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5 X-ray Pulsar-Based Navigation: Theories and Experiments
better than 10 m, and the ultimate accuracy of the XPNAV can also reach 10 m level
in the future [2]. Furthermore, for the traditional celestial navigation based on star
optical sensors, its basic observables are angles. Since a small angle measurement
error will bring up a great orbit error for spacecrafts, the accuracy using the celestial
navigation is typically the level of several kilometers, even tens of kilometers.
Considered that the X-ray signals radiated by the pulsars are extremely weak, the
signal integration time is long in order to get the pulse profiles with enough high
Signal-to-Noise Ratio (SNR). Moreover, the measurement accuracy of the pulse
arrival-time for the XPNAV is not as high as that of the pseudo-code arrival-time
for the GNSS, and currently in practice, it is still difficult for the spacecraft to carry
multiple X-ray detection devices and track multiple pulsars at the same time. Therefore, using the geometric orbit determination method to determine the orbit accuracies
of the spacecrafts can only reach the level of thousands of meters, or even lower. From
the perspective of practical engineering application, the dynamics method is used for
the spacecraft’s orbit determination, which is helpful to improving the accuracy of
estimating the navigation parameters, to make the orbit propagation in real time, and
to need not to simultaneously observe more than four pulsars.
The XPNAV is a feasible approach to achieve long-term and high-precision
autonomous navigation for spacecrafts, with the performance advantages that the
traditional navigation technologies cannot be comparable to. There are five significant technical-features for the XPNAV. The first feature is the full information, that
is, the XPNAV can provide ten-dimensional navigation information for spacecrafts,
including three-dimensional position, three-dimensional velocity, three-dimensional
attitude and one-dimensional time. The second feature is the all space, that is, the
XPNAV can be applied to the whole solar system, even the whole universe, from
the near-earth orbit to deep space, and interstellar flight spacecrafts, as well as the
rovers on the planetary surfaces without dense atmosphere, so as to achieve seamlessly autonomous navigation for all space flight missions. The third feature is the
long term, that is, the XPNAV using the SSB as the space-time reference point can
provide the navigation information service for the navigation constellations under
an “absolute” reference frame, so as to solve the problem of the whole rotation of
navigation constellations and realize the autonomous operation of the constellation
during a long time. The fourth feature is the high accuracy, that is, the XPNAV can
ultimately achieve the orbit determination accuracy of 10 m, the time synchronization
accuracy of 1 ns, and the attitude measurement accuracy of 3 arc-seconds, which are
incomparable to the traditional celestial navigation technology. The fifth feature is
the autonomy, that is, due to the X-ray signals radiated by pulsars as natural beacons,
the XPNAV has the characteristics of information integrity, operating in real time,
no emitting signals required by spacecrafts, no relying on the ground stations and
long-term automatic operation. So, the pulsar navigation is an effective approach to
realize the autonomous navigation for spacecrafts.
5 X-ray Pulsar-Based Navigation: Theories and Experiments
better than 10 m, and the ultimate accuracy of the XPNAV can also reach 10 m level
in the future [2]. Furthermore, for the traditional celestial navigation based on star
optical sensors, its basic observables are angles. Since a small angle measurement
error will bring up a great orbit error for spacecrafts, the accuracy using the celestial
navigation is typically the level of several kilometers, even tens of kilometers.
Considered that the X-ray signals radiated by the pulsars are extremely weak, the
signal integration time is long in order to get the pulse profiles with enough high
Signal-to-Noise Ratio (SNR). Moreover, the measurement accuracy of the pulse
arrival-time for the XPNAV is not as high as that of the pseudo-code arrival-time
for the GNSS, and currently in practice, it is still difficult for the spacecraft to carry
multiple X-ray detection devices and track multiple pulsars at the same time. Therefore, using the geometric orbit determination method to determine the orbit accuracies
of the spacecrafts can only reach the level of thousands of meters, or even lower. From
the perspective of practical engineering application, the dynamics method is used for
the spacecraft’s orbit determination, which is helpful to improving the accuracy of
estimating the navigation parameters, to make the orbit propagation in real time, and
to need not to simultaneously observe more than four pulsars.
The XPNAV is a feasible approach to achieve long-term and high-precision
autonomous navigation for spacecrafts, with the performance advantages that the
traditional navigation technologies cannot be comparable to. There are five significant technical-features for the XPNAV. The first feature is the full information, that
is, the XPNAV can provide ten-dimensional navigation information for spacecrafts,
including three-dimensional position, three-dimensional velocity, three-dimensional
attitude and one-dimensional time. The second feature is the all space, that is, the
XPNAV can be applied to the whole solar system, even the whole universe, from
the near-earth orbit to deep space, and interstellar flight spacecrafts, as well as the
rovers on the planetary surfaces without dense atmosphere, so as to achieve seamlessly autonomous navigation for all space flight missions. The third feature is the
long term, that is, the XPNAV using the SSB as the space-time reference point can
provide the navigation information service for the navigation constellations under
an “absolute” reference frame, so as to solve the problem of the whole rotation of
navigation constellations and realize the autonomous operation of the constellation
during a long time. The fourth feature is the high accuracy, that is, the XPNAV can
ultimately achieve the orbit determination accuracy of 10 m, the time synchronization
accuracy of 1 ns, and the attitude measurement accuracy of 3 arc-seconds, which are
incomparable to the traditional celestial navigation technology. The fifth feature is
the autonomy, that is, due to the X-ray signals radiated by pulsars as natural beacons,
the XPNAV has the characteristics of information integrity, operating in real time,
no emitting signals required by spacecrafts, no relying on the ground stations and
long-term automatic operation. So, the pulsar navigation is an effective approach to
realize the autonomous navigation for spacecrafts.