[time-nuts] Precise positions for GPSDOs

[Stewart Cobb]

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