This document discusses sensor options for terminal lunar descent and proposes using multiple laser rangefinders (LRFs) along with a lunar descent sensor (LDS) camera as a low-cost alternative to LIDAR. It reviews historical lunar landing sensors and finds that LRFs offer advantages over altimeters for precision landing. Using terrain-relative navigation with LRF and camera updates could provide accurate position and attitude estimates for landing. Outreach is requested to collaborate on guidance, navigation and control advances for lunar missions.

1. Choosing sensors for
terminal lunar descent
By Chandrabhraman: https://github.com/chandrabhraman
We are interested in sharing our work in Open and looking for collaborating with projects comprising the latest advancements in GNC
for moon. Reach out on chandrabhraman@gmail.com

2. Sample configuration
Honeywell IMU+ Honeywell HG 8500 +Lunar Descent Camera
Honeywell IMU
Honeywell HG8500
Gyro error coefficients 1 sigma Accelero error coeffs
Vendor IMU Output rate Bias (deg/hr)
Random walk
(deg/sqrt(hr)) Scalefactor Bias (micro-g) Scalefactor
Thermal range
(Deg celsius) Mass (kg)
Honeywell HG9900 300 Hz 0.003 0.002 5 PPM 25 100 -54 to 71 2.94
Vendor Altimeter Accuracy Range
Update
rate Range bias
Range
noise
Operational
Temperature
(Celsius) Beam divergence Size(WxHxL)
Weight
(kg)
Honeywell HG8500
max(+/- 2 feet ,+/-
2% actual altitude) 2.44 km 25 Hz
+/- 2%
altitude -
-54 deg to +95
deg' -
8.64cm x 8.64 cm x
14.22 cm 4.54 kg

3. Problems with configuration
Altimeter uncertainty +/- 2 feet too high w.r.t. requirements for a touchdown engine cutoff logic
Altimeter uncertainty affects camera algorithm
Uncertainty with inertial frame derived attitude (modeling error in velocities is a problem)
6 DoF information on r,v,q,w are well known but frame definition is very poor, no handover from
inertial position estimates accurate or possible.

4. Historical survey: Surveyor
RADVS (Radar Altimeter and Doppler Velocimeter sensor)
Solid ejected, RADVS operational slant range: 50000 ft to 14 ft (TD cutoff)
Controlled the final descent by throttling the vernier engines
4 narrow radar beams (3 oriented 25 deg off roll axis & 1 along the roll axis)
Refer: http://www.hq.nasa.gov/alsj/a12/Surveyor-III-MIssionRpt1967028267.pdf

5. Historical survey: Hayabusa
HAYABUSA had successfully touchdown based on its
navigation sensor including LIDAR
LIDAR detected the range from 50km to 35 m
4 x LRFs (laser rangefinders) provide attitude alignment
Target marker based optical navigation to cancel
horizontal velocity (help from LRF)
Refer: http://dedead.free.fr/projet/bk_02_001.pdf

6. Historical survey: Chang’e 3
State based: Laser Ranging System and Microwave
range sensor. Gamma Ray Altimeter to provide precise
altitude till the 4 m cutoff point
GRA altimetry based cutoff
Optical terrain imagery based hazard avoidance
Gravity turning guidance
Attitude control is achieved by a 150N thruster.
Attitude determination achieved by gyro prediction
Navigation based on measurement-updated IMU

7. Options
Vendor Altimeter Accuracy Range
Update
rate Range bias
Range
noise
Operational
Temperature
(Celsius)
Beam
divergence Size(WxHxL)
Weight
(kg)
ASC
Golden Eye
GE-2800-SC +/- 10 cm 3 km <10 Hz +/- 10 cm +/- 15 cm 0 deg to 35 deg
14 cm x 14.5 cm
x 21.6 cm 6.5 kg
Honeywell HG8500
max(+/- 2 feet ,+/-
2% actual altitude) 2.44 km 25 Hz
+/- 2%
altitude -
-54 deg to +95
deg'
8.64cm x 8.64
cm x 14.22 cm 4.54 kg
ASC SpaceCub 1 - 2.25 km 20 Hz O(cm) 15 deg FOV
ASC SpaceCub 2 - 150 m 20 Hz O(cm)
L3 Com
DP-ELRF I
MLRF +/- 4 m
20 km,
39.9 km
range gate 1 Hz +/- 2 m -
-40 deg to 71
deg
0.017 deg to
0.037 deg =
3.78 m footprint
at perilune
6.85 cm x 4.82
cm x 12.7 cm 0.47 kg
Bosch/
BELA/ L3
Com
Laser distance
meter
+/- 1 cm 50-350 m 1 Hz 0.1 kg

8. Tradeoff
1. Multiple LRFs+LDS camera sensor =
Poor Man’s LIDAR
2. Qualify many such LRFs (cost reduction
of 10-100x). Works well for small duration
missions like lunar transfer.

9. Other Advantages of using laser
rangefinders
1. The laser altimeter serves as a redundant sensor to the LRF which could be used to initiate
states at the terminal descent point.
2. Testing the LRFs is not expected to take major efforts
3. The canting angle for the laser rangefinders can be optimized to find the right set of terrain
relative state
4. Update rates of LRFs are very good
5. The canting angle of LRFs can be optimized to get the most accurate lateral velocity using the
LDS FOV as a comparative input sensor

10. Use Terrain-relative Navigation
• Update position and attitude
• Propagate with IMU using the
observations from LRF and
terrain camera
• Work on refined measurement
model going on

11. Good luck!