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4 Navigations from Ground to Space
to upload the commands and navigation message every day to maintain operation
for a long time. So, the GPS system is usually considered to be a kind of semiautonomous navigation way for the LEO satellites. From the basic characteristics
of the autonomous navigation for spacecrafts, the spacecrafts have not achieved the
autonomous navigation in the real sense at present, and most spacecrafts still rely
on the ground TT&C system to complete the navigation tasks. It fully shows the
complexity and difficulty of autonomous navigation technology for spacecrafts.
For the navigation constellations, the requirements for their orbit determination
and time synchronization accuracies are higher than other spacecrafts. Therefore, it
is very significant to achieve the autonomous navigation for the navigation constellations, and its benefits are: to reduce the number of ground control stations; to be
able to keep the normal operation of the constellation without the worldwide monitor
stations; to reduce the frequency of upload information from the injection stations to
the navigation satellites; to reduce the cost of the construction and long-term maintenance of the constellation; to monitor the integrity of navigation information in
real time and enhance the availability of the system; to reduce the dependence of
the navigation satellites on the ground control network, and enhance the anti-jam
and autonomous survivability of the system. Furthermore, with the support of the
ground control network, the crosslink measurements within the constellation provide
an independent means to examine the ephemeris and clock parameters of the satellites, and through joint data processing, further improve the system performance and
navigation accuracy.
Generally, there are two key indicators used to evaluate the performance of the
autonomous navigation for navigation constellations: one is autonomous operational
time and the other is User Ranging Error (URE). The URE refers to the root mean
square of the broadcast ephemeris errors and clock biases of the navigation satellites
projected along the line-of-sight direction. The URE dynamically changes over time,
whose reasons are: on the one hand, the elevation angles of the satellites are always
changing; on the other hand, the predicted ephemeris errors and clock biases increase
over time. As a result, the predicted ephemeris and clock parameter accuracies are
closely associated with the Age of Data (AOD) for the navigation satellites.
4.9.2 Autonomous Navigation of GPS Constellation
In the early 1980s, the GPS system was still in the phase of engineering test. On
the basis of launching the GPS Block I satellites and conducting the navigation test,
the JPO proposed specific requirements for the development of payloads of the new
generation GPS Block II satellites: they could provide high-precision navigation
and positioning services, and also have the real-time monitoring capability of global
nuclear test. As a result, researchers from the University of Texas, Aerospace Corporation, National Bureau of Standards, International Business Machines Corporation
(IBM) and Rockwell Automation Inc. soon proposed a nuclear explosion monitoring
proposal based on GPS inter-satellite communication link. The basic process of the
4 Navigations from Ground to Space
to upload the commands and navigation message every day to maintain operation
for a long time. So, the GPS system is usually considered to be a kind of semiautonomous navigation way for the LEO satellites. From the basic characteristics
of the autonomous navigation for spacecrafts, the spacecrafts have not achieved the
autonomous navigation in the real sense at present, and most spacecrafts still rely
on the ground TT&C system to complete the navigation tasks. It fully shows the
complexity and difficulty of autonomous navigation technology for spacecrafts.
For the navigation constellations, the requirements for their orbit determination
and time synchronization accuracies are higher than other spacecrafts. Therefore, it
is very significant to achieve the autonomous navigation for the navigation constellations, and its benefits are: to reduce the number of ground control stations; to be
able to keep the normal operation of the constellation without the worldwide monitor
stations; to reduce the frequency of upload information from the injection stations to
the navigation satellites; to reduce the cost of the construction and long-term maintenance of the constellation; to monitor the integrity of navigation information in
real time and enhance the availability of the system; to reduce the dependence of
the navigation satellites on the ground control network, and enhance the anti-jam
and autonomous survivability of the system. Furthermore, with the support of the
ground control network, the crosslink measurements within the constellation provide
an independent means to examine the ephemeris and clock parameters of the satellites, and through joint data processing, further improve the system performance and
navigation accuracy.
Generally, there are two key indicators used to evaluate the performance of the
autonomous navigation for navigation constellations: one is autonomous operational
time and the other is User Ranging Error (URE). The URE refers to the root mean
square of the broadcast ephemeris errors and clock biases of the navigation satellites
projected along the line-of-sight direction. The URE dynamically changes over time,
whose reasons are: on the one hand, the elevation angles of the satellites are always
changing; on the other hand, the predicted ephemeris errors and clock biases increase
over time. As a result, the predicted ephemeris and clock parameter accuracies are
closely associated with the Age of Data (AOD) for the navigation satellites.
4.9.2 Autonomous Navigation of GPS Constellation
In the early 1980s, the GPS system was still in the phase of engineering test. On
the basis of launching the GPS Block I satellites and conducting the navigation test,
the JPO proposed specific requirements for the development of payloads of the new
generation GPS Block II satellites: they could provide high-precision navigation
and positioning services, and also have the real-time monitoring capability of global
nuclear test. As a result, researchers from the University of Texas, Aerospace Corporation, National Bureau of Standards, International Business Machines Corporation
(IBM) and Rockwell Automation Inc. soon proposed a nuclear explosion monitoring
proposal based on GPS inter-satellite communication link. The basic process of the