Hi, While we are chatting about how GPS works and its accuracy, I would like add a few interesting tidbits.
A good part of the accuracy come from taking Einstein's Relativity into account. Special Relativistic effects like properly calculating doppler shifts and relative motion are important, but also General Relativistic effects need to be applied. For example, Special Relativity states that moving clocks run slower, but General Relativity states that clocks run slower in a gravitational field. Satellites are moving fast compared to someone on Earth so this makes their clocks to run slower. But satellites feel less of the earth's gravity so our Earth-bound clocks run slower. Since a satellite speed, while fast to us is slow relative to the speed of light, our Earthly clocks end up running slower than clocks on GPS satellites (~50 microseconds/day which amounts to about a 7 nautical mile spread!). Also, light (i.e. GPS signals) do not travel in a straight path as one assumes in triangulating a "fix". Rather matter curves space around it, so GPS signals actually bend (i.e. gravitational lensing). The latter effect, which is tiny compared to the former, was actually proved using a sextant of sort, by measuring a star's position during a solar eclipse in 1919. While these effects are ordinarily insignificant for life on Earth, they are important on the scale of GPS accuracy. I am sure that Einstein did not have GPS in mind when he wrote down the theory of Relativity, but I'll still thank him nonetheless. - Paul E. 1979 C&C 29 Mk1 S/V Johanna Rose Carrabelle, FL On Jan 29, 2013, at 8:44 AM, cnc-list-requ...@cnc-list.com wrote: > Message: 5 > Date: Tue, 29 Jan 2013 05:55:03 +0000 > From: "Brent Driedger" <bren...@highspeedcrow.ca> > To: "Leslie Paal" <lpaalc...@yahoo.com>, cnc-list@cnc-list.com > Subject: Re: Stus-List More on GPS accuracy > Message-ID: > > <499177535-1359438905-cardhu_decombobulator_blackberry.rim.net-1984730907-@b17.c8.bise6.blackberry> > > Content-Type: text/plain; charset="Windows-1252" > > I'm enjoying this in depth GPS education. > I recall some scuttlebutt in Sail magazine a year or two ago warning that > most of the birds in the system were approaching their "best before date" of > over 25 years and without getting immediate replacement the system would be > down a few leaving some holes or temporary signal loss in some locations in > the coming years. Have you heard any updates to this rumor? > > Brent Driedger > s/v Wild Rover > C&C 27-5 > Sent from my BlackBerry? smartphone on the MTS High Speed Mobility Network > > -----Original Message----- > From: Leslie Paal <lpaalc...@yahoo.com> > Sender: "CnC-List" <cnc-list-boun...@cnc-list.com> > Date: Mon, 28 Jan 2013 12:04:01 > To: cnc-list@cnc-list.com<cnc-list@cnc-list.com> > Reply-To: Leslie Paal <lpaalc...@yahoo.com>, cnc-list@cnc-list.com > Subject: Stus-List More on GPS accuracy > > _______________________________________________ > This List is provided by the C&C Photo Album > http://www.cncphotoalbum.com > CnC-List@cnc-list.com
_______________________________________________ This List is provided by the C&C Photo Album http://www.cncphotoalbum.com CnC-List@cnc-list.com
