Applicability of Random-Modulation CW Techniqueon LED Mini-Lidar for Mars RoverPrane Mariel B. Ong1, Tatsuo Shiina1, Naohiro Manago1, Hiroaki Kuze1, Hiroki Senshu2,Naohito Otobe3, George Hashimoto4, and Yasuhiro Kawabata5
1
Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba, 263-8522, Japan,Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino-shi, 275-0016 Japan
3
Fukuoka University, 8-19-1 Nakakuma, Jonan-ku, Fukuoka, 814-0180 Japan
4
Okayama University, 1-1-1 Tsushima-naka, Kita-ku, Okayama, 700-8530 Japan
5
Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki, 305-0052 Japan
2Abstract: A compact LED lidar was constructed and field-tested with the aim to observe the Mars’ dust devils. It was constructed to fit
a 10 cm-cube dimension limit. In its present form, it has a 0.75W (7.5nJ/10ns) pulse power, and 500 kHz pulse repetition frequency.
These parameters were optimized so rapid measurement of dust flow in the near range is feasible. Although, it is limited to nigttime
measurement, the pseudo random modulated continuous wave (RM-CW) technique will increase its signal-to-noise ratio (SNR) andrealize a breakthrough in daytime measurement. The PRBS-8 code length with 8ns pulse width parameters were shown to meet therequirements for the LED lidar.
Key Words: LED lidar, RM-CW, Mars Rover
1. Introduction
An LED mini lidar system was constructed to fit a Mars roverwith the purpose of measuring quantitatively the “dust devils”
on Mars surface1). Although the motion and size of the observed
Martian dust devils can be estimated from images obtained fromsatellites and rovers, still the quantitative information from theseestimates were poorly constrained2). Hence, in situ
measurements of near-ground atmospheric and dust activities on
Mars will be useful in the study of this phenomenon.
Given the size limitation of a Mars rover, and the extreme
surface temperature condition on Mars, our team have examined
the use of LED (light emitting diode) as the light source of themini lidar system to be installed in a Mars rover. Aside frombeing energy efficient, heat sink independent, and tough against
circumference change, previous researches on LED lidar have
proved that the LED lidar can be used in near-ground Earthatmosphere monitoring1,3-7). The beam radiation angle is widecompared to its laser counterpart, but this wide beam divergenceof the transmitter will establish a constant overlap in the nearfield with the receiver’s narrow field of view (F.O.V.). Hence,the system design has initially been focused on the robustness
against tough treatment, transportability (10 cm-cube limit), andcapability for near-range monitoring (~30 m) of dust activities.
2. Setup
To date, there are two compact LED lidars that werefabricated given a 10 cm-cube limit: version 1, and version 2 (seeFig. 1). Version 2 was the improved version of version 1,specifically, in terms of compactness, design, and stability. Theirspecifications are provided in Table 1. A high pulse repetition
frequency was set to improve the SNR due to the low pulse
energy (7.5nJ/10ns). To be able to follow the high repetition
frequency (500 kHz) of the transmitter, the photon counter wasfabricated using a high-speed FPGA (Spartan 6, Xilinx) with atime resolution as low as 1 ns or 0.15 m distance resolution
equivalent. In addition, the high-speed photon counter has thefeature to generate the pseudo random modulated sequence(PRBS) with linear feedback shift register (LFSR) of 8 to 15-bit
length (L-bits). The PRBS is the most commonly used RM code
underlying the RM-CW lidar, which was first demonstrated byTakeuchi, et al.8,9) in a Mie scattering lidar. In addition, RM-CW
technique is one technique that can recover a weak signal buried
from a random noise, thus, can be utilized to further increase theSNR of the current LED lidar system. The power consumptionof the current photon counter is 2 W for 1 channel DAQ, but it
was verified that it can still be reduced to 1 W.

Table 1 Parameters of the transmitter and the receiver.
Transmitter

Cassegrain Receiver

NUV-LED
Light Source NCSU034B(Nichia Corp.)
Wavelength
385 nmPulse Width

10 ns

Pulsed Power0.75 W

Pulse-Repetition
Frequency

500 kHz

Beam Divergence

70 mradBeam Size

3 cm

①Barrel

5 cm

F.O.V.

3 mradPrimary Mirror Aperture 10 cm
Primary Mirror Focal
72.25 mm
LengthSecondary Mirror
2.5 cm
Diameter
Secondary Mirror Focal-25 mm
Length②transmitter

3 cm
10 cm
10 cm

10 cm

10 cm

3 cm
receiverreceivertransmitter

Fig. 1 Two versions of the compact LED lidar for near-range
ground experiment:  version 1, and  version 2.
3. System Performance of the 10 cm-cube LED Lidar
In the wind tunnel experiment that was conducted in Japan
Meteorological Agency in Tsukuba, the LED lidar was able tomonitor the smoke’s flow, diffusion and convection from aglycerine smoke generator. A time-distance smoke profiles arepresented in Fig. 2. The smoke generator was positioned 16 m
away from the LED lidar, and directed towards it. It can be seenin Fig. 2-1-A that there is a strong backscattered echo from about
15 m and gradually decreased over time, then registered a slightincrease after 40s at about 2-3 m in front of the lidar system.
Given this information, we can deduce that the smoke underwent
diffusion and convection processes, as illustrated in Fig. 2-2-A.
Similar scenario is presented Fig. 2-1-B with a different windspeed, 0.5 m/s. Due to a faster wind speed, it cuts down theconvection process to a shorter period.

8ns
convection(upward)0.05m/sconvection(downward)
0.05m/sconvection(upward)0.042m/s

Fig. 2 Smoke profiles in two perspectives: (1) height vs time
graph and (2) possible smoke convections, given two different
wind speeds: (A) 0.3m/s and (B) 0.5m/s, at fixed smoke interval,
20s.

Fig. 3 Dust profile from both LED lidar ver. 2 data and dust
counter data (Inage-Kaigan, Chiba, Japan).
In the field-test experiment in Inage-Kaigan, Chiba, wemonitored the dust particles near the ground surface. The windcondition was about 6 m/s northward, which was directed
downwind with respect to the lidar system. To be able to monitor
in such condition, the LED lidar was placed a few tens
centimeters above the ground with an elevation angle close to
the horizontal. Figure 3 shows a good correlation between thelidar data and the dust counter data. There was no detected dust
farther than 20 m. It is an indication that the dust might havealready settled to the ground right away.
In the current LED lidar setting, it can only monitor during
nighttime. A limitation due to its low pulse peak power giving it
a low SNR. One possible solution to this is to install the RMCW technique in the current LED lidar.
4. RMCW
For a RM-CW lidar, the typical source of the binary sequenceis the shift register of L-bit length. The length (2L-1) of these
PRBS-L (i.e. PRBS-1, PRBS-2, etc.), defines how long themodulated signal will be before it cycles again. In our current
setup, the LED is set to transmit a pulse width of 10ns with apulse repetition frequency of 500kHz. These parameters wherechosen to be able to rapidly observe dust particles in the nearrange. To be able to modulate the PRBS in our LED transmittercircuit and still have the same repetition frequency as before,
several factors were considered: (1) PRBS length, and (2) circuit
tolerance and limitation. A section of the generated PRBS-8
waveform of length 255 bits is presented in Fig. 4. This justshows the capability of the high-speed photon counter togenerate an 8ns (Δt) minimum pulse width necessary toreproduce the 500kHz pulse repetition frequency (in PRBS it isequivalent to 1 cycle iteration). Its derivation is illustrated in Fig.
5. Upon verification that we can implement the necessary
random modulated code, we proceeded with the transmittercircuit improvement to be able to accommodate the random
modulated code, which shifted the scheme from peak powertransmission to average power transmission.

Fig. 4 A section of the PRBS-8 generated waveform with 8 ns
minimum width generated using the PRBS generator feature of
the high-speed photon counter.

Fig. 5 Illustration of a pseudo-random modulated signal oflength 15 bits (PRBS-4) leveled to a pulse modulation for visuallength comparison. A sample computation of one cycle iteration(period, T) of a 255 bit (PRBS-8) code length is also shown.
5. Summary
Two compact LED lidars were fabricated to fit the required
10 cm-cube dimension limit for the Mars rover. They were bothtested and both have shown promising results in near-range dust
echo measurement. Although, the measurement is still restricted
to nighttime, the possibility of utilizing the random modulated
CW technique was shown to be theoretically possible for therequirement of the LED lidar. This could potentially increase theSNR of the LED lidar. Currently, we are improving thetransmitter circuit to accommodate the random modulated code.
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