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7 Guidance, Navigation and Control Technology …
of the slow descending. In the vertical direction, the velocity and acceleration was
controlled when the altitude was above 20 m; when the altitude was less than 20 m,
only the acceleration was controlled and the commanded acceleration was slightly
less than the local lunar gravity acceleration, in order to improve the reliability of
the lunar lander during descent.
7.5.5 Hazard Detection and Safe Landing Area Selection
Method [9]
The hazard detection method was based on the 2-D or 3-D image data obtained
by the sensor to identify the size of the obstacle and the slope gradient. The safe
landing point selection method was to determine the safety radius of the safe landing
site based on the identified obstacle characteristics, and select the landing site with
the largest safety radius (defined as the safest) as the target landing site. The hazard
detection and the safe landing point selection logic for the Chang’E-3 the lunar lander
during powered descent process are as follows.
1. Large Hazard Detection and Safe Landing Area Selection Based on Optical
Image
The rough hazard detection based on optical images was to identify large obstacles
using image features of lunar rocks and craters to determine safe landing area.
The ratio of the shortest dimension to the longest dimension of the lunar rock is
1 ~ 0.2. The shape of the lunar rock is generally circular, rectangular pit shape or
corroded bubble shape. The height is generally 1/2 of the diameter, and the standard
albedo is 14 ~ 22%. Thus, the characteristics of the lunar surface rock image are: (1)
The surface of the rock has a distinct bright target feature, the brightness is 2 times
greater than the background; (2) there is a clear shadow area next to the bright target;
(3) there is strong contrast between light area and shadow area, shown as edges; (4)
The normal direction of the edge is consistent with the direction of the solar vector
projection.
The slope of the moon crater is generally 25° ~ 50°, with an average of 35°. The
outside of the crater is generally 3° ~ 8° with an average of 5°. The image features of
the crater can be summarized as follows: (1) bright areas appear on the side towards
the Sun; (2) shadow areas appear on the non-lighted dark areas; (3) arcs appear on
the outer edges of dark areas.
Based on these characteristics of lunar rocks and craters, rough hazard identification and safe landing zone selection algorithms were designed: (1) image histogram
analysis; (2) K-means clustering; (3) over-highlighted hazard identification; (4) overdark hazard identification; (5) texture hazard identification; (6) using helix search
algorithm to determine the safety radius of each cell (as shown in Fig. 7.13); (7)
according to the safety radius, select a candidate safe landing point; (8) assess the
7 Guidance, Navigation and Control Technology …
of the slow descending. In the vertical direction, the velocity and acceleration was
controlled when the altitude was above 20 m; when the altitude was less than 20 m,
only the acceleration was controlled and the commanded acceleration was slightly
less than the local lunar gravity acceleration, in order to improve the reliability of
the lunar lander during descent.
7.5.5 Hazard Detection and Safe Landing Area Selection
Method [9]
The hazard detection method was based on the 2-D or 3-D image data obtained
by the sensor to identify the size of the obstacle and the slope gradient. The safe
landing point selection method was to determine the safety radius of the safe landing
site based on the identified obstacle characteristics, and select the landing site with
the largest safety radius (defined as the safest) as the target landing site. The hazard
detection and the safe landing point selection logic for the Chang’E-3 the lunar lander
during powered descent process are as follows.
1. Large Hazard Detection and Safe Landing Area Selection Based on Optical
Image
The rough hazard detection based on optical images was to identify large obstacles
using image features of lunar rocks and craters to determine safe landing area.
The ratio of the shortest dimension to the longest dimension of the lunar rock is
1 ~ 0.2. The shape of the lunar rock is generally circular, rectangular pit shape or
corroded bubble shape. The height is generally 1/2 of the diameter, and the standard
albedo is 14 ~ 22%. Thus, the characteristics of the lunar surface rock image are: (1)
The surface of the rock has a distinct bright target feature, the brightness is 2 times
greater than the background; (2) there is a clear shadow area next to the bright target;
(3) there is strong contrast between light area and shadow area, shown as edges; (4)
The normal direction of the edge is consistent with the direction of the solar vector
projection.
The slope of the moon crater is generally 25° ~ 50°, with an average of 35°. The
outside of the crater is generally 3° ~ 8° with an average of 5°. The image features of
the crater can be summarized as follows: (1) bright areas appear on the side towards
the Sun; (2) shadow areas appear on the non-lighted dark areas; (3) arcs appear on
the outer edges of dark areas.
Based on these characteristics of lunar rocks and craters, rough hazard identification and safe landing zone selection algorithms were designed: (1) image histogram
analysis; (2) K-means clustering; (3) over-highlighted hazard identification; (4) overdark hazard identification; (5) texture hazard identification; (6) using helix search
algorithm to determine the safety radius of each cell (as shown in Fig. 7.13); (7)
according to the safety radius, select a candidate safe landing point; (8) assess the