5.6 Methods of Large-Scale Navigation
351
a process in which all photon energies are averaged synchronously in a desired pulse
cycle [0, 2π] or [0, 1]. If the pulse period is divided into m equal time intervals, then
each interval stands for a phase box of the pulse profile curve within a cycle, usually
known as a “bin”. The n photons are allocated to the corresponding bins according to
the folding time sequence. As a result, some bins will have more photons, indicating
strong photon radiation flux, while the number of photons in other bins is relatively
scarce, indicating that they have weak photon radiation flux. Finally, a recognizable
pulse profile curve can be produced [24, 28], which is similar to the photon histogram
drawn during a pulse cycle. The smaller is the bin’s value, the narrower the histogram
block and the higher the resolution of pulse profile curve. Otherwise, the resolution
is lower. Certainly, it is possible that the selected value of bin is too small to get more
photons and clear profile.
The pulse profile is the unique identifier of pulsar, which is obtained by the
synchronous average of a large number of photons. The relevant pulse parameters,
such as the amplitude, peak number, period and its rate, as well as noise characteristics, can be measured through the pulse profile. In this sense, the pulse profiles are
also the basic observables for the XPNAV. Only by extracting the pulse profile can
the observed pulsar be identified, and its relevant parameters in the onboard database
gotten to carry out the autonomous navigation computation for the spacecraft. Generally, the average pulse profile curve obtained during a short observation time, such as
tens of minutes, is called measuring pulse profile, while the pulse profile obtained
by long-term observation data processing is called standard pulse profile. Although
the measuring pulse profile is with low SNR, it has the inherent characteristics of
the pulse profile for identifying the pulsar. The standard pulse profile has a high
SNR, which is a reference template of recognizing the pulsar for the XPNAV. In the
XPNAV, the soft X-ray radiation spectrum range of 1−15 keV is generally selected
to obtain more X-ray photons per unit time per unit detection area, which is beneficial
to shorten the update time of measurement of the autonomous navigation filtering
for spacecrafts.
5.6.3.2 Pulse Arrival-Time Measuring
The pulse arrival-time is a basic observable to determine the position and time parameters of spacecrafts. Only by comparing the measured pulse arrival-time with the
predicted one can the time delay from the SSB to the spacecraft along the line of
sight of the pulsar be obtained, and then the measurement equations are established
to solve the navigation parameters. Supposed that the measuring pulse profile is
obtained by synchronous average of a large number of single pulses within enough
long observation time, which can characterize the inherent properties of the pulse
profile generated within the same energy spectrum range, thus the pulse arrival-time
can be measured by the cross-correlation processing between the measuring and
standard pulse profiles.
From the studied results, it is shown that for a pulsar, the measuring pulse profile
is the best approximation of the standard pulse profile, but there are the differences
351
a process in which all photon energies are averaged synchronously in a desired pulse
cycle [0, 2π] or [0, 1]. If the pulse period is divided into m equal time intervals, then
each interval stands for a phase box of the pulse profile curve within a cycle, usually
known as a “bin”. The n photons are allocated to the corresponding bins according to
the folding time sequence. As a result, some bins will have more photons, indicating
strong photon radiation flux, while the number of photons in other bins is relatively
scarce, indicating that they have weak photon radiation flux. Finally, a recognizable
pulse profile curve can be produced [24, 28], which is similar to the photon histogram
drawn during a pulse cycle. The smaller is the bin’s value, the narrower the histogram
block and the higher the resolution of pulse profile curve. Otherwise, the resolution
is lower. Certainly, it is possible that the selected value of bin is too small to get more
photons and clear profile.
The pulse profile is the unique identifier of pulsar, which is obtained by the
synchronous average of a large number of photons. The relevant pulse parameters,
such as the amplitude, peak number, period and its rate, as well as noise characteristics, can be measured through the pulse profile. In this sense, the pulse profiles are
also the basic observables for the XPNAV. Only by extracting the pulse profile can
the observed pulsar be identified, and its relevant parameters in the onboard database
gotten to carry out the autonomous navigation computation for the spacecraft. Generally, the average pulse profile curve obtained during a short observation time, such as
tens of minutes, is called measuring pulse profile, while the pulse profile obtained
by long-term observation data processing is called standard pulse profile. Although
the measuring pulse profile is with low SNR, it has the inherent characteristics of
the pulse profile for identifying the pulsar. The standard pulse profile has a high
SNR, which is a reference template of recognizing the pulsar for the XPNAV. In the
XPNAV, the soft X-ray radiation spectrum range of 1−15 keV is generally selected
to obtain more X-ray photons per unit time per unit detection area, which is beneficial
to shorten the update time of measurement of the autonomous navigation filtering
for spacecrafts.
5.6.3.2 Pulse Arrival-Time Measuring
The pulse arrival-time is a basic observable to determine the position and time parameters of spacecrafts. Only by comparing the measured pulse arrival-time with the
predicted one can the time delay from the SSB to the spacecraft along the line of
sight of the pulsar be obtained, and then the measurement equations are established
to solve the navigation parameters. Supposed that the measuring pulse profile is
obtained by synchronous average of a large number of single pulses within enough
long observation time, which can characterize the inherent properties of the pulse
profile generated within the same energy spectrum range, thus the pulse arrival-time
can be measured by the cross-correlation processing between the measuring and
standard pulse profiles.
From the studied results, it is shown that for a pulsar, the measuring pulse profile
is the best approximation of the standard pulse profile, but there are the differences