PS07
UWB

Ultra-Wide Band

Development of a Localization Method Using LandmarkConstruction by Releasing UWB (Ultra-Wide Band) Tag
for a Small Lunar Rover2,2,2,Makoto WAKUI1, Kimitaka WATANABE2, Kazuto TANAKA2 and Masataka KAWAGUCHI2
1

1
2

1.

Doshisha University#2,

Graduate School of Life and medical Sciences, Doshisha University#1,

Introduction
In recent years, lunar exploration missions have been planned by governments and private companies

around the world, and new technological developments and international competition have been taking place.
The development of small lunar exploration rovers has been vigorously promoted for lunar exploration. LEV1 and LEV-2 rovers were mounted on Smart Lander for Investigating Moon (SLIM). LEV-1 performed
leapfrogging on the lunar surface and communication with the ground station1), and LEV-2 successfully took
photographs of the SLIM after landing on the surface and transmitted the image data2). Carnegie Mellon
University, in collaboration with NASA, has developed the 2 kg IRIS Lunar Rover which aims to reach thelunar surface3) and Daimon Corporation has released YAOKI, an ultra-compact 2-wheeled lunar rover with amass of 498 g4). Transporting a large number of these small lunar rovers to the Moon is expected to enable
lower-risk and more efficient exploration compared to using a single large rover. Assuming an exploration
mission for sample collection and in-situ analysis by multiple small rovers; first, the lander lands near theexploration target, avoiding obstacles. For example, SLIM has a pinpoint landing accuracy of approximately
10 m5), and the lander is expected to land dozens of meters from the planned landing site, taking into accountobstacle avoidance during landing. Next, multiple rovers disembark from the lander and run dozens of meters
to the target exploration site, where the rovers collect samples and perform in-situ analysis and return to thelander with the collected samples. The rovers are expected to run a total of several hundred meters during theabove missions.
The functions which are useful for such a mission include the ability to estimate the rover's self-position
from the lander and to place landmarks on the lunar surface that have been identified as having absolutepositions. If the rover can estimate its own position, it will be possible to achieve reliable sample return andavoid running out of battery power by taking a long detour. In addition, previous research on swarm
exploration has reported that it is possible to estimate the relative positions of rovers by measuring the strengthof communication between them6). Therefore, it has been suggested that by determining the absolute position
1 of 9of one rover, the absolute positions of the other rovers can also be determined, potentially enabling efficientand safe group exploration. It has been shown that if there are landmarks with known absolute positions on
the lunar surface, the rover's self-positioning accuracy can be improved by periodically returning to thelandmarks7). Furthermore, the placement of landmarks at a point where the target resource is buried can attract
other rovers and improve the exploration efficiency.
Localization methods can be divided into relative localization and absolute localization. Relative localization
is a method of continuously adding up relative displacements and has the problem of increasing error
accumulation8). Absolute localization has been studied using camera-based terrain matching9) and sun sensorbased celestial navigation10); but no research has achieved both low-load processing without using a cameraand position estimation accuracy within a few meters. The localization method using communication between
multiple landers and rovers11) faces the issue of high installation costs for communication bases (landmarks).
Additionally, the method that uses communication between four UWB (Ultra-Wide Band) anchors installedon the lunar surface and UWB tags attached to rovers12) does not clearly specify the installation method for thecommunication bases.
Therefore, this study aims to develop a rover which can perform low-load processing and high-precision
localization, and which can install landmarks at a low cost. The rover will be equipped with an Ultra-WideBand (UWB) tag ejection mechanism and a sensor capable of acquiring rover attitude. A method of rover
localization will be developed by placing the tag as a landmark on the lunar surface. In this study the rover
specifications and the position of the tag ejection to the lunar surface at the time of rover localization aredetermined, and the accuracy of the rover localization is examined.
2.

Development of a Small Lunar Rover and Self-Positioning Methods

2.1. Design of a Small Lunar Rover
Figure 1 shows the specifications of the small lunar rover. The system is controlled by Spresense (Sony), thedrive motors are servo motors XM430-W210 (ROBOTIS), and the motor controller is Dynamixel Shield(ROBOTIS). Two UWB anchors will be folded on the top of the rover and will deploy at a distance of 1 m
between the anchors when the rover descends from the lander. The rover's attitude will be obtained bymeasuring the rover's heading angle from the geomagnetic field and angular velocity using the IMU (InertialMeasurement Unit) sensor. It is assumed that the rover's attitude will be measured using a sun sensor to detectthe direction of the sun during lunar operations.

Figure 1. Small Lunar Rover.

2 of 92.2. Design of UWB tags and Anchors
UWB wireless communication uses very short pulses and communicates over an ultra-wide frequency
bandwidth, resulting in low power consumption and high ranging accuracy13). The UWB anchors (receivers)
to be deployed from the side of the rover and the UWB tags (transmitters) to be emitted from the rear of therover will be manufactured using the UWB modules, DWM1000 (Qorvo).
2.3. Operational Sequence on the Lunar Surface
Figure 2 shows the sequence of the rover's localization by tag release on the lunar surface. First, the roverdescends from the lander and releases a UWB tag at the landing point of the lander (original position).
Subsequently, the rover estimates its position by measuring the distance between the tag and the anchor, as
well as by assessing its attitude. Thereafter, it proceeds to move 5 meters in a direction perpendicular to thetarget point, where it deploys a new tag. The tag position on the lunar surface is determined by measuring thedistances between the tags and anchors and the rover attitude. Then the rover begins to run towards the target
point, and it continues to determine its position from the two tags during running. As the communication
distance between the tags and anchors increases, communication strength decreases and delays occur14). To
prevent this, the rover runs in a zigzag manner dropping a tag every 30 m to establish a new communicationbase and estimate its own position. After the rover arrives at the sample collection point, when the rover'sanalyzer detects a buried resource, the rover drops the tag on the spot and calls other rovers and efficient
sampling operations will be realized. After the rover finishes collecting samples, it returns to the lander while
acquiring its own location using the installed tag.

(a) ①Getting off the lander(b) ②Releasing UWB tag
Figure 2. Operating sequence.

3 of 9(c) ③Rover localization(d) ④Releasing UWB tag and Determining tag localization(e) ⑤Rover localization(f) ⑥Releasing UWB tag and Determining tag localization(g) ⑦Rover localization(h) ⑧Releasing UWB tag and Determining tag localization
Figure 2. Operating sequence.

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2.4. Field Experiment
In this study, the distance between the tags will be reduced in order to facilitate field experiments in aconfined space. First, the rover will drop a tag at the original position (landing point), and then drop a newtag 5 m in the direction perpendicular to the target point. After the rover has determined the tag's position, itbegins zigzagging while estimating its own position, and drops three tags every 3 m, which is one-tenth of thedistance used in the lunar sequence. The rover makes a U-turn at a point 1 m ahead from the position of thethird tag and returns to the original position while acquiring its own position using the installed tags.
2.5. Localization Accuracy
The accuracy of localization using this method was examined. The error in localization during lunar
operations was calculated based on a measured distance error of 150 mm, which is specified in the datasheetof the UWB module used, without considering measured errors due to extended communication distance orobstacles. First, the rover estimates its position using the locations of the two tags dropped at the landing site.
Figure 3 illustrates the positions of the rover (anchor) and the tags. The yellow and blue lines represent thestraight lines connecting the tags and anchors, with the length of each line corresponding to the distance
between the tags and anchors. Figure 4 provides an enlarged view of the area around the rover in Figure 3,
where the anchors attached to both sides of the rover are depicted as green squares, and the center of the roveris represented by a black circle. Consider the scenario where the rover's true position lies centrally among thethree anchor pairs and the three rover center positions depicted in Figure 4. In Figure 4, we consider a scenario
where the true positions of the three anchor pairs and the three rover center positions are located at the center.
When the measured error in the distance between the tag and the anchor is +150 mm, the estimated anchor
position is located 150 mm beyond the extensions of the yellow and blue lines, resulting in the green squareat the top of Figure 4, with the rover center position corresponding to the black circle at the top. In this case,the estimated position of the rover is determined to be 149.0 mm in front of the true position, based on theangle and length of the yellow and blue lines (the distance between the tag and anchor). Similarly, when theerror is -150 mm, the estimated anchor position is located 150 mm short of the endpoints of the yellow andblue lines, resulting in the green square at the bottom of Figure 4, with the rover center position corresponding
to the black circle at the bottom. In this scenario, the estimated position of the rover is determined to be 149.0
mm behind the true position, based on the angle and length of the yellow and blue lines (the distance between
the tag and the anchor). Afterward, the rover drops a tag every 30 meters as it zigzags, and its position isdetermined from the two most recently deployed tags. Figure 5 shows the rover (anchor) position and tagposition, while Figure 6 presents an enlarged view of the area around the rover." The estimated position of therover deviates 149.3 mm from its ground truth position in the forward direction when the measured error is+150 mm and 148.7 mm in the backward direction when the error is -150 mm. Considering that the rover drops
the tags three times during the zigzag running, the sum of the accumulated errors in the direction of runningis expressed by the following equation.
𝜀= 149.0 + 149.3 × 3 = 596.9 𝑚𝑚

𝜀= −149.0 + (−148.7) × 3 = −595.1 𝑚𝑚

5 of 9Where 𝜀𝑚+ is the error in localization at a measured distance error of +150 mm and 𝜀𝑚− is the error inlocalization at a measured distance error of -150 mm.

Figure 3. UWB tags and UWB anchors(rover) position at the start of the running when operating on the lunar surface.

Figure 4. Enlarged view of the anchor in Figure 3.

Figure 5. UWB tags and UWB anchors(rover) position at the start of the zigzag running when operating on the lunar surface.
6 of 9Figure 6. Enlarged view of the anchor in Figure 5.

Next, the error in localization during lunar operations was calculated. The same calculation was conductedby changing the tag drop interval from 30 m to 3 m during zigzag running (Figures 7-10), and the sum of theaccumulated errors in the direction of running is expressed by the following equation.
𝜀= 115.6 + 113.5 × 3 = 456.1 𝑚𝑚

𝜀= −112.4 + (−113.1) × 3 = −451.7 𝑚𝑚

Where 𝜀𝑓+ is the error in localization at a measured distance error of +150 mm and 𝜀𝑓− is the error inlocalization at a measured distance error of -150 mm.

Figure 7. UWB tags and UWB anchors(rover) position at the start of the running in field experiments.

7 of 9Figure 8. Enlarged view of the anchor in Figure 7.

Figure 9. UWB tags and UWB anchors(rover) position at the start of the zigzag running in field experiments.

Figure 10. Enlarged view of the anchor in Figure 9.
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3.

Future WorkIn the future, UWB anchors and tags will be manufactured, IMU sensors for attitude acquisition will becalibrated, and these will be mounted on the rover. Field experiment will be conducted, and the obtained
accuracy of localization is compared with the accuracy of localization derived from the ranging accuracy of
the UWB module to confirm that the theoretical accuracy is obtained. Furthermore, localization is evaluatedby triangulation, particle filter, and extended Kalman filter methods, and the true trajectory and the acquired
trajectory are compared to determine which method provides the most accurate localization.
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