In Special Issue on the Challenge of Global Time in Large-Scale Distributed Real-Time Systems, Journal of Real-Time Systems, Vol 12, n. 3, November 1997, Kluwer Academic Publishers.
The main features of our clock synchronization scheme can be summarized as follows: hybrid external/internal synchronization protocol improves effectiveness of synchronization; heterogeneous failure semantics for clocks and processors improves previous lower bounds on processors; two-level hierarchy improves scalability. The root of the hierarchy is the GPS satellite constellation, which ``sprays'' its reference time over a set of nodes provided with GPS receivers, one per local network. The second level of the hierarchy performs internal synchronization, further ``spraying'' the external time inside the local network.
Also available extended report (gzip postscript).