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 _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.