4.7 The Navigations for Spacecrafts
247
direction based on the active attitude control or using the environmental torque. For
a dual-spin stabilized satellite composed of the spinning wheel (the rotor) and the
de-spun platform, the spin axis is orientated by the rotor rotating rapidly about it,
and connected with the bearing of the de-spun platform each other. With the spin
axis orientating, the de-spun platform is controlled by the bearing motors to obtain
a three-axis stabilization effect. For this reason, the payloads are usually assembled
on the de-spun platform in practical applications.
In general, the spacecraft’s orbit and attitude controls are associated with each
other. Before implementing the orbit control, the spacecraft’s attitude should meet the
requirements for its orbit control. On the contrary, the implementation of the attitude
maneuver for the spacecraft requires corresponding orbit conditions. Of course, for
some specific case or special space missions, the orbit and attitude controls can be
carried out independently. For example, the Earth space environment exploration
satellite, which operates in the Kepler orbit, can meet the requirements for space
exploration, without the need of implementing the orbit control, and only with the
requirements for the attitude control. According to the different sources of force and
torque for the spacecraft’s orbit and attitude controls, the controls for the spacecraft
are divided into two categories: one is the active control, the control of consuming
the spacecraft’s own energy to generate the control force and moment, which is
generally realized by the closed-loop control system, composed of the spacecrafts
and the ground facilities; another is the passive control, using space environment
and spacecraft dynamics characteristics to provide the control force and moment,
without consuming the energy for the spacecraft itself, such as using aerodynamic
force and moment, solar radiation pressure, gravity-gradient moment and magnetic
moment, all of which are also able to achieve the orbit and attitude controls for
spacecrafts.
From practical applications, the active controls are the major means for the spacecraft controls, and usually include two kinds of control systems: autonomous control
system and space-ground large loop control system. The autonomous control system
is composed of the sensors, computers, controllers, actuators, and orbit and attitude
dynamics units, carried by the spacecrafts, not relying on the ground TT&C stations,
the spacecrafts are able to autonomously complete the control tasks, as shown in
Fig. 4.8. For example, the autonomous controls are used in the de-spun controls
for the dual-spin stabilized satellites, the attitude controls for the three-axis stabilized satellites and the east–west station keeping controls for the GEO satellites. The
space-ground large loop control system is composed of the onboard remote control
receiving units, actuators, orbit and attitude dynamics units, sensors and telemetry
sending units, as well as the ground telemetry receiving equipment, tracking and
measuring facilities for the orbits and attitudes, control center and remote control
sending devices, as shown in Fig. 4.9. For example, the space-ground large loop
controls are used in the attitude controls for the single-spin and dual-spin stabilized
satellites, and the orbit controls for most of the spacecrafts. It is mainly because all
of the spacecrafts have not achieved really autonomous navigation at present.
247
direction based on the active attitude control or using the environmental torque. For
a dual-spin stabilized satellite composed of the spinning wheel (the rotor) and the
de-spun platform, the spin axis is orientated by the rotor rotating rapidly about it,
and connected with the bearing of the de-spun platform each other. With the spin
axis orientating, the de-spun platform is controlled by the bearing motors to obtain
a three-axis stabilization effect. For this reason, the payloads are usually assembled
on the de-spun platform in practical applications.
In general, the spacecraft’s orbit and attitude controls are associated with each
other. Before implementing the orbit control, the spacecraft’s attitude should meet the
requirements for its orbit control. On the contrary, the implementation of the attitude
maneuver for the spacecraft requires corresponding orbit conditions. Of course, for
some specific case or special space missions, the orbit and attitude controls can be
carried out independently. For example, the Earth space environment exploration
satellite, which operates in the Kepler orbit, can meet the requirements for space
exploration, without the need of implementing the orbit control, and only with the
requirements for the attitude control. According to the different sources of force and
torque for the spacecraft’s orbit and attitude controls, the controls for the spacecraft
are divided into two categories: one is the active control, the control of consuming
the spacecraft’s own energy to generate the control force and moment, which is
generally realized by the closed-loop control system, composed of the spacecrafts
and the ground facilities; another is the passive control, using space environment
and spacecraft dynamics characteristics to provide the control force and moment,
without consuming the energy for the spacecraft itself, such as using aerodynamic
force and moment, solar radiation pressure, gravity-gradient moment and magnetic
moment, all of which are also able to achieve the orbit and attitude controls for
spacecrafts.
From practical applications, the active controls are the major means for the spacecraft controls, and usually include two kinds of control systems: autonomous control
system and space-ground large loop control system. The autonomous control system
is composed of the sensors, computers, controllers, actuators, and orbit and attitude
dynamics units, carried by the spacecrafts, not relying on the ground TT&C stations,
the spacecrafts are able to autonomously complete the control tasks, as shown in
Fig. 4.8. For example, the autonomous controls are used in the de-spun controls
for the dual-spin stabilized satellites, the attitude controls for the three-axis stabilized satellites and the east–west station keeping controls for the GEO satellites. The
space-ground large loop control system is composed of the onboard remote control
receiving units, actuators, orbit and attitude dynamics units, sensors and telemetry
sending units, as well as the ground telemetry receiving equipment, tracking and
measuring facilities for the orbits and attitudes, control center and remote control
sending devices, as shown in Fig. 4.9. For example, the space-ground large loop
controls are used in the attitude controls for the single-spin and dual-spin stabilized
satellites, and the orbit controls for most of the spacecrafts. It is mainly because all
of the spacecrafts have not achieved really autonomous navigation at present.