342
5 X-ray Pulsar-Based Navigation: Theories and Experiments
spacecrafts in the Riemannian space can autonomously determine its navigation
parameters. It is a new space navigation approach based on four-dimensional spacetime transformation, which is defined here as large-scale navigation.
5.6.1 Navigational Modes and Basic Observables
According to the different types of navigation parameters and achieving order, the
autonomous navigation of spacecrafts based on the X-ray pulsars can be divided into
four parameter-determined modes, including time, attitude, velocity and position.
Determining the time parameters is helpful to getting the pulse profile, identifying
the pulsar’s characteristics, measuring the photon’s arrival-time and updating the
onboard clock parameter estimation; determining the attitude parameters is to get
the spacecraft’s orientation relative to the orbit coordinate system, so as to judge
whether the spacecrafts are rotating or rolling normally; determining the velocity
parameters is to get the motion directions and rate parameters of spacecrafts, so as
to carry out the orbit maneuver for spacecrafts, and generally the pulsars which are
approximately perpendicular to the orbital plane can be selected for navigation to
reduce the Doppler frequency shift effect; determining the position parameters is to
improve the estimation accuracy of position and velocity parameters, which is closely
related to photon arrival-time measurement and time transformation. The parameterdetermined order from the time to attitude, velocity and position is suitable for recovering the spacecraft’s navigation system in start-up mode or correction mode, and
advantage to solving all navigation parameters. Of course, the order of determining
the time and attitude parameters may be changed according to the requirement for
actual mission, and sometimes the onboard clock itself provides enough accurate
time information. In fact, the spacecraft’s position, velocity and time parameters are
often coupled with one another for the system state estimation based on the Kalman
filtering.
According to selecting the navigation reference points, the autonomous navigation
of spacecraft based on the X-ray pulsars can be divided into absolute navigation and
relative navigation. The absolute navigation refers to the process of directly determining the three-dimensional position and velocity components of the spacecrafts
relative to the BCRS and guiding them to arrive at the target orbits or in celestial bodies. The relative navigation refers to the process of solving the navigation
parameters of the spacecraft relative to a reference object frame, and the reference
object may be another spacecraft, observation station, Lagrange point or one of the
solar system planets.
Using the X-ray pulsars to determine the navigation parameters of spacecrafts,
no matter which navigation mode is adopted, they are achieved by using the Xray detectors installed on the spacecrafts to detect the X-ray signals radiated from
pulsars and get the basic observables. The X-ray signals from pulsars contain a
variety of navigation information, such as the pulse-arriving time, Doppler frequency
5 X-ray Pulsar-Based Navigation: Theories and Experiments
spacecrafts in the Riemannian space can autonomously determine its navigation
parameters. It is a new space navigation approach based on four-dimensional spacetime transformation, which is defined here as large-scale navigation.
5.6.1 Navigational Modes and Basic Observables
According to the different types of navigation parameters and achieving order, the
autonomous navigation of spacecrafts based on the X-ray pulsars can be divided into
four parameter-determined modes, including time, attitude, velocity and position.
Determining the time parameters is helpful to getting the pulse profile, identifying
the pulsar’s characteristics, measuring the photon’s arrival-time and updating the
onboard clock parameter estimation; determining the attitude parameters is to get
the spacecraft’s orientation relative to the orbit coordinate system, so as to judge
whether the spacecrafts are rotating or rolling normally; determining the velocity
parameters is to get the motion directions and rate parameters of spacecrafts, so as
to carry out the orbit maneuver for spacecrafts, and generally the pulsars which are
approximately perpendicular to the orbital plane can be selected for navigation to
reduce the Doppler frequency shift effect; determining the position parameters is to
improve the estimation accuracy of position and velocity parameters, which is closely
related to photon arrival-time measurement and time transformation. The parameterdetermined order from the time to attitude, velocity and position is suitable for recovering the spacecraft’s navigation system in start-up mode or correction mode, and
advantage to solving all navigation parameters. Of course, the order of determining
the time and attitude parameters may be changed according to the requirement for
actual mission, and sometimes the onboard clock itself provides enough accurate
time information. In fact, the spacecraft’s position, velocity and time parameters are
often coupled with one another for the system state estimation based on the Kalman
filtering.
According to selecting the navigation reference points, the autonomous navigation
of spacecraft based on the X-ray pulsars can be divided into absolute navigation and
relative navigation. The absolute navigation refers to the process of directly determining the three-dimensional position and velocity components of the spacecrafts
relative to the BCRS and guiding them to arrive at the target orbits or in celestial bodies. The relative navigation refers to the process of solving the navigation
parameters of the spacecraft relative to a reference object frame, and the reference
object may be another spacecraft, observation station, Lagrange point or one of the
solar system planets.
Using the X-ray pulsars to determine the navigation parameters of spacecrafts,
no matter which navigation mode is adopted, they are achieved by using the Xray detectors installed on the spacecrafts to detect the X-ray signals radiated from
pulsars and get the basic observables. The X-ray signals from pulsars contain a
variety of navigation information, such as the pulse-arriving time, Doppler frequency