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