4.9 Autonomous Navigation Technology for Spacecrafts
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cannot meet the practical application requirements yet. If the ground stations regularly transmit the ranging signals and the orientation parameters to the constellation,
and the satellites autonomously carry out the information processing, the whole rotation error accumulated with time will be eliminated or restrained, which is known as
the “ground-based anchor” technique [26]. However, the solution of establishing
satellite-ground links violated the principle that the autonomous navigation satellites
do not rely on the ground stations to operation for a long time.
In addition, the Earth’s rotation rate and polar wander parameters need to be known
in the navigation and positioning process, so as to calculate the rotation angle of the
Earth-Centered Earth-Fixed (ECEF) coordinate system relative to the ECI, and add
it to the navigation message. Therefore, the user receivers can obtain the position and
velocity components of the satellites in the ECEF in real time to solve the navigation
parameters. Due to the non-uniformity error of the Earth’s rotation rate and the
residual of polar wander, the ECEF will drift relative to the ECI, resulting in user’s
navigation accuracy to degrade. The autonomous navigation of the constellation
based on the inter-satellite crosslink is also unable to solve the problem that the
non-uniformity error and polar residual are accumulated with time. In fact, the error
of Earth’s rotation rate is the difference between the UT1 and UTC, and its drift is
approximately 1 s per year, where the UT1 is the Universal Time (UT) corrected by
the polar wandering error, and the UTC is a Coordinated Universal Time using the
length of a second defined by the International Atomic Time (TAI) and synchronizing
with the UT approximately. In order to keep the consistency between the UT1 and
UTC time system, it is necessary to make a leap second correct for the UTC each
year. The global monitoring data of the Earth rotation is analyzed and processed by
the Bureau International des Poids et Measures (BIPM), and the difference between
the UT1 and UTC is published regularly. Moreover, the non-uniformity error and
polar residual of the Earth rotation can be predicted for a long time, and the users
on the Earth’s surface can also use the modern communication networks to directly
download relevant parameters from the BIPM website for correction.
According to the design specifications for the GPS Block IIR and IIR-M satellites’ autonomous navigation, it was required that the URE is less than 6 m during
180 days, and the navigation and positioning accuracy of users will not be degraded
significantly. However, from the analysis of the post-processing results of the actual
inter-satellite ranging data, it was shown that the whole rotation error of the constellation increases rapidly over time. It is difficult to meet the original design requirements. For this reason, in the design of the GPS Block IIF satellite, the autonomous
navigation requirement had been changed, that is, the URE is less than 2 m, with the
autonomous navigation capability of 60 days [27].
In summary, it is seen that the ground-based anchor technique does not meet the
definition of spacecraft’s autonomous navigation. From the perspective of the development of GPS constellation autonomous navigation, its technical vein is gradually
formed, from the inter-satellite links to the ground-based anchor, and finally “spacebased anchor”. The so-called space-based anchor is a technology that is taking the
X-ray signals radiated by pulsars as natural beacons, and the navigation satellites
269
cannot meet the practical application requirements yet. If the ground stations regularly transmit the ranging signals and the orientation parameters to the constellation,
and the satellites autonomously carry out the information processing, the whole rotation error accumulated with time will be eliminated or restrained, which is known as
the “ground-based anchor” technique [26]. However, the solution of establishing
satellite-ground links violated the principle that the autonomous navigation satellites
do not rely on the ground stations to operation for a long time.
In addition, the Earth’s rotation rate and polar wander parameters need to be known
in the navigation and positioning process, so as to calculate the rotation angle of the
Earth-Centered Earth-Fixed (ECEF) coordinate system relative to the ECI, and add
it to the navigation message. Therefore, the user receivers can obtain the position and
velocity components of the satellites in the ECEF in real time to solve the navigation
parameters. Due to the non-uniformity error of the Earth’s rotation rate and the
residual of polar wander, the ECEF will drift relative to the ECI, resulting in user’s
navigation accuracy to degrade. The autonomous navigation of the constellation
based on the inter-satellite crosslink is also unable to solve the problem that the
non-uniformity error and polar residual are accumulated with time. In fact, the error
of Earth’s rotation rate is the difference between the UT1 and UTC, and its drift is
approximately 1 s per year, where the UT1 is the Universal Time (UT) corrected by
the polar wandering error, and the UTC is a Coordinated Universal Time using the
length of a second defined by the International Atomic Time (TAI) and synchronizing
with the UT approximately. In order to keep the consistency between the UT1 and
UTC time system, it is necessary to make a leap second correct for the UTC each
year. The global monitoring data of the Earth rotation is analyzed and processed by
the Bureau International des Poids et Measures (BIPM), and the difference between
the UT1 and UTC is published regularly. Moreover, the non-uniformity error and
polar residual of the Earth rotation can be predicted for a long time, and the users
on the Earth’s surface can also use the modern communication networks to directly
download relevant parameters from the BIPM website for correction.
According to the design specifications for the GPS Block IIR and IIR-M satellites’ autonomous navigation, it was required that the URE is less than 6 m during
180 days, and the navigation and positioning accuracy of users will not be degraded
significantly. However, from the analysis of the post-processing results of the actual
inter-satellite ranging data, it was shown that the whole rotation error of the constellation increases rapidly over time. It is difficult to meet the original design requirements. For this reason, in the design of the GPS Block IIF satellite, the autonomous
navigation requirement had been changed, that is, the URE is less than 2 m, with the
autonomous navigation capability of 60 days [27].
In summary, it is seen that the ground-based anchor technique does not meet the
definition of spacecraft’s autonomous navigation. From the perspective of the development of GPS constellation autonomous navigation, its technical vein is gradually
formed, from the inter-satellite links to the ground-based anchor, and finally “spacebased anchor”. The so-called space-based anchor is a technology that is taking the
X-ray signals radiated by pulsars as natural beacons, and the navigation satellites