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From: Geoffrey S. Mendelson on 10 May 2010 05:39 Morten Reistad wrote: >>Yes. However, the satellites are flying fast enough that they "age" at a >>slightly different speed than people on earth. So they adjusted the >>clock rate of the GPS satellites to compensate for this so that the run >>at the same speed as on earth. > > They are also at a different height in the gravity well we live in, > and that affects aging as well. The signals reaching us also go through > a gradient in that gravity well; and ISTR they compensate for that > too. Is this adjustment in the satellites or in the receivers? Geoff. -- Geoffrey S. Mendelson, Jerusalem, Israel gsm(a)mendelson.com N3OWJ/4X1GM New word I coined 12/13/09, "Sub-Wikipedia" adj, describing knowledge or understanding, as in he has a sub-wikipedia understanding of the situation. i.e possessing less facts or information than can be found in the Wikipedia.
From: Charlie Gibbs on 10 May 2010 13:11 In article <hs7ni6$30p$1(a)news.eternal-september.org>, Groleau+news(a)FreeShell.org (Wes Groleau) writes: > On 05-09-2010 18:14, Geoffrey S. Mendelson wrote: > >> As for the data, orignaly it was good to 10 meters (30 feet) without >> the encryption keys or other data. There were companies in the US >> selling correction data which was sent of the subcarrier of FM >> broadcast stations, and GPS units that used LORAN* data to compensate. > > Why LORAN? OMEGA superceded LORAN long ago. I think OMEGA receivers were quite a bit more expensive than LORAN. It was LORAN, not OMEGA, that started appearing in models priced for general aviation use before GPS blew everything away. -- /~\ cgibbs(a)kltpzyxm.invalid (Charlie Gibbs) \ / I'm really at ac.dekanfrus if you read it the right way. X Top-posted messages will probably be ignored. See RFC1855. / \ HTML will DEFINITELY be ignored. Join the ASCII ribbon campaign!
From: Morten Reistad on 10 May 2010 17:38 In article <slrnhufkuu.5ge.gsm(a)cable.mendelson.com>, Geoffrey S. Mendelson <gsm(a)mendelson.com> wrote: >Morten Reistad wrote: > >>>Yes. However, the satellites are flying fast enough that they "age" at a >>>slightly different speed than people on earth. So they adjusted the >>>clock rate of the GPS satellites to compensate for this so that the run >>>at the same speed as on earth. >> >> They are also at a different height in the gravity well we live in, >> and that affects aging as well. The signals reaching us also go through >> a gradient in that gravity well; and ISTR they compensate for that >> too. > >Is this adjustment in the satellites or in the receivers? GPS and cousins really need to use the full set of formulas from general relativity. The degree of precision needed for time is in the order of 1E-10 seconds on a ~70 millisecond signal path. GPS has to take relativity into account for gravity field, satellite speed, rotational drag, and gravity gradient for the signal. It also has to compute athmospheric deflection of the signal. This has to be done as a general operating procedure. With modern receivers there is hardware to do this, though. Geostationary station keeping is a lot simpler. The same effects are operating there, but they represent a constant difference from newtonian mechanics, so the calculation can be done using newtonian mechanics with a constant offset. A satellite can be pretty far off (in kilometers) from the ideal station point without much effect on the stability or the signal strength. Most such satellites are actually in orbit around the geostationary ellipse. -- mrr
From: Bill Marcum on 10 May 2010 22:33 ["Followup-To:" header set to alt.folklore.computers.] On 2010-05-06, Huge <Huge(a)nowhere.much.invalid> wrote: > On 2010-05-06, Charles Richmond <frizzle(a)tx.rr.com> wrote: >> Charlie Gibbs wrote: >>> >>> [snip...] [snip...] [snip...] >>> >>> Smart is believing only half of what you hear. >>> Brilliant is knowing which half. >>> >> >> "Believe nothing you hear, and only half of what you see." > > Damn, I thought that was Lou Reed. > Or was it Lou Rawls? (Or Marvin Gaye, or Gladys Knight). I can't help being confused.
From: Bill Marcum on 10 May 2010 22:28
["Followup-To:" header set to alt.folklore.computers.] On 2010-05-06, Huge <Huge(a)nowhere.much.invalid> wrote: > On 2010-05-04, Joe Pfeiffer <pfeiffer(a)cs.nmsu.edu> wrote: >> Ian Gregory <ianji33(a)googlemail.com> writes: >> >>> On 2010-05-04, Joe Pfeiffer <pfeiffer(a)cs.nmsu.edu> wrote: >>> >>>> When we can see that something can be done, arbitrarily decreeing that >>>> it's "impossible" for a machine to do it is, as Patrick points out, >>>> bizarre. >>> >>> According to folklore the laws of aerodynamics prove that the bumblebee >>> should be incapable of flight but scientists never claimed that they had >>> evolved an anti-gravity organ or anything like that. It was always clear >>> that we simply didn't have an adequate grasp of aerodynamics, fluid >>> dynamics, biomechanics etc to explain such a complex phenomenon. >> >> http://www.paghat.com/beeflight.html > > Illumination provided by cows? > By the light of the silvery moo. |