It is precisely the time dilation effect that they measure. As long as two clocks are comoving, the only difference in their rates is due to the difference in t

It is precisely the time dilation effect that they measure. As long as two clocks are comoving, the only difference in their rates is due to the difference in their gravitational potential. Without a clock very far from any gravitating body, it's impossible to measure the absolute depth of the gravitational potential. This paper was downright incredible when it came out: http://www.sciencemag.org/content/329/5999/1630.short http://www.sciencemag.org/content/329/5999/1630.short NIST PDF here: tf.boulder.nist.gov/general/pdf/2447.pdf