The Parkes Pulsar Timing Array

The aims of the Parkes Pulsar Timing Array project are to (1) make a direct detection of gravitational waves, (2) improve the solar system planetary ephemeris and (3) develop a pulsar-based timescale. In this paper we describe the project, explain how the data are collected and processed and describe current research. Our current data sets are able to place an upper bound on the gravitational wave background that is the most stringent to date.