in the middle of a road. 

Note that google earth does orthorectification on the imagery. If you knew where the imagery had the least correction, that might be a place where the position data is accurate. If a tall structure looks tilted, then you know the image has had a lot of post processing.

-----Original Message-----
From: Chris Albertson <albertson.chris at gmail.com>
Sender: time-nuts-bounces at febo.com
Date: Thu, 2 May 2013 08:19:17 
To: Discussion of precise time and frequency measurement<time-nuts at febo.com>
Reply-To: Discussion of precise time and frequency measurement
	<time-nuts at febo.com>
Subject: Re: [time-nuts] Precise positions for GPSDOs

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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-- 

Chris Albertson
Redondo Beach, California
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