Global travel-time data from the ISC Bulletin were used to determine the function τ(p) for core phases. Estimates were obtained for the AB, BC, DE, and EF branches of the PKP phase and also for the SKS and SKKS phases. The data were all analyzed in a uniform manner to yield 31 estimates of τ(p) and its uncertainty at the 99.9 per cent confidence level in a range of p which covers most of the core. The effects of the mantle were removed with the PREM velocity models.
The extremal bounds on the P velocity in the core were estimated at the joint confidence level of 97 per cent. The average width of the velocity bounds is about 0.6 km/sec, with the major source of uncertainty coming from the SKS and SKKS data. The outer core-inner core transition zone is well constrained, and the results favor a simple fairly abrupt increase in velocity.