Sir Isaac Newton

[xelasnave]

Hi Julian
Thanks for your interest and your important contribution to the thread.
Given that I wonder why Genreal Relativity and Special Relativity are apparently employed in GPS.
More reading.
Thanks again.
Alex

[julianh72 (Julian)]

Originally Posted by xelasnave

Given that I wonder why Genreal Relativity and Special Relativity are apparently employed in GPS.

Two fundamental reasons:

a) The whole GPS system is about measuring location, so it is vital that the system knows the locations of all of the satellites to the highest possible precision. Missing your "target" on the Moon by 100 mm may be of no real consequence for the Apollo program, but relativistic errors will add up over time for an orbiting satellite.

b) More importantly, the system works by measuring the time lag for very accurately timed signals from the satellites to reach your receiver, so it is essential that the clocks on the GPS satellites are accurately synchronised. Time dilation effects definitely matter when you are keeping track of time to pico-second accuracy on a whole constellation of satellites, all of which are moving in their own orbits, within the Earth's "gravity well".

Every nanosecond of error would correspond to about 300 mm of error in calculated position, which would be catastrophic for a system which can measure location to centimetre-precision or better (when using survey-grade differential GPS equipment), so the time-keeping system has to allow for relativistic effects.

[xelasnave]

Thanks Julian.
I read up on it earlier and realised I knew about it but forgot some of it..as you do when you get to my age and try to cramb too much in...
I was going to explain it but already have to think again about the roles each of Special Relativity and General Relativity play...Special has to do with speed and General to do with space time being less bent☺ you have too add one subtract the other and carry the 4...☺
Part is complex but there is an interesting part the receiver plays which I have already forgoten but I am sure some one will mention it.

Maybe I should read it all again☺

I had intended to cut and paste and link and maybe I will do so but a little busy at the moment.

Thanks again for your contribution and I am sure this will be very helpful to members I certainly found it helpful.

I was thinking today about a system without satellites simply using mobile phones all over the world and their relationship to transmitters...but then I saw a butterfly and thought how interesting they are...it was a display...sad but interesting.
Alex

Here is a link for folk who would like a relatively☺ simple explanation which has links to Relativity lectures and to something on the engineering.

http://www.astronomy.ohio-state.edu/...Unit5/gps.html