UT1 is definitively established by the IERS (now called the International Earth Rotation and Reference Systems Service). UT1 is referenced to the ITRF (the International Terrestrial Reference Frame) which is basically a set of points whose coordinates realize the ideal International Terrestrial Reference System (ITRS) There can be many realizations of the ITRS -- WGS84 is another one. Over the years, WGS84 has been adjusted to move closer to the ITRF which has turned out to be a better realization of the IERS. The longitude origin of the ITRS is that which was initially given by the forerunner of the IERS, the Bureau International de l'Heure (BIH) at the epoch 1984.0. For some background on the relationship between the BIH zero meridian and that of the Airy transit circle at Greenwich, see this interesting Web page: <http://www.nmm.ac.uk/server/show/conWebDoc.416>. By the way, the U.S. military probably no longer has a better model of the Earth's gravity field than that of the civilian community. The gravity model which accompanies WGS84 was fully declassified in the early 1990s. Subsequently, U.S. DoD worked with the U.S. National Geodetic Survey and NASA to produce the superior and publicly available Earth Gravity Model 1996. This is a global model. There are regional models of different parts of the Earth which are even better.
-- Richard Langley On Tue, 24 May 2005, Frank King wrote: >Dear Wee-Meng > >You raise some very interesting points... > >> When I read about longitudes/latitudes in GPS articles, >> there are loads of different types of projection used. > >Indeed there are. These all represent different models >of the shape of the Earth and the best one is kept secret >by the U.S. Military who probably know the shape of the >Earth better than anyone else! > >> In my GPS, if a point is specified using the wrong >> projection, it may be way off. > >Sadly this is true. We in the U.K. like to think that >Longitude 0 degrees has been fixed since 1884 by the >position of Airy's Transit Circle telescope at Greenwich. > >For some purposes this is still true but certainly not >for all purposes. Even the much-acclaimed British >Ordnance Survey Maps use a different Longitude 0 (for >the simple reason that the Ordnance Survey started >long before 1884 and they haven't wanted to change!). > >The WGS84 model was established by the U.S. who used >a secondary longitude (probably one in Washington) as >a reference during the refining of the model. When >they finished, it turned out that 0 degrees on the >WGS84 model is about 6 arc-seconds to the EAST of >the longitude of the Airy Transit Circle. > >I am writing all this from memory so someone else may >correct this figure of 6 arc-seconds. > >If my memory is right this translates into about >0.4 seconds of time or (very roughly) 100 metres >at the latitude of Greenwich. > >I don't think diallists should worry too much about >this error. UTC is allowed to differ from UT1 by over >twice this amount and I expect most diallists use UTC >for checking sundials without correcting to UT1. [A >few serious pedants like me make this correction!] > >To those readers who are familiar with the difference >between UT1 and UTC I should like to have it confirmed >that UT1 STILL uses the Airy Transit Circle as defining >reference longitude 0 degrees. > >In short: is it still the case that 12h UT1 is the >instant of superior transit of the mean sun at Airy's >Transit Circle? I am fairly sure the answer is yes but >I would be happier if some expert could confirm this. > >Frank King >Cambridge, U.K. > >- > =============================================================================== Richard B. Langley E-mail: [EMAIL PROTECTED] Geodetic Research Laboratory Web: http://www.unb.ca/GGE/ Dept. of Geodesy and Geomatics Engineering Phone: +1 506 453-5142 University of New Brunswick Fax: +1 506 453-4943 Fredericton, N.B., Canada E3B 5A3 Fredericton? Where's that? See: http://www.city.fredericton.nb.ca/ =============================================================================== -
