My superficial knowledge of the subject comes from an A in a grad class at 
Caltech on Relativity, long before GPS.  No problem in measuring interval 
between events at the same place, with a relativistic correction. The issue 
here is synchronization of clocks at different points in space (CERN and 
Italy).  This is doable, of course, but not trivial. Still hopin' Mr. Natural 
will enlighten me! 

Incidentally, most "explanations" of GPS are vaguely incorrect.  You CAN'T 
directly measure the time a signal takes to get to you, sitting at a mountain 
lake, from a Teapot in the sky, because you can't synchronize your $199 Gelsons 
GPS unit.  It doan really know when the signal was sent.  There's a tricky 
little calculation involving the difference in arrival time at your station 
between calibrated signals from two located, synchronized Teapots.  It does 
assume you are at one place at a given instant -- oftentimes the case with me.  

GPS, like most things in this world, is much smarter than its interpreters!  

Peter Lissaman, Da Vinci Ventures 

Expertise is not knowing everything, but knowing what to look for. 

1454 Miracerros Loop South, Santa Fe, New Mexico 87505,USA 
tel:(505)983-7728 

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