Microsecond accuracy at multiple sites: is it possible without GPS?

Critical infrastructure timing systems rely upon GPS clocks simply because microsecond accuracy is easy to achieve with GPS and difficult to achieve without it. This paper has discussed how GPS clocks meet critical infrastructure timing requirements, why they are vulnerable, and some potential backup strategies. It seems clear that achieving microsecond accuracy at thousands of geographically dispersed sites without relying on GPS clocks is a very challenging problem. It seems equally clear that solving this problem is in the best interest of the United States.