5.5 Theory of Pulsar Timing System
341
pulsar’s parameters inevitably absorbs part of the instability of atomic time into the
fitted parameters, leading to the deviation of the pulsar’s parameters, and finally the
obtained timing residuals cannot fully reflect the actual difference.
In the past 10 years, the PTA has been developing rapidly over the world, and more
than 10 PTA stations have been put into operation. According to the standard operation procedures, the PTA is composed of the radio telescopes, each of which is an
array element in the pulsar timing observation and carries out long-term observation
and data processing for the selected millisecond pulsars. Using the long-term observation data from the Parkes Pulsar Timing Array (PPTA), an ensemble pulsar time was
established by George Hobbs et al., in 2012 [27], and then the clock bias sequences
of the ensemble pulsar time compared with the TAI. A total of 19 millisecond-radio
pulsars are used for establishing the TT (PPTA11), in which the shortest observation
time span is 8.1 years and the longest span is 17.0 years. The TT (PPTA11) denotes
the TT achieved by using the data from the PPTA until 2011, which is the ensemble
pulsar time. Generally, the BIPM adjusts the reference frequency standard every few
years, resulting in the maximum difference between the TT (BIPM11) and TT (TAI)
around 1998, where TT (TAI) denotes the TT achieved by the TAI, and TT (BIPM11)
denotes the TT achieved by the TAI that was corrected by the BIPM in 2011. From
Hobbs’ results, it can be seen that from 1994 to 1998, the difference between the TT
(PPTA11) and TT (TAI) is large, with the maximum bias of 1300 ns or so; since 1998,
the difference between the TT (PPTA11) and TT (TAI) has gradually decreased, with
the maximum difference of about 500 ns, and the long-term instability of the TAI has
been detected by the ensemble pulsar time; the TT (PPTA11) is well consistent with
the TT (BIPM11), and both change trends are same, with the maximum difference
of 300 ns after 1998, which indicates the effectiveness of the TAI corrected by the
BIPM.
With the standardization and scale of the millisecond pulsar timing observation,
the mature application of coherent de-dispersion technology, the improvement and
perfection of software and hardware, and the increase of timing observation station
and observational time span, the pulsar timing observation accuracy will be improved,
so as to refine the short-term and long-term stability of the pulsar time. For the
XPNAV, the stability of the X-ray pulsar timing will be better than that of the radio
pulsar timing because there is no dispersion effect in the interstellar propagation of
X-ray photons. By using the pulsar timing models with high stability, the pulse TOA
can be accurately predicted, which is conducive to the high-precision autonomous
orbit determination and time synchronization for spacecrafts.
5.6 Methods of Large-Scale Navigation
The pulsars are high-speed rotating neutron stars, far away from the solar system,
reaching thousands of light-years, even tens of thousands of light-years. The XPNAV
takes the general relativity as the fundamental theory, and by using the XPNAV, the
341
pulsar’s parameters inevitably absorbs part of the instability of atomic time into the
fitted parameters, leading to the deviation of the pulsar’s parameters, and finally the
obtained timing residuals cannot fully reflect the actual difference.
In the past 10 years, the PTA has been developing rapidly over the world, and more
than 10 PTA stations have been put into operation. According to the standard operation procedures, the PTA is composed of the radio telescopes, each of which is an
array element in the pulsar timing observation and carries out long-term observation
and data processing for the selected millisecond pulsars. Using the long-term observation data from the Parkes Pulsar Timing Array (PPTA), an ensemble pulsar time was
established by George Hobbs et al., in 2012 [27], and then the clock bias sequences
of the ensemble pulsar time compared with the TAI. A total of 19 millisecond-radio
pulsars are used for establishing the TT (PPTA11), in which the shortest observation
time span is 8.1 years and the longest span is 17.0 years. The TT (PPTA11) denotes
the TT achieved by using the data from the PPTA until 2011, which is the ensemble
pulsar time. Generally, the BIPM adjusts the reference frequency standard every few
years, resulting in the maximum difference between the TT (BIPM11) and TT (TAI)
around 1998, where TT (TAI) denotes the TT achieved by the TAI, and TT (BIPM11)
denotes the TT achieved by the TAI that was corrected by the BIPM in 2011. From
Hobbs’ results, it can be seen that from 1994 to 1998, the difference between the TT
(PPTA11) and TT (TAI) is large, with the maximum bias of 1300 ns or so; since 1998,
the difference between the TT (PPTA11) and TT (TAI) has gradually decreased, with
the maximum difference of about 500 ns, and the long-term instability of the TAI has
been detected by the ensemble pulsar time; the TT (PPTA11) is well consistent with
the TT (BIPM11), and both change trends are same, with the maximum difference
of 300 ns after 1998, which indicates the effectiveness of the TAI corrected by the
BIPM.
With the standardization and scale of the millisecond pulsar timing observation,
the mature application of coherent de-dispersion technology, the improvement and
perfection of software and hardware, and the increase of timing observation station
and observational time span, the pulsar timing observation accuracy will be improved,
so as to refine the short-term and long-term stability of the pulsar time. For the
XPNAV, the stability of the X-ray pulsar timing will be better than that of the radio
pulsar timing because there is no dispersion effect in the interstellar propagation of
X-ray photons. By using the pulsar timing models with high stability, the pulse TOA
can be accurately predicted, which is conducive to the high-precision autonomous
orbit determination and time synchronization for spacecrafts.
5.6 Methods of Large-Scale Navigation
The pulsars are high-speed rotating neutron stars, far away from the solar system,
reaching thousands of light-years, even tens of thousands of light-years. The XPNAV
takes the general relativity as the fundamental theory, and by using the XPNAV, the