  Given sufficient sensitivity, pulsar timing observations can make a direct
detection of gravitational waves passing over the Earth. Pulsar timing is most
sensitive to gravitational waves with frequencies in the nanoHertz region, with
the most likely astronomical sources being binary super-massive black holes in
galaxy cores. The Parkes Pulsar Timing Array project uses the Parkes 64-m radio
telescope to make precision timing observations of a sample of about 20
millisecond pulsars with a principal goal of making a direct detection of
gravitational waves. Observations commenced about one year ago and so far
sub-microsecond timing residuals have been achieved for more than half of these
pulsars. New receiver and software systems are being developed with the aim of
reducing these residuals to the level believed necessary for a positive
detection of gravitational waves.
