4.8 Ground Navigation Networks for Spacecrafts
253
TT&C, so as to make up for the limitations of the land area, with the advantages
of large range of activity, flexible deployment and high coverage of TT&C. The
aircraft-based TT&C station is a kind of mobile TT&C station that is achieved
by the special aircraft, mainly using for the TT&C tasks of the manned spacecraft
in the section launching into the orbit and the returning section, to ensure the twoway voice communication between the control center and the spacecraft, to receive
and record telemetry information, and to send the commands to the spacecraft when
necessary. The satellite-based TT&C station is a mobile TT&C station achieved by
Tracking and Data Relay Satellite (TDRS) in the GEO, and its space-ground TT&C
link consists of the user spacecraft, relay satellite and ground TT&C terminals, of
which the TT&C terminals are connected with the control center through the ground
communication link to exchange the TT&C information. Generally, for the LEO
spacecrafts with an inclination of 45° or so, the TT&C coverage of each orbit can
be obtained by deploying three tracking ships on the oceans at three appropriate
locations, respectively, which have the different geographic longitudes while the
values of their latitudes nearly equal to the inclination value. Moreover, for the
spacecrafts in the medium and low Earth orbits, only using two relay GEO satellites,
with the located-longitude spaced 130° apart, whose information is transmitted by
the same ground terminal station, the TT&C coverage for more than 85% of the
orbital arc-segments can be achieved.
4.8.1.3 Classifications of Ground Navigation Networks
The TT&C network is a special network for tracking, measuring and controlling the
spacecrafts after launching into orbits, and focuses on the navigation, control and
management for the spacecraft’s platform and the support services for the astronauts.
Therefore, from the perspective of general navigation concept, the TT&C network is
essentially the ground navigation network for spacecrafts, and from the perspective
of special navigation concept, the ground navigation network is the TT&C network
only with the function of tracking and measuring spacecrafts. In this book, although
the concept of general navigation is used to define the ground navigation network,
only the tracking measurement and control for spacecrafts are discussed in detail.
According to the different spaceflight missions, the ground navigation network can
be divided into three basic types, respectively, providing services for the spacecrafts
in near-earth space, the navigation constellations and the deep-space explorations.
The navigation network for the spacecrafts in near-earth space is usually a tracking
and measuring network for the spacecrafts in the LEO, MEO, GEO and HEO, except
for the navigation satellites. Due to the restriction of the straight-line propagation of
electromagnetic wave and the curvature of the Earth, using single TT&C station, the
tracking duration of each circle is generally about 4−7 min for the LEO spacecrafts,
about 15 min for the MEO ones and more than 1 h for the GEO or HEO ones. The
TT&C frequencies are L-band, S-band and C-band, and the diameter of microwave
TT&C antennas is generally no more than 10 m. Based on the design idea of “one
253
TT&C, so as to make up for the limitations of the land area, with the advantages
of large range of activity, flexible deployment and high coverage of TT&C. The
aircraft-based TT&C station is a kind of mobile TT&C station that is achieved
by the special aircraft, mainly using for the TT&C tasks of the manned spacecraft
in the section launching into the orbit and the returning section, to ensure the twoway voice communication between the control center and the spacecraft, to receive
and record telemetry information, and to send the commands to the spacecraft when
necessary. The satellite-based TT&C station is a mobile TT&C station achieved by
Tracking and Data Relay Satellite (TDRS) in the GEO, and its space-ground TT&C
link consists of the user spacecraft, relay satellite and ground TT&C terminals, of
which the TT&C terminals are connected with the control center through the ground
communication link to exchange the TT&C information. Generally, for the LEO
spacecrafts with an inclination of 45° or so, the TT&C coverage of each orbit can
be obtained by deploying three tracking ships on the oceans at three appropriate
locations, respectively, which have the different geographic longitudes while the
values of their latitudes nearly equal to the inclination value. Moreover, for the
spacecrafts in the medium and low Earth orbits, only using two relay GEO satellites,
with the located-longitude spaced 130° apart, whose information is transmitted by
the same ground terminal station, the TT&C coverage for more than 85% of the
orbital arc-segments can be achieved.
4.8.1.3 Classifications of Ground Navigation Networks
The TT&C network is a special network for tracking, measuring and controlling the
spacecrafts after launching into orbits, and focuses on the navigation, control and
management for the spacecraft’s platform and the support services for the astronauts.
Therefore, from the perspective of general navigation concept, the TT&C network is
essentially the ground navigation network for spacecrafts, and from the perspective
of special navigation concept, the ground navigation network is the TT&C network
only with the function of tracking and measuring spacecrafts. In this book, although
the concept of general navigation is used to define the ground navigation network,
only the tracking measurement and control for spacecrafts are discussed in detail.
According to the different spaceflight missions, the ground navigation network can
be divided into three basic types, respectively, providing services for the spacecrafts
in near-earth space, the navigation constellations and the deep-space explorations.
The navigation network for the spacecrafts in near-earth space is usually a tracking
and measuring network for the spacecrafts in the LEO, MEO, GEO and HEO, except
for the navigation satellites. Due to the restriction of the straight-line propagation of
electromagnetic wave and the curvature of the Earth, using single TT&C station, the
tracking duration of each circle is generally about 4−7 min for the LEO spacecrafts,
about 15 min for the MEO ones and more than 1 h for the GEO or HEO ones. The
TT&C frequencies are L-band, S-band and C-band, and the diameter of microwave
TT&C antennas is generally no more than 10 m. Based on the design idea of “one