336
5 X-ray Pulsar-Based Navigation: Theories and Experiments
For data processing of the ensemble pulsar time, the following points should be
paid attention to:
(1) The original timing observation data and unified theoretical model are used to
ensure the consistency of the residuals. In addition, the residual data and its
model parameters are obtained to examine the difference between the theoretical models. Especially for the observation data of the same pulsar obtained
from different stations, the influence of the deviation between observation
systems should be eliminated at first.
(2) It should be avoided that the system error resulted from residual deterministic
trend is mixed in noise.
(3) The equal interval observation data is used to facilitate data processing.
(4) In order to avoid the step of time series caused by the change of the number of
ensemble pulsars, a correction value should be added, that is
AT (t) − PT en = AT (t + δ t) −
PT
en (t + δ t) + a
,
(5.88)
where a is the added correction value, and PT
en is the ensemble pulsar time calculated
after increasing or decreasing the pulsar’s number.
The pulsar time is different from the atomic timescale and needs not to adjust
the frequency, because in the process of fitting the parameters by using the timing
residuals, the deterministic trend part of frequency variation has been eliminated.
Since the long-term stability of the ensemble pulsar time is better than that of the
atomic time, it is possible to transfer the accuracy of atomic time from one timesegment to another as long as the continuity of observation is maintained. In order
words, if the frequency reference fails temporarily, the ensemble pulsar time can
be used to maintain the accuracy of atomic time. The future frequency reference
has higher accuracy, and after years of stable operation, it is possible to provide a
new definition of second. The accuracy of the ensemble pulsar time can be used for
a reverse transfer, thus improving the accuracy of the previous atomic timescale.
Certainly, this is also limited by the instability of pulsar time itself. In addition, the
long-term stability of atomic time can be analyzed by the residual values of AT–PT,
and the related parameters of pulsar time are obtained by fitting the AT–PT values.
It is shown that the fitting method of least squares can be used to filter out all the
short period terms with the period of less than 1 year and the long period terms with
the same order of magnitude as the observation time span. Obviously, only when
the observation time span is more than two years, the stability of AT–PT can be
effectively estimated and the long-term stability of atomic time be analyzed. So, it is
an effective approach to improving the stability of timescale by using the long-term
accumulated pulsar observation data to establish the ensemble pulsar time.
5 X-ray Pulsar-Based Navigation: Theories and Experiments
For data processing of the ensemble pulsar time, the following points should be
paid attention to:
(1) The original timing observation data and unified theoretical model are used to
ensure the consistency of the residuals. In addition, the residual data and its
model parameters are obtained to examine the difference between the theoretical models. Especially for the observation data of the same pulsar obtained
from different stations, the influence of the deviation between observation
systems should be eliminated at first.
(2) It should be avoided that the system error resulted from residual deterministic
trend is mixed in noise.
(3) The equal interval observation data is used to facilitate data processing.
(4) In order to avoid the step of time series caused by the change of the number of
ensemble pulsars, a correction value should be added, that is
AT (t) − PT en = AT (t + δ t) −
PT
en (t + δ t) + a
,
(5.88)
where a is the added correction value, and PT
en is the ensemble pulsar time calculated
after increasing or decreasing the pulsar’s number.
The pulsar time is different from the atomic timescale and needs not to adjust
the frequency, because in the process of fitting the parameters by using the timing
residuals, the deterministic trend part of frequency variation has been eliminated.
Since the long-term stability of the ensemble pulsar time is better than that of the
atomic time, it is possible to transfer the accuracy of atomic time from one timesegment to another as long as the continuity of observation is maintained. In order
words, if the frequency reference fails temporarily, the ensemble pulsar time can
be used to maintain the accuracy of atomic time. The future frequency reference
has higher accuracy, and after years of stable operation, it is possible to provide a
new definition of second. The accuracy of the ensemble pulsar time can be used for
a reverse transfer, thus improving the accuracy of the previous atomic timescale.
Certainly, this is also limited by the instability of pulsar time itself. In addition, the
long-term stability of atomic time can be analyzed by the residual values of AT–PT,
and the related parameters of pulsar time are obtained by fitting the AT–PT values.
It is shown that the fitting method of least squares can be used to filter out all the
short period terms with the period of less than 1 year and the long period terms with
the same order of magnitude as the observation time span. Obviously, only when
the observation time span is more than two years, the stability of AT–PT can be
effectively estimated and the long-term stability of atomic time be analyzed. So, it is
an effective approach to improving the stability of timescale by using the long-term
accumulated pulsar observation data to establish the ensemble pulsar time.