  Locating sources on the sky is one of the largest challenges in gravitational
wave astronomy, owing to the omni-directional nature of gravitational wave
detection techniques, and the often intrinsically weak signals being observed.
Ground-based detectors can address the pointing problem by observing with a
network of detectors, effectively triangulating signal locations by observing
the arrival times across the network. Space-based detectors will observe
long-lived sources that persist while the detector moves relative to their
location on the sky, using Doppler shifts of the signal to locate the sky
position. While these methods improve the pointing capability of a detector or
network, the angular resolution is still coarse compared to the standards one
expects from electromagnetic astronomy. Another technique that can be used for
sky localization is null-stream pointing. In the case where multiple
independent data streams exist, a single astrophysical source of gravitational
waves will appear in each of the data streams. Taking the signals from multiple
detectors in linear combination with each other, one finds there is a two
parameter family of coefficients that effectively null the gravitational wave
signal; those two parameters are the angles that define the sky location of the
source. This technique has been demonstrated for a network of ground-based
interferometric observatories, and for 6-link space interferometers. This paper
derives and extends the null-stream pointing method to the unique case of
pulsar timing residuals. The basic method is derived and demonstrated, and the
necessity of using the method with multiple sub-arrays of pulsars in the pulsar
timing array network is considered.
