A Measurement Model for Precision Pulsar Timing
  This paper describes a comprehensive measurement model for the error budget
of pulse arrival times with emphasis on intrinsic pulse jitterand plasma
propagation effects (particularly interstellar scattering), which are
stochastic in time and have diverse dependences on radio frequency. To reduce
their contribution, timing measurements can be made over a range of frequencies
that depends on a variety of pulsar and instrumentation-dependent factors that
we identify. A salient trend for high signal-to-noise measurements of
millisecond pulsars is that time-of-arrival precision is limited either by
irreducible interstellar scattering or by pulse-phase jitter caused by variable
emission within pulsar magnetospheres. A cap on timing errors implies that
pulsars must be confined to low dispersion measures (DMs) and observed at high
frequencies. Use of wider bandwidths that increase signal-to-noise ratios will
degrade timing precision if nondispersive chromatic effects are not mitigated.
The allowable region in the DM-frequency plane depends on how chromatic timing
perturbations are addressed. Without mitigation, observations at 1.4~GHz or
5~GHz are restricted to $\DM\lesssim 30$ and $\lesssim 100~\DMu$, respectively.
With aggressive mitigation of interstellar scattering and use of large
telescopes to provide adequate sensitivity at high frequencies (e.g. Arecibo,
FAST, phase 1 of the SKA, and the SKA), pulsars with DMs up to 500~$\DMu$ can
be used in precision timing applications. We analyze methods that fit arrival
times vs. frequency at a given epoch prior to multi-epoch fitting. While the
terms of greatest astrophysical interest are achromatic (e.g. orbital and
gravitational wave perturbations), measurements may ultimately be limited by
similarly achromatic stochasticity in a pulsar's spin rate.
