To successfully detect gravitational waves with pulsar timing arrays, we need
to have a comprehensive understanding of the physical origins and statistical
characteristics of the noise in pulse arrival times and identify mitigation
methods to reduce the noise. In this paper we will review radiometer noise,
phase jitter noise and timing noise in the noise budget of pulsar timing and
show various efforts used to reduce them. We will briefly discuss the results
of an overall assessment of the components and physical causes of the timing
residuals for millisecond pulsars in the North American Nanohertz Observatory
for Gravitational Waves (NANOGrav).Comment: 8 pages, to be published in the proceeding of the 10th LISA Symposiu