>I suspect even with GPS failure they could run entirely on atomic clocks, it would just increase their uncertainty Using GPS is an operational convenience, no

>I suspect even with GPS failure they could run entirely on atomic clocks, it would just increase their uncertainty Using GPS is an operational convenience, not a necessity, I would think. If GPS didn't exist, they would have to have a master atomic clock, say, in Mountain View. Then they would have to bring each remote clock to Mountain View, synch it with the master, and then ship it (running continuously) to its final destination. I remember perusing an HP catalog in the late eighties. You could buy a portable "traveling clock" for around $40,000. It was intended to be used to synchronize remote clocks. Here is a 1965 article from the Hewlett-Packard Journal, which describes the process: http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1965-04.pdf http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1965-04.pdf