  Millisecond and binary pulsars are the most stable astronomical standards of
frequency. They can be applied to solving a number of problems in astronomy and
time-keeping metrology including the search for a stochastic gravitational wave
background in the early universe, testing general relativity, and establishing
a new time-scale. The full exploration of pulsar properties requires that
proper unbiased estimates of spin and orbital parameters of the pulsar be
obtained. These estimates depend essentially on the random noise components
present in pulsar timing residuals. The instrumental white noise has
predictable statistical properties and makes no harm for interpretation of
timing observations, while the astrophysical/geophysical low-frequency noise
corrupts them, thus, reducing the quality of tests of general relativity and
decreasing the stability of the pulsar time scale.
