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7 Guidance, Navigation and Control Technology …
data of gyro and accelerometer according to the mass and inertia characteristics of
the lander. If the output data exceeded the upper limit, the output data of the dynamic
model was used instead of the output of the sensor.
According to the diagnosis allowable sign of telecommand, it was determined
whether to perform fault diagnosis on the MRVS. The diagnosis method was: under
the normal communication condition, if all the beam data of the microwave velocity
measurement were continuously invalid, the MRVS was considered to be faulty. The
diagnostic handling method was: switch to the backup MRVS when the main MRVS
was fault and the status word showed that the MRVS could be chosen autonomously.
According to the star tracker status word of telecommand and communication
error counter, the corresponding star tracker sensor was selected to work. When a
star tracker sensor’s validity status word was valid and there was no communication
error, the star tracker’s output was considered to be valid. When any of the star
tracker outputs assigned by telecommand was valid, the star sensor was used. When
a plurality of star sensors was normal, one output was sequentially selected as a
measurement data for attitude determination. Otherwise, the gyro data was used to
determine the attitude.
LTU communication fault diagnosis occurred during each control period. A single
communication failure was a communication error, and consecutive control period
errors were communication failure. If the LTU with active and standby cold backup
failed to communicate, it was switched to the backup LTU. If the LTU was not backed
up, the application software would not handle it autonomously after the communication failure, and if more than one LTU on duty was found to be in communication
failure within one control period, then the control computer was switched.
3. GNC System Autonomous Failure Diagnosis and Handling Methods
(1) Diagnosis and handling methods of the main deceleration: If the attitude tolerance of the main deceleration caused by the disturbance torque was greater than
the control torque continuously, a strategy of reducing the disturbance torque
by using the thrust force of 5000 N was required. Based on the consideration of
reducing the consumption of propellant, the target state of 3 km was changed
to the target state of 100 m for the lunar lander above 4.5 km and the thrust of
the main engine was changed from 7500 N to 5000 N, and the structure of the
PEG law was unchanged, i.e., only the guidance target should be replaced.
If altitude of the lunar lander was between 4.5 km and 3 km, the 7500 N main
engine was switched to 5000–1500 N throttling thrust, and target status was
changed to 100 m hovering status by using the designed safe mode guidance
law.
(2) Diagnosis and handling methods of the approaching: at the beginning of the
approaching, the nominal trajectory of the approaching was calculated based
on the navigation data at current time. During approaching, if the deviation
between the actual trajectory and the nominal trajectory was large, the safe
mode guidance law was used.
7 Guidance, Navigation and Control Technology …
data of gyro and accelerometer according to the mass and inertia characteristics of
the lander. If the output data exceeded the upper limit, the output data of the dynamic
model was used instead of the output of the sensor.
According to the diagnosis allowable sign of telecommand, it was determined
whether to perform fault diagnosis on the MRVS. The diagnosis method was: under
the normal communication condition, if all the beam data of the microwave velocity
measurement were continuously invalid, the MRVS was considered to be faulty. The
diagnostic handling method was: switch to the backup MRVS when the main MRVS
was fault and the status word showed that the MRVS could be chosen autonomously.
According to the star tracker status word of telecommand and communication
error counter, the corresponding star tracker sensor was selected to work. When a
star tracker sensor’s validity status word was valid and there was no communication
error, the star tracker’s output was considered to be valid. When any of the star
tracker outputs assigned by telecommand was valid, the star sensor was used. When
a plurality of star sensors was normal, one output was sequentially selected as a
measurement data for attitude determination. Otherwise, the gyro data was used to
determine the attitude.
LTU communication fault diagnosis occurred during each control period. A single
communication failure was a communication error, and consecutive control period
errors were communication failure. If the LTU with active and standby cold backup
failed to communicate, it was switched to the backup LTU. If the LTU was not backed
up, the application software would not handle it autonomously after the communication failure, and if more than one LTU on duty was found to be in communication
failure within one control period, then the control computer was switched.
3. GNC System Autonomous Failure Diagnosis and Handling Methods
(1) Diagnosis and handling methods of the main deceleration: If the attitude tolerance of the main deceleration caused by the disturbance torque was greater than
the control torque continuously, a strategy of reducing the disturbance torque
by using the thrust force of 5000 N was required. Based on the consideration of
reducing the consumption of propellant, the target state of 3 km was changed
to the target state of 100 m for the lunar lander above 4.5 km and the thrust of
the main engine was changed from 7500 N to 5000 N, and the structure of the
PEG law was unchanged, i.e., only the guidance target should be replaced.
If altitude of the lunar lander was between 4.5 km and 3 km, the 7500 N main
engine was switched to 5000–1500 N throttling thrust, and target status was
changed to 100 m hovering status by using the designed safe mode guidance
law.
(2) Diagnosis and handling methods of the approaching: at the beginning of the
approaching, the nominal trajectory of the approaching was calculated based
on the navigation data at current time. During approaching, if the deviation
between the actual trajectory and the nominal trajectory was large, the safe
mode guidance law was used.