GPS and relativity

In article ,
Stan Gosnell wrote:
(Everett M. Greene) wrote in
:

According to an article in the November 2004 edition of
Scientific American, the GPS satellites' clocks have a
7 microsec per day effect due to relativistic motion
and a 45 microsec per day effect due to the lower
gravity in orbit. The two effects partially cancel, so
the net adjustment is 38 microseconds.

It is left as an exercise for the student to determine
the effect of an uncompensated time value on position
values.

It's many miles. Nanosecond accuracy is required for the current ~10
meter accuracy. Linear extrapolation would lead to ~10 kilometers.

An easy back-of-the-envelope way to look at this is that light travels
roughly a foot per nanosecond. 38 microseconds = 38000 nanoseconds.
Thus, in 38 microseconds, light will travel about 38000 feet, or about
7 miles.

-- Dane

Others have covered this amply already it's really not about hor far
light goes in 38us. However, I do want to plug one of my favorite
internet tools: The Google Calculator.

If you google on "38 microseconds * c in miles" it'll print out 7.079
for you. It does dimensional analysis and has a half decent library of
constants, too. By using 'in' at the end of the calculation you get to
specify what form you'd like to see your result.

-- dave j