  We propose a data processing technique to cancel monopole and dipole noise
sources (such as clock and ephemeris noises respectively) in pulsar timing
array searches for gravitational radiation. These noises are the dominant
sources of correlated timing fluctuations in the lower-part ($ \approx 10^{-9}
- 10^{-8}$ Hz) of the gravitational wave band accessible by pulsar timing
experiments. After deriving the expressions that reconstruct these noises from
the timing data, we estimate the gravitational wave sensitivity of our proposed
processing technique to single-source signals to be at least one order of
magnitude higher than that achievable by directly processing the timing data
from an equal-size array. Since arrays can generate pairs of clock and
ephemeris-free timing combinations that are no longer affected by correlated
noises, we implement with them the cross-correlation statistic to search for an
isotropic stochastic gravitational wave background. We find the resulting
optimal signal-to-noise ratio to be more than one order of magnitude larger
than that obtainable by correlating pairs of timing data from arrays of equal
size.
