  The K2K (KEK E362) long-baseline neutrino oscillations experiment requires
synchronization of clocks with ~100 nsec accuracy at the near and far detector
sites (KEK and Super-Kamiokande, respectively), which are separated by 250 km.
The Global Positioning System (GPS) provides a means for satisfying this
requirement at very low cost. In addition to low-resolution time data (day of
year, hour, minute, second), commercial GPS receivers output a 1 pulse per sec
(1PPS) signal whose leading edge is synchronized with GPS seconds rollovers to
well within the required accuracy. For each beam spill trigger at KEK, and each
event trigger at Super-Kamiokande, 50 MHz free-running Local Time Clock (LTC)
modules at each site provide fractional-second data with 20 nsec ticks. At each
site, two GPS clocks run in parallel, providing hardware backup as well as data
quality checks.
