Abstract.Arrival times of some millisecond pulsars can be measured with submicrosecond precision. This allows unprecedented measurements of the rotation of these stars, astrometric parameters including proper motion and parallax, motion in binary and complex dynamical systems, perturbations resulting from propagation through small scale turbulence in the interstellar plasma and other effects. In addition to the independent parameters which are required to model each pulsar, there is a set of global parameters that affect the array of pulsars in a correlated manner. These parameters involve the international atomic time scale, the ephemeris of the Earth's orbit and the gravitational wave background and have monopole, dipole and quadrupole and higher order angular signatures, respectively.