Clock drift. Not strictly a Linux question.

[Vu Pham]

On Fri, 2007-02-16 at 07:59 -0800, Tony Alfrey wrote:
> Roger Oberholtzer wrote:
> > It's Friday. Time for a weekend-think-about-it question.
> > 
> > I think I am doing my maths correct. I am checking a PC's clock against a 
> > high-end Trimble receiver (> $2000), using the pulse per second signal. 
> > Unless I am doing something wrong, I seem to see a 0.02 % linear (over the 
> > time I have looked) drift in the PC's clock (via gettimeofday()) compared to 
> > the pulse from the GPS. 
> 
> Silly boy, this is clearly a general relativity problem.  The pulse 
> you're getting from the GPS is from a geosynchronous satellite, at a 
> radius of some 25,000 miles, whereas your box is in Sweden (I seem to 
> remember?), where the Earth's crust is particularly thin and so you are 
> close to the mantle.  Therefore, the gravitational field at the location 
> of your box is considerably higher than at the geosynchronous satellite. 
>   And from general relativity, we know that clocks run slower in higher 
> gravitational fields.
> 
> This is also why Swedes age slower than the rest of us.
> 

Hmm, my calculation shows the difference is only 2.e-14 second drift for
every 10 seconds of the satellites, assuming all satellites at the same
distance to the Earth center. 0.02% is too big. :)

Vu