-----Original Message----- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of J. Forster Sent: Wednesday, July 15, 2009 10:54 AM To: time-nuts@febo.com Subject: Re: [time-nuts] [Fwd: Accurate Thunderbolt position]
"Mark Sims" <hol...@hotmail.com> wrote: >> >> My Ashtech Z12 system has located my g-spot to within 4mm lat/lon >> and 27mm altitude. I forwarded the full post to a good friend who knows a LOT about GPS and herewith is his reply: <snip of excellent explanation> > >> A GPS guy I know comments that when you start talking down in the >> sub-meter sorts of accuracies, particularly for absolute >> measurements... > > I wish people would not imagine that GPS measurements are "absolute." > If you don't determine position with respect to reference points on > the ground by DGPS, then you are determining position with respect to > a particular combination of satellite orbital position coordinates and > clock-offset parameters that you got from real-time broadcasts or > perhaps later via the Internet from someone. Those position- > coordinate and clock-offset parameter values were determined by > someone who _assumed_ position-coordinate values for certain ground > stations. (Real-time broadcasts suffer substantially from > extrapolation; and short-time orbit-determinations usually also > involve a significant amount of extrapolation.) There ain't no such > thing as absolute position, any more than there is "absolute time." > All position and all time measurements are _relative_ to some man-made > and man-maintained "standard." If you are talking about state-of-the- > art, research-grade measurements, then it is essential to understand > the relevant standards. Yes indeed.. > > >> there's a whole raft of factors that are all of the same general >> magnitude >> that you need to take into account: tidal deformation, ionosphere, >> multipath, thermal distortion of your antenna, changes in the cable >> due to >> temperature, etc.etc.etc.> > > The factors recited in the above-quoted paragraph are _NOT_ all of the > same magnitude. Not even close. > When I was talking to my GPS guy, we were talking single channel, non-geodetic receivers and antennas, and his point was that there's a lot of stuff that you need to be aware of when you go the next step beyond a simple handheld GPS in your car. As you've mentioned, dual frequency, good antennas, etc. are all a given when doing mm scale measurements. The tidal deformation one was interesting to me, because I hadn't thought that it was that big. Model-out-able, certainly, but interesting none-the-less. _______________________________________________ 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.