[LEAPSECS] LEAPSECS Digest, Vol 51, Issue 22

[Warner Losh]

On 02/07/2011 11:12, Tony Finch wrote:

> On Mon, 7 Feb 2011, Steve Allen wrote:

>> Until there is an ensemble of cesium chronometers not on the surface of

>> the earth there is no easy way to measure the potential depth to 1e-10,

>> so the corrections currently being used to compensate for the NIST and

>> PTB chronometers being about a mile high are as good as things can get.

> The PTB campus is at about 75m above sea level.


Boulder, where the NIST campus is, is 1700m above seal level, more or 
less.  I know they are measuring clocks there because some gear that my 
former employer made, along with a bunch of software I wrote, is sitting 
there gathering measurements on 24 different cesium clocks and providing 
data to the BIPM...


> Also, doesn't the GPS count as an ensemble of atomic clocks?


They are.  And they are steered to UTC(earth) not UTC(orbit) and the 
various relativistic effects are also compensated for (Sagnac, Doppler 
and acceleration being the main three).  These three effects are large 
enough to measure with cesiums in airplanes flying a loop over a fixed 
course, and would present unacceptably large errors in the GPS solution 
were they neglected.

Warner