oxford,. .you're both saying the same thing. The phase difference depends on the time in transit which, with constant speed, depends on the distance travelled. For a time measuring device doomed to fail, I'd say it's doing rather well!. .. .I'm beginning to realise that my quest for knowledge will have to go on elsewhere though. That explanation is simple indeed and that's the problem - it's not detailed. <img border="0" title="" alt="[Frown]" src="frown.gif" /> . .. .Cheers,. . /ft