  Frequency metrology outperforms any other branch of metrology in accuracy
(parts in $10^{-16}$) and small fluctuations ($<10^{-17}$). In turn, among
celestial bodies, the rotation speed of millisecond pulsars (MSP) is by far the
most stable ($<10^{-18}$). Therefore, the precise measurement of the time of
arrival (TOA) of pulsar signals is expected to disclose information about
cosmological phenomena, and to enlarge our astrophysical knowledge. Related to
this topic, Pulsar Timing Array (PTA) projects have been developed and operated
for the last decades. The TOAs from a pulsar can be affected by local emission
and environmental effects, in the direction of the propagation through the
interstellar medium or universally by gravitational waves from super massive
black hole binaries. These effects (signals) can manifest as a low-frequency
fluctuation over time, phenomenologically similar to a red noise. While the
remaining pulsar intrinsic and instrumental background (noise) are white. This
article focuses on the frequency metrology of pulsars. From our standpoint, the
pulsar is an accurate clock, to be measured simultaneously with several
telescopes in order to reject the uncorrelated white noise. We apply the modern
statistical methods of time-and-frequency metrology to simulated pulsar data,
and we show the detection limit of the correlated red noise signal between
telescopes.
