252
7 Guidance, Navigation and Control Technology …
In Fig. 7.12, the parameter adaptation section was to ensure that the engine thrust
specific impulse and the initial thrust ratio were estimated to converge to the true value
as the accelerometer data accumulated; the target adaptation section was to adjust
the target position and velocity, and after the quick adjustment was embedded at the
end of deceleration, the predicted terminal position and velocity was continuously
approaching the target position and velocity; the two parts were combined to ensure
that the position and velocity after quick adjustment converged to the target value
gradually.
2. Guidance Law for Quick Attitude Adjustment Phase
At the end of the main deceleration, the attitude of the lunar lander was still close to the
horizontal, the main engine was still working in the maximum thrust mode, and the
thrust acceleration also reached the maximum; while the subsequent approaching
required the attitude of the lunar lander to be close to vertical, the main engine
worked at a low thrust level, and the altitude, velocity and acceleration should meet
a certain relationship. It could be seen that it was difficult to directly connect the
end status of the main deceleration and the initial status of the approach phase. In
order to smoothly transition from the main deceleration to the approach phase, it
was necessary to quickly adjust the attitude and thrust of the lunar lander to meet
the status requirements at the entrance of the approach phase. According to the
uniform transition requirements of the thrust of the main engine and the attitude of
the lunar lander, a guidance law with the linear variation of the thrust magnitude and
direction was proposed. The guidance parameters were determined by the actual state
constraints of the final deceleration and the initial state of the approach phase. The
same guidance law was also used to predict the quick deceleration process during the
main deceleration to ensure that the actual state change of the quick adjustment was
basically consistent with the main deceleration, thereby satisfying the constraints of
the entrance status of the approach phase.
3. Guidance Law for Approaching
The main task of the approaching was to image the landing area and perform rough
obstacle avoidance. The terminal relative velocity to lunar surface was close to 0 m/s.
It was needed to ensure that the Optical Imaging Sensor could image the landing area
and complete the rough obstacle avoidance during the approach phase. Therefore,
the approach guidance law must be able to meet constraints of the position, velocity,
attitude, and initial altitude and initial velocity of the guidance target. In order to
satisfy the above constraints, based on the fourth-order polynomial guidance law,
an improved polynomial guidance algorithm was proposed for the approach phase.
When multiple constraints were satisfied, the time to go could be analytically calculated without iterative solution, so that the calculation was simplified and the stability
of the algorithm was improved. At the same time, in order to ensure the high reliability of the guidance law for approach phase, a protection method for time to go,
altitude and velocity out of tolerance were proposed. By design of the constraints in
each direction, it was ensured that the Optical Imaging Sensor could always observe
the landing area. After processing the image of the lunar surface, the safe landing
7 Guidance, Navigation and Control Technology …
In Fig. 7.12, the parameter adaptation section was to ensure that the engine thrust
specific impulse and the initial thrust ratio were estimated to converge to the true value
as the accelerometer data accumulated; the target adaptation section was to adjust
the target position and velocity, and after the quick adjustment was embedded at the
end of deceleration, the predicted terminal position and velocity was continuously
approaching the target position and velocity; the two parts were combined to ensure
that the position and velocity after quick adjustment converged to the target value
gradually.
2. Guidance Law for Quick Attitude Adjustment Phase
At the end of the main deceleration, the attitude of the lunar lander was still close to the
horizontal, the main engine was still working in the maximum thrust mode, and the
thrust acceleration also reached the maximum; while the subsequent approaching
required the attitude of the lunar lander to be close to vertical, the main engine
worked at a low thrust level, and the altitude, velocity and acceleration should meet
a certain relationship. It could be seen that it was difficult to directly connect the
end status of the main deceleration and the initial status of the approach phase. In
order to smoothly transition from the main deceleration to the approach phase, it
was necessary to quickly adjust the attitude and thrust of the lunar lander to meet
the status requirements at the entrance of the approach phase. According to the
uniform transition requirements of the thrust of the main engine and the attitude of
the lunar lander, a guidance law with the linear variation of the thrust magnitude and
direction was proposed. The guidance parameters were determined by the actual state
constraints of the final deceleration and the initial state of the approach phase. The
same guidance law was also used to predict the quick deceleration process during the
main deceleration to ensure that the actual state change of the quick adjustment was
basically consistent with the main deceleration, thereby satisfying the constraints of
the entrance status of the approach phase.
3. Guidance Law for Approaching
The main task of the approaching was to image the landing area and perform rough
obstacle avoidance. The terminal relative velocity to lunar surface was close to 0 m/s.
It was needed to ensure that the Optical Imaging Sensor could image the landing area
and complete the rough obstacle avoidance during the approach phase. Therefore,
the approach guidance law must be able to meet constraints of the position, velocity,
attitude, and initial altitude and initial velocity of the guidance target. In order to
satisfy the above constraints, based on the fourth-order polynomial guidance law,
an improved polynomial guidance algorithm was proposed for the approach phase.
When multiple constraints were satisfied, the time to go could be analytically calculated without iterative solution, so that the calculation was simplified and the stability
of the algorithm was improved. At the same time, in order to ensure the high reliability of the guidance law for approach phase, a protection method for time to go,
altitude and velocity out of tolerance were proposed. By design of the constraints in
each direction, it was ensured that the Optical Imaging Sensor could always observe
the landing area. After processing the image of the lunar surface, the safe landing