Simultaneous Data + Power Laser Link Investigation with Applications for Lunar

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MASTER'S THESIS

Simultaneous Data + Power Laser Link Investigation with Applications for Lunar

Exploration Missions

Maxime Sixdeniers 2014

Master of Science (120 credits) Space Engineering - Space Master

Luleå University of Technology

Department of Computer Science, Electrical and Space Engineering

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Acknowledgements

Foremost, I would like to express my sincere gratitude to my supervisor Frank Steinsiek.

Thanks to him I had the chance to work on a topic that deeply interests me within the space system division of Airbus Defense and Space (Airbus DS). He also made possible the partnership between Airbus DS and the robotic department of the German Research Center for Artificial In- telligence (DFKI), where I was provided with great research facilities.

Sincere thanks are also addressed to Dipl.-Ing. Roland Sonsalla, my supervisor at DFKI, for his support and guidance throughout the work and for his helpful feedback and comments during the redaction of the thesis. Additional thanks go to Dr.-Ing. Ingo Scholz, who acted as my second supervisor at DFKI, and took care of organizing the partnership with Airbus DS.

At Airbus DS, I would also like to thank Wolfgang Wulfken for all the useful technical advice that helped orient my work in the right direction and for the time spent in the lab together. At DFKI, appreciation goes to Dipl.-Ing. Florian Cordes, for his patience and advice when it came to work with micro-controllers. Eventually, I would like to thank all the persons in DFKI’s electronic lab who were always keen to support me when needed.

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Abstract

Lasers are used nowadays in a wide range of fields with applications for scientific, military, medical, industrial or commercial purposes. Amongst the numerous associated technologies, laser power beaming and free space optical communication have known intense research in the past years in order to develop systems capable to transmit wirelessly either high amount of power or data at tremendous speed. The work presented in this document is divided between a theoretical and an experimental part. The theoretical part presents a special mission scenario defined for lunar rover exploration where an innovative power supply chain set up by a team of collab- orating robots is introduced. A first study comes to the conclusion that this power supply chain as currently proposed is not viable to reach some of the scientific objectives planned by the scenario. A second study assesses the benefits of using laser power beaming as a support for the supply chain. Its outcome shows that with this new configuration the exploration is made possible and several advantages are identified. The experimental part of the work combines laser power beaming and free space optical communication to offer a new perspective and develop a proof-of-concept exhibiting simultaneous power and data transmission capabilities. The review of each of these technologies leads to the identification of the functional blocks required for a dual system. Hardware is then selected before carrying on the development process with the implementation of each individual block. In the next step, the first experiments putting together some of the blocks are performed. Continuous wave and pulsed operation of a laser diode are demonstrated. A first successful experiment testing demodulation capabilities is also presented. Further experiments towards the realization of the proof-of-concept are eventually explained in order to outline the steps leading to the creation of an operational system.

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List of Illustrations

Figures

2.1 Electromagnetic Spectrum [Goddard, 2013]. . . 12

2.2 LPB general functional blocks diagram. . . 15

3.1 FSO general functional blocks. . . 18

4.1 First elements of the reconfigurable system developed during RIMRES: Sherpa, CREX and payload-items [Roehr et al., 2013]. . . 21

4.2 Current mission scenario in the Amundsen crater [Sonsalla et al., 2013]. . . 22

4.3 Landing site used as input for mission scenario (B) and associated points of interest [Sonsalla et al., 2013]. . . 30

5.1 Estimated evolution of LPB technology in terms of power vs. distance according to [LaserMotive, 2012]. . . 36

5.2 SkyCrane’s SA design process. Corresponding SkyCrane SA size and mass to provide required output power is plotted vs. SFR, SherpaFM and Opportunity’s battery charging time. Assumed end-to-end laser link efficiency: 25% . . . 39

5.3 Free LoS evaluation for communication within Amundsen crater [Sonsalla et al., 2013]. . . 42

5.4 Proposed communication architecture for TransTerrA’s mission scenario [Sonsalla et al., 2013]. . . 42

6.1 LPB and FSO functional blocks diagrams (repeated). . . 45

6.2 Dual LPB/FSO system functional blocks diagram. . . 45

6.3 Laser diodes and optomechanical components. . . 47

6.4 Driver demo board with FP 650 nm laser diode mounted. . . 48

6.5 InGaAs photodiode in the foreground with two Si photodiodes. . . 49

6.6 µCcrumb board and XBee RF transceivers . . . 50

6.7 UART 8-N-1 frame format. . . 51

6.8 ASCII character ”A” (binary ”01000001”) sent with 8-N-1 frame format. . . 51

6.9 Baudrate and Frequency. . . 52

6.10 Visualizing ”A*” on an oscilloscope. . . 53

6.11 28 kHz waveform generated with µC timer . . . 54

6.12 OOK demodulation circuit. . . 55

6.13 LTC3108 DC/DC step-up converter. . . 57

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6.14 LTC3108 on PCB with photodiode and super-capacitor. . . 57

7.1 CW configuration, laser diode unmounted (left) and mounted (right). . . 61

7.2 Pulsed operation and duty cycle. . . 62

7.3 Pulsed configuration, laser diode unmounted (left) and mounted (right). . . 64

7.4 Input square wave at the transmitter (channel 3) and triangle wave observed at Vo1 at the receiver (channel 2). . . 65

7.5 Measuring photodiode’s short circuit current. . . 67

8.1 Receiver electronic circuit for simultaneous power and data transmission. . . 72

8.2 Transmitter electronic circuit implementing the complete proof-of-concept functional blocks . . . 73

8.3 Receiver electronic circuit implementing the complete proof-of-concept functional blocks . . . 73

9.1 Receiver element including photodiodes used in different modes. . . 76

9.2 LPB/FSO compatible receiver module. . . 77

Tables

4.1 Cumulated distance traveled by the exploration rover and distance of each of the shuttle’s support trips (one way and round trip distance provided) [Sonsalla et al., 2013]. . . . 23

4.2 Rovers under evaluation . . . 25

4.3 Rover Specifications. . . 27

4.4 Evaluation Leg 1 . . . 28

4.5 Leg 2 Subsections . . . 29

4.6 Evaluation Leg 2, outside PSR . . . 29

4.7 Rover performance per battery discharge cycle and corresponding number of resupplies required from the shuttle for the exploration rovers to traverse each PSR subsection of Leg 2 (as indicated in Table 4.5) . . . 31

5.1 PSR exploration performance of MER Opportunity when supported by LPB for energy supply purposes. Different battery charging times are evaluated, impacting on the SkyCrane’s required SA output power. . . . 40

5.2 PSR exploration performance of SFR when supported by LPB for energy supply purposes. Different battery charging times are evaluated, impacting on the SkyCrane’s required SA output power. . . . 40

5.3 PSR exploration performance of SherpaFM when supported by LPB for energy supply purposes. Different battery charging times are evaluated, impacting on the SkyCrane’s required SA output power. . . . 40

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Nomenclature

µC Micro-Controller

ADC Analog-to-digital converter AM Amplitude Modulation ASK Amplitude-Shift Keying COTS Commercial Off-The-Shelf CREX CRater EXplorer

CTC Clear Timer on Compare CW Continuous Wave DC Direct Current DFB Distributed Feedback EMI ElectroMechanical Interface EPS Electrical Power Subsystem

FASTER Forward Acquisition of Soil and Terrain Data for Exploration Rover FP Fabry-Perot

FSK Frequency Shift Keying

FSO Free-Space Optical communication

Ge Germanium

InGaAs Indium Gallium Arsenide IR Infra-Red

ISM Industrial, Scientific and Medical

LASER Light Amplification by Stimulated Emission of Radiation LD Laser Diode

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LED Light Emitting Diode Li-Ion Lithium-Ion

LoS Line-of-Sight

LPB Laser Power Beaming MER Mars Exploration Rover

MPT Microwave Power Transmission OOK On/Off Keying

Op-amp Operational amplifier PCB Printed Circuit Board

PSR Permanently Shadowed Regions PV Photovoltaic

QAM Quadrature Amplitude Modulation

RIMRES Reconfigurable Integrated Multirobot Exploration System Rx Reception

SA Solar Arrays

SBSP Space Based Solar Power SED Specific Energy Density SFR Sample Fetching Rover Si Silicon

Tx Transmission

UART Universal Asynchronous Receiver/Transmitter UAVs Unmanned Aerial Vehicles

UV Ultra-Violet

VCSELs Vertical External-Cavity Surface-Emitting Lasers

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Introduction

In the realm of optical waves, laser power beaming and data transmission are two existing and well-proven technologies. They use properties inherent to optical waves to transmit either power or data wirelessly. The reason for using shorter wavelengths is on the one hand to transmit power with relevant plug-to-plug efficiency, which is not possible with longer wavelengths. On the other hand, when used to transfer an information signal, the data rates that can be achieved are signifi- cantly higher than radio-frequency or microwave links.

The thesis aims to set up the first development steps towards the creation of a proof-of-concept whose goal is to demonstrate the possibility of using the same optical laser link to transmit simultaneously power and data. The use of one single laser link to transmit wirelessly both power and data is not documented in the literature and is therefore of interest. Furthermore, the applicability of laser power beaming (LPB) is discussed within the reference mission scenario identified for TransTerrA, a project focusing on rover exploration and currently ongoing at the robotic institute of the German Research Center for Artificial Intelligence (DFKI).

The thesis work is divided into distinct parts. Chapter 1 states the different objectives. In Chapter 2 and 3, an introduction to the technologies used nowadays to transmit power or data with the help of optical waves is given. Chapter 4 first introduces TransTerrA’s mission scenario before presenting a study evaluating the feasibility of the current power supply chain planned by this mission scenario. Chapter 5 assesses the benefits of using LPB as a support for the power supply chain. In Chapter 6, the steps required for the development of the proof-of-concept experiments are presented. The descriptions and results of the experiments performed are then found in Chapter 7. Eventually, Chapter 8 and 9 conclude with the lessons learned, the next steps to go through with the development of the proof-of-concept and examples of further investigations that can be based on a first operating system.

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Chapter 1 Objectives

The use of laser beams to transmit data or power is not new. The work presented in this document is the first step in an effort to demonstrate the possibility of combining these two technologies. The motivation behind this new perspective is the creation of a system with the capabilities of beaming a significant amount of power while simultaneously transmitting data at very high data rate.

The mission scenario developed in the frame of TransTerrA is used to discuss the applicability of LPB. Indeed, demonstrating the cooperation of a team of robots for exploration purposes is one of the objectives of TransTerrA. By performing two evaluation studies, without and with LPB available as a support for the logistic chain, it is possible to determine to which extent the use of this technology is beneficial within TransTerrA’s reference mission set-up.

Shortly stated, the two objectives of the work therefore are:

• Two feasibility studies on project TransTerrA, without and with LPB included in the mission scenario.

• Development of the first steps towards the creation of a system allowing simultaneous laser transmission of power and data.

1.1 TransTerrA Evaluation

1.1.1 TransTerrA

One of the challenges that TransTerrA’s exploration rover has to face is the exploration of permanently shadowed regions (PSR). The exploration rover is planned to enter three times such regions in order to carry out scientific investigations. Solar power cannot be harvested in these areas and therefore different means of energy supply must be envisaged. One solution is to used the logistic chain planned by mission TransTerrA so that a small shuttle rover, with low power requirements and higher velocity, resupplies the exploration rover with fresh battery items when required. The first evaluation study determines the feasibility of such an approach. In order to set up a realistic frame, the capabilities of exploration rovers currently operating or under concept studies are used to perform the evaluation.

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1.1.2 TransTerrA+

The second evaluation study aims to investigate TransTerrA+, where LPB is used to support TransTerrA’s mission scenario. While the rover paths proposed by TransTerrA for the exploration rover remain unchanged, the main difference concerns the initial deployment of the rovers. In this case the lander must also be used as a power beaming station and therefore lands itself onto an area with maximum sunlit illumination to harvest solar energy. This evaluation helps determin- ing how the use of this technology for such a mission could be beneficial in terms of e.g. battery size, required powers, exploration range or mission duration. Both the exploration rover and the lander are looked at during the assessment of TransTerrA+.

1.2 Development of proof-of-concept experiments

To achieve simultaneous transmission of power and data, several small-scale proof-of-concept experiments must be developed. A laser link is made of two main parts separated physically: a transmitter and a receiver element. Reviewing the technologies associated to LPB and laser data transmission, it is possible to determine what capabilities are to be implemented at both ends of the link. The corresponding functional blocks can then be identified in order to select the hardware necessary to start the development.

Through the development of the experiments, the different capabilities fundamental for the future implementation of the system can be tested one after the other. They constitute the objectives to reach and are listed hereafter:

• At the transmitter side:

– Generation of an information signal.

– Modulation of laser beam by information signal.

– Radio-frequency receiver.

• At the receiver side:

– Demodulation of laser beam to retrieve information signal.

– Harvesting of incoming optical power through conversion into electrical power.

– Radio-frequency transmitter.

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Chapter 2 Laser Power Beaming

LPB belongs to a very small group of technologies that have been developed to transmit relatively high amount of power wirelessly over large distance. Indeed, microwave power transmission (MPT) is the only other method where research is actively ongoing to achieve this same goal.

Despite the fact that these methods use the same fundamental phenomenon of electromagnetism, the technologies differ widely in implementation and characteristics. This chapter first presents the common physical background behind LPB and MPT, before emphasizing on what makes them different one from another. Detailed information regarding LPB technology is then given, in order to identify the functional blocks required for the implementation of such system.

2.1 Background

LPB and MPT use electromagnetic radiation i.e. the energy of a photon to transmit power from one point to another without the use of conductors. Instead, a direct Line-of-Sight (LoS) is required between transmitter and receiver to be able to wirelessly transmit power.

The photon energy E is directly linked to its frequency ν thanks to the Plank constant h: E = h · ν. In optical communication terminology, the wavelength λ of the radiation is more frequently used. Wavelength is directly related to frequency through the speed of light c: ν = _λ^c, and therefore

E = h · c

λ (2.1)

From this simple but nonetheless fundamental equation it is easy to realize than the smaller the wavelength, the higher the photon energy, directly classifying radiation into several types.

The electromagnetic spectrum is illustrated in Figure 2.1, where it can be seen what these different types of radiation are:

• Very high energetic radiation, such as Gamma rays, X-rays or far UV radiation, are absorbed by the atmosphere and therefore cannot be used for applications where part of the channel goes through the air. Due to their high energy, radiation emitting at such wavelengths can also be very harmful and must be manipulated with extreme caution. Technologies using this part of the electromagnetic spectrum for wireless power transmission are not available.

• At the other end of the spectrum, radio waves (excluding microwaves) exhibit the longest wavelengths with the lowest electromagnetic energy. For these two reasons, the transmitter

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and receiver elements would require antennas of tremendous sizes in order to transmit a significant amount of power. Consequently, such radiation can be discarded for wireless transmission of high amount of power.

• As of now, optical waves and microwaves (which can be considered as high energy radio waves) remain the most promising regions of the spectrum that can be used to wirelessly transmit power. Numerous investigations and experiments have been carried out for several decades and the first terrestrial applications appeared in the past years.

Figure 2.1: Electromagnetic Spectrum [Goddard, 2013].

2.2 Differences between LPB and MPT

Different means must be employed to achieve wireless power transmission whether optical or microwave wavelengths are used. It is due to the nature of the radiation itself, and implies that each method has advantages and drawbacks with respect to the other. They are five main points in which the characteristics exhibited by LPB and MPT differ due to both the different physical properties of optical and micro waves and the technologies developed to use this two types of radiation to wirelessly transmit power.

Efficiency LPB plug-to-plug efficiency (direct current (DC) in to direct current out) is comparatively low. LaserMotive [2012] indicates that current design can reach up to 25% overall efficiency and were expected to exceed 30% in the following years, eventually approaching 50%. MPT systems however, thanks to the development of related technologies for several decades, can demonstrate very high overall efficiency. In particular at 2.45 GHz, the frequency used by most MPT systems as it belongs to the 2.4 GHz - 2.5 GHz band reserved for Industrial, Scientific, and Medical (ISM) applications, efficiency over 90% are achieved [Mohammed et al., 2010].

Size The size of the different elements making up a LPB or MPT system varies widely. LPB uses near-UV, visible and IR radiation to transmit power. These shorter wavelengths enable to use different types of laser to convert electricity into a laser beam at a specific wavelength, containing the optical energy. The narrow collimated beam produced can be focused on

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small receiver elements, photovoltaic (PV) cells, to convert the optical energy back into an electric current. Both lasers and PV cells can be kept to relatively small sizes, resulting in the creation of compact systems, easing the implementation. MPT uses phased array transmitting antennas as they offer the advantages of being electrically steered, no moving parts are necessary to point the beam in a certain direction. At the receiver, a rectenna is used to convert microwave energy into direct current and consists in the combination of an antenna and a rectifier circuit. The principle of a rectenna is to use multiple antenna elements spread over a wide area to capture more energy. According to the power that needs to be received, its diameter can reach dimension up to several kilometers.

Interference Interference with existing communication systems can turned out to be a critical driver when designing a long range system using electromagnetic radiation. LPB systems do not face this issue since optical wavelengths and related very high frequencies are not used by current communication systems and therefore do not interfere with them. The frequency bands used by MPT links are encountered into the broader range of bands used for commercial applications, such as communication or broadcasting. Implementing a MPT link in a location where the same frequency is used for a different application would create interferences between the two systems.

Safety issue When looking at LPB, localized heating caused by collimated beams of high power density is potentially hazardous if the exposure is prolonged for a certain time. Near-IR wavelengths are also dangerous for the eye because they are just beyond the visible range but are still focused by the eye. Very low level of power can create irreversible damage.

Longer IR wavelengths are less dangerous because not focused by the eye. They are considered eye-safe for exposure level considerably higher than visible and near-IR wavelengths [LaserMotive, 2012]. In the case of MPT, the power density of the beam falls well below in- ternational safety standard with intensities no greater that what can be felt by the Sun, and therefore do not pose safety problems. It is important to mention that these issues are not relevant for space-to-space applications.

High atmosphere attenuation Short optical wavelengths used by LPB are considerably attenu- ated in the atmosphere due to molecular absorption and scattering. Local weather conditions, such as rain, fogs or aerosols can even temporarily blocked a laser link. For ground to space or space to ground applications, clouds also act as a barrier. The longer wavelengths of MPT systems make them much less sensitive to molecular absorption or scattering in the atmosphere and cloud results almost invisible. This is also one of the reasons that explains why overall link efficiency is higher for MPT than LPB.

2.3 LPB technology

2.3.1 State of the art

LPB is currently at an early stage of research and development with applications in a wide range of fields in the military and industrial sectors. Potential examples include powering small UAVs, unattended sensors, underwater vehicles, telecommunication towers, hybrid or all-electric air- craft. Providing power in case of disaster, where the supporting electrical infrastructure has been destroyed, would also be a possible application [LaserMotive, 2012]. One of the ultimate goal of

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LPB (and MPT) is the creation of space based solar power plants that would harvest solar energy in orbit before beaming it to Earth. Many concept studies were performed on the implementation of this technology called Space Based Solar Power (SBSP) but as of now, the technological challenges and costs involved by such a program have made impossible the start of an actual project [URSI, 2007; Lu and McClanahan, 2009].

Several LPB experiments have been documented. The record set by LaserMotive Inc. in 2009 as part as NASA Power Beaming Challenge demonstrated the possibility of transmitting up to 1 kW of power to a receiver over 1 km, with a laser link efficiency of more than 10 percent. Further experiments in 2010 saw the 5-minute battery of a small quadrotor helicopter being recharged during the flight thanks to LPB, enabling the flight to last approximately 12 hours [LaserMotive, 2012].

The main limitation of LPB is due to the low efficiency to convert electricity energy into optical energy and vice-versa. Attenuation in the atmosphere is another limitation, which is not relevant for space to space applications. Investigating different types of lasers and developing special receiver cells are the two main ongoing research activities working on increasing the overall efficiency of laser links.

An active area of research is laser pumping i.e. how the energy is transfered from an external source into the gain medium of the laser. The most common type of laser produced, the laser diode (LD), is an electronically-pumped semiconductor laser. It is now used in LPB systems due to recent advances in technology which made them sufficiently powerful and inexpensive. VCSELs (vertical cavity surface-emitting lasers) are semiconductors lasers which can reach efficiencies as high as 50% and up to 100 W [Seurin et al., 2009]. However, to reach higher output power, lasers can be pumped using optical energy (lamp or laser). This technology is applied in fiber lasers which can reach output powers of several kW for efficiencies up to 25% [Wright et al., 2012].

On the receiver side of LPB systems, PV cells especially developed to be sensitive to monochromatic radiation can reach efficiencies higher than traditional triple-junction cells used to harvest solar energy. Amongst these different cells, the dedicated laser power converter developed to exhibit peak efficiency at 1550 nm is shown to reach up to 45% efficiency at room temperature [Mukherjee et al., 2012]. This efficiency can reach up to 80% when the PV cell is maintained at low temperature.

2.3.2 Functional blocks

The main elements of a LPB system consists in the laser, which acts as the transmitter and converts electricity into a collimated beam and the receiver element, typically a highly-efficient PV cell which converts the optical energy back to electrical energy. However, several functional blocks, illustrated in Figure 2.2, are required around these elements to allow for power transmission.

Power supply Primary DC High Power supply is required to power the laser.

Laser driver Lasers are very sensitive device and can easily be damaged or burnt if the operational specifications are not respected. A laser driver is an electronic circuit that allows to control the laser output power and ensures that the voltage and current maximum ratings are not exceeded.

Laser Laser devices operate the conversion between electrical and optical energy. Different technologies can be used, such as semi-conductors lasers or fiber lasers, enabling to reach differ-

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ent output power and beam qualities.

Cooling system A laser heats up as stimulated light is emitted which first reduces the quality of the output and eventually leads to irreversible damages. A cooling system is therefore necessary for any setup whose power is not very low. Thermally regulated mounts can be used for laser diodes for example.

Beam-shaping optics In order to achieve transmission over long distances, beam-shaping optics is necessary to concentrate the energy on a single narrow beam. It is achieved mainly thanks to the use of a collimation system. The quality of the beam at the laser output and the collimation system determines how narrow the beam can be. This makes the use of LPB more hazardous than MPT since the power density_W

m² is much higher in this case. The collimated beam can then be directed and focused on the array of PV cells belonging to the receiver element.

PV cell A large area photodiode used in photovoltaic mode - therefore called PV cell and similar to the ones used on solar arrays - is necessary to convert the optical energy back into electrical energy after the free-space transmission. Unlike the broadband spectrum of the Sun, the laser beam monochromatic radiation enables to have much more efficient cells than the ones encountered in solar arrays. Depending on the material used, these cells have a peak sensitivity at one specific wavelength, which should be the operating wavelength. Material is critical since only photons with sufficient energy will lead to significant photocurrents.

Typical materials include Silicon (Si) for applications in the wavelength range 190 nm to 1.1 µm, Germanium Ge from 400 nm to 1.7 µm and Indium gallium arsenide (InGaAs) from 800 nmto 2.6 µm [Held, 2008].

Power control, battery and load Electronics is needed to regulate the incoming power. It consists mainly of DC/DC converters to match the output voltage from the PV cells to the voltage required to charge the batteries or directly supply power to the loads.

Figure 2.2: LPB general functional blocks diagram.

2.3.3 Wavelength used for the transmission

Theoretically, visible and IR wavelengths are suitable to transmit power wirelessly. However, safety issue and efficiency of available components have oriented ongoing research towards IR radiation. Commercial Off-The-Shelf (COTS) components, including a number of efficient PV cells, are now even available at certain wavelengths in the near IR, e.g. 1054 or 1550 nm. Wavelengths

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ranging from 400 nm to 1.400 nm can create thermal injury to the retina [Sliney, 2011] and therefore mid-IR is sometimes preferred over near-IR when designing a system.

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Chapter 3 Laser Data Transmission

3.1 State of the art

Free-space optical communication (FSO) refers to optical communication links used to transmit data without the use of wires. The frequency of optical waves used by such systems enables to reach data rate much higher than current radio waves technologies. Unlike LPB links where a high amount of power must be transmitted to retrieve significant amounts at the receiver, there is no need with FSO to transmit high quantity of power as power must be high enough only to preserve the integrity of the signal during the transmission. Typically, hundreds of milliwatts are required to achieve transmission over a few kilometers and up to a few watts for long distance experiments, depending on the application on Earth or in space.

Several ground to ground experiments using lasers have demonstrated the potential of such technology over distances in the order of a few kilometers. For example, MOSTCOM [2013] developed a system capable of reaching 10 Gbps over a distance of up to 2.5 km.

Regarding ground to space experiment, the two-way distance record for optical communica- tions was achieved by the Mercury laser altimeter instrument aboard the Messenger spacecraft, which was able to communicate over a distance of 24 million km [Space, 2006]. A more recent data transmission system was also demonstrated by NASA in 2013 where an image of the Mona Lisa was beamed to the Lunar Reconnaissance Orbiter, a satellite orbiting the Moon around 400 000 km away, using laser pulses [NASA, 2013]. In terms of long-distance high-speed achievement, a study envisages to use eight satellites orbiting at about 12,000 km to create a total system capacity of 6 Tbps, corresponding to download speed of 200 Gbps, which is about 100 times faster that what can be achieved with today’s communication satellites’ radio links [TechnologyReview, 2013].

3.2 Functional Blocks

In order to transmit data, an information signal must be applied to the laser beam. The process of applying information to an electromagnetic wave, which acts as carrier for the signal, is called modulation. Current telecommunication methods use radio-frequency waves to carry the signal to be transmitted. Optical modulation refers to a signal being transmitted thanks to optical waves produced by a laser. To achieve this, some of the functional blocks from Figure 2.2 must be modi- fied. Figure 3.1 illustrates the changes.

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Modulation At the transmitter end, the laser driver, on top of providing regulations capabilities as explained in Section 2.3.2, must also have the capabilities to modulate the laser output.

The complexity of the electronic circuit performing this task depends on the type of information signal to be transmitted and the chosen modulation scheme.

Photodiode mode The photovoltaic mode used for the photodiode element at the receiver end of a LPB system can also be used in FSO system to convert the optical power into a current.

However, for high-speed transmission system, fast response time must be achieved and this is the reason why photodiodes are often used in this case in a photoconductive mode where a reverse voltage bias is applied to reduce the response time. More detailed explanation about the difference between photovoltaic and photoconductive modes is provided in Sec- tion 6.3.3.

Demodulation A dedicated electronic circuit is required at the receiver to demodulate the information signal. The demodulation circuit depends directly on the specific modulation scheme that is used for the transmission.

Figure 3.1: FSO general functional blocks.

3.3 Modulation/Demodulation

Modulation methods can be divided into two main categories, depending on what kind of signal is to be transmitted:

Digital modulation Such methods are used to transmit a digital signal i.e. a signal representing a sequence of discrete values corresponding for example to a sequence of bits.

Analogue modulation Analogue modulation aims to transmit an analogue signal i.e. a continuous signal such as an audio or TV stream.

Whether a digital or analogue signal is transmitted, the modulation methods are either called coherent or non-coherent.

Non-coherent or incoherent modulation Non-coherent techniques do not use the phase of the carrier wave to modulate the signal. No synchronization between the transmitter and receiver is required. For an analogue signal, amplitude modulation (AM) is an example of incoherent modulation: the information signal is modulated as intensity variations onto the carrier wave. For digital signal, the corresponding method is called amplitude-shift keying

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(ASK): the discrete values to be transmitted are represented by different amplitude levels onto the carrier signal. Incoherent detection or demodulation is the process of retrieving a signal that has been incoherently modulated on the wave carrier. For AM or ASK, only the amplitude of the carrier wave is ascertained and required to retrieve the information. No phase information is recovered.

Coherent modulation Coherent techniques use the knowledge of the phase of the carrier wave to modulate the signal. When the signal is coherently modulated, the detection method required to retrieve it is not the same that the comparatively simpler one used for incoherent signals. Carrier phase recovery is required at the receiver, which must be synchronized with the transmitter, in order to be able to retrieve the signal. Quadrature amplitude modulation (QAM) is an example of a modulation scheme used to transmit both analogue and digital signal. It uses AM for analogue signal and ASK for digital signal to transmit simultaneously two signals. This is achieved by generating two carrier waves that are out of phase with each other by 90^◦. To the cost of higher complexity, coherent modulation methods enable to reach higher data rates than incoherent methods.

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Chapter 4 Feasibility Study on TransTerrA’s Reference Mission Scenario

4.1 TransTerrA: Innovative Multirobot Exploration Scenario

Project TransTerrA aims to demonstrate how several robots can cooperate together to achieve common objectives. With the aim of assessing systems and technologies developed to achieve this cooperation, both for TransTerrA or in the frame of previous projects such as RIMRES (Re- configurable Integrated Multirobot Exploration System), a complete lunar mission scenario was defined in accordance to a number of scientific objectives related to lunar exploration. The new exploration scheme proposed by TransTerrA could lead to more efficient and cost-effective ways to explore planetary bodies in the solar system. Terrestrial applications could also benefit from the final outcome of the project.

The scenario involves the deployment of a team of robots. The complex missions planned for the future, such as sample return or preparation for new manned missions, will need to be prepared by several automated systems beforehand in order to ensure their success. TransTerrA’s mission scenario demonstrates how several robots cooperate to achieve an efficient logistic chain between a base camp, a rover and at least one shuttle whose task is to supply the rover.

An exploration rover called Sherpa and the scout rover called CREX (CRater EXplorer) have been developed between 2009 and 2012 in the frame of RIMRES [Roehr et al., 2013]. The particu- larity of these rovers is that they are equipped with a specific electro-mechanical interface (EMI) that enables them to physically interact. The EMI is composed of a male (passive) and a female (active) part that provide secure mechanical, signal and power connections between the two connected systems. CREX can be docked to Sherpa and thus transported with it. Payload items equipped with a similar EMI can also be docked to the rovers, providing them with new capabilities. Currently, two fully-operational payload items can be connected to Sherpa and CREX:

Battery module Used in the current stage of the project to represent energy-harvesting payload- items, this module is to be further developed to actually harvest energy from a given source.

For example, an improved module equipped with an array of photovoltaic cells could harvest energy from the Sun.

Camera module This item is used to simulate scientific payload-items. Such functional items can have integrated power source or need to be connected to a battery module. This configura-

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tion was used during RIMRES to successfully demonstrate the operation of the communication channel between the payload-item and Sherpa’s on-board computer. As a result, when docked to Sherpa, the payload-item becomes an integrated part of the whole system.

Therefore, it becomes possible to interconnect systems to develop the logistic chain used as a new exploration scheme thanks to the cooperation of a team of robots. This chain enhances the performance currently achievable with standalone rovers by conferring (semi-)autonomous capabilities to the system. Figure 4.1 shows the exploration rover Sherpa, holding two payload items connected to its articulated arm thanks to the EMI, with CREX underneath it (undocked).

Figure 4.1: First elements of the reconfigurable system developed during RIMRES: Sherpa, CREX and payload-items [Roehr et al., 2013].

4.2 Mission scenario

TransTerra’s mission scenario plans to apply these new technologies to reach scientific objectives falling into the scope of lunar exploration. Sonsalla et al. [2013] evaluated several science concepts in order to determine which scientific lunar mission would be most suitable for the technology demonstration. The assessment led to the choice of a landing site near the south pole of the Moon, with the aim of exploring the Amundsen crater. This choice is partly due to the fact that 9% of the Amundsen crater exhibits PSR of scientific interest. This landing site is therefore used as an input for the definition of the exploration mission intended by TransTerrA. A first mission design concept has been developed to cover areas of interest in the Amundsen crater:

• Approaching the central peak

• Traveling into PSR

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• Climbing the Amundsen crater rim

Figure 4.2 illustrates a possible mission scenario in the Amundsen crater used to demonstrate the operation of the logistic chain through the cooperation of a team of robots exploring the crater.

The main elements of this mission scenario includes the exploration rover Sherpa, the shuttle rover which enables to perform cooperative tasks and the payload items previously described.

Furthermore, stationary base camps are deployed providing infrastructure to support the logistic chain. The mission concept also involves a home base for the logistic chain, which will serve as main supply and link to the ground station. This function is to be assumed by the lander, independently of the chosen landing system. It provides depot for base camps and payload items which can be taken from the lander by the exploration rover.

Figure 4.2: Current mission scenario in the Amundsen crater [Sonsalla et al., 2013].

Points b1 to b7 represent points of scientific interests. Two different legs are to be followed by the exploration rover, which include the visit of the points of interests, the deployment of the base camps, sampling and rendezvous with the shuttle. An executive summary of the mission scenario is as followed [Sonsalla et al., 2013]:

• Rover Leg 1

1. After landing and commission phase, the exploration rover equipped with a base camp heads towards b1 located on the slopes of the central peak.

2. At b1, the exploration rover deploys the base camp and takes regolith samples that are stored in a modular payload item.

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3. The exploration rover then heads towards b2 where it takes more regolith samples and rendezvous with the shuttle which comes from the lander. The shuttle can resupply the rover with battery payload items and exchange the filled sampling payload elements with new ones. Subsequently, both rovers return to the lander, the exploration rover to fetch the second base camp assembly and the shuttle to bring back the samples.

• Rover Leg 2

4. The exploration rover enters PSR and heads towards b3 and b4 to take geological samples.

5. The exploration rover leaves PSR towards b6. Its curved path sees another rendezvous with the shuttle and the deployment of the second base camp.

6. The shuttle returns samples and battery payload to the lander, while the exploration rover enters PSR again in order to sample at b6 and b7 where the last rendezvous with the supplying rover is planned for this mission scenario .

Sonsalla et al. [2013] indicates the cumulated estimated distance traveled by the exploration rover as well as the distance of each of the shuttle’s support trips, as presented in Table 4.1.

Table 4.1: Cumulated distance traveled by the exploration rover and distance of each of the shuttle’s support trips (one way and round trip distance provided) [Sonsalla et al., 2013].

Position(Rover) b1 b2 Lander b3 b4 base 2 b6 b7

Distance traveled [km] 10.47 16.98 22.39 27.04 31 35.46 40.37 47.75 Trip(Shuttle) Lander to b2 Lander to base 2 base 2 to b7

Distance traveled [km] 5.23 10.42 8.96 18.06 11.09 22.08

4.3 Evaluation Study

A review of the main rover requirements for planetary exploration is first given to help under- standing the important features driving the evaluation. Existing rovers as well as rovers under development are then reviewed to determine how they would perform in the frame of TransTerrA’s mission scenario.

4.3.1 Exploration rover requirements

The first exploration rover to successfully land on an extra-terrestrial surface was the Soviet Luno- khod 1. It reached the Moon in November 1970 and remained in contact with Earth until Septem- ber 1971, establishing a durability record that will last for 30 years [Wikipedia-1]. Since then, technologies related to exploration rovers never ceased to get more performant. The reason to carry on intense research work in this frame is that rovers exhibit a number of advantages over stationary landers when it comes to the study of planetary surfaces and soils. They are not re- stricted to a single landing site, can be remotely controlled towards areas of interest and even

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work autonomously. Three main requirements are inherent to the use of such automated vehicles in the context of space exploration:

Reliability Once the rover is set up onboard the spacecraft that will bring it to its scientific target and placed in the rocket’s fairing, it becomes impossible to physically access it. From this point on, it must cope with all aspects of the harsh environment it will face while traveling to and operating on the planetary body to explore. This is why, similarly to all objects sent to space, an exploration rover must be highly reliable to make sure it will reach its scientific objectives and remained operational during the desired mission’s lifetime.

Compactness Depending on the mission objectives, a compromise must be done between the total size and thus mass of the rover and its scientific capabilities. A larger rover needs a larger spacecraft to carry it, which directly increases the launch cost. On top of that, the rover is to be packed on a spacecraft during the travel, to occupy as little space as possible.

Several elements are then deployed when at destination, such as solar arrays, wheels or antennas. These two reasons explain why compactness is one of the most important driver when designing an exploration rover.

Autonomy A rover needs to be able to operate autonomously during a certain period of time. It is not possible to operate the rover remotely in real-time since the time it takes for the signal to travel from Earth prevents real-time communication. Moon rovers can be controlled using near real-time communication since the time for signal to travel back and forth is ”only”

2.5 seconds. However, sending a signal to Mars takes between 3 and 21 minutes, therefore autonomy becomes very critical. Rovers must be able to identify obstacles and hazards on their own, and subsequently take decision, in order to speed up the reconnaissance process without human input.

A trade-off between these requirements eventually leads to a compromise between the rover’s scientific capabilities, exploration range and expected durability. Bearing them in mind, several existing and planned rovers were chosen in order to evaluate their performance in the frame of TransTerrA’s mission scenario. They were chosen for two reasons:

Sufficient available information When basics data such as size or mass are easily encountered, it can turn out to be difficult to find more detailed information necessary for the continuation of the study. For example, no sufficient information were available to include in the study the ESA ExoMars rover planned to be launched in 2018.

Rover size The class of the rovers reviewed should be roughly similar to the exploration rover planned for TransTerrA.

As a result two exploration rovers were picked, all meant for Mars exploration. A first important remark is that all rovers in this mass range use solar arrays (SA) as power source, unlike the comparatively massive Mars Science Laboratory Curiosity (900 kg) which uses the expensive radioisotope power systems alternative [CNES, 2012]. The exploration rover that is to be used in the frame of TransTerrA’s mission scenario is supposed to be an update of the Sherpa rover, equipped with SA, called SherpaFM. The performance of this exploration rover has not been yet evaluated and therefore the following study includes it in the assessment process. Table 4.2 sums

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up the rovers under evaluation, the references used are indicated in Section 4.3.3. The outcome of this first study is used as a baseline to evaluate the performance of exploration rovers in a mission scenario which does not include LPB technology as a support for the logistic chain.

Table 4.2:Rovers under evaluation

Rover Agency Status Mass [kg] Power Source

Opportunity (MER) NASA Operational (since 2004) 180 SA

SFR (Sample Fetching Rover) ESA Concept study ≈73 SA

SherpaFM DFKI Concept study ≈160 SA

In order to assess the performance in the mission scenario shown in Figure 4.2, some information concerning the operational environment, namely the Amundsen crater, are needed. Several technical data from the rovers are also required.

4.3.2 Amundsen crater operational environment

On Mars, a rover typically uses its power source to move and explore during the day, slowly charging its batteries at the same time. When the target waypoint of the day is reached, the rover stops, optimizes the orientation of its solar arrays with respect to solar radiation to ensure fast battery charging. This must be done before the Sun sets so that batteries are charged when the rover enters the eclipse period and goes into sleeping mode until the next day. The battery slowly discharges during the night while providing enough power to maintain the rover at a correct temperature.

A typical operational day for a rover located in the Amundsen crater near the south pole of the Moon sees a completely different environment. One of the first reason is that day/night cycles are different on the Moon, with most of the regions seeing 14.77 days of sunlight followed by 14.77 days of night. This is not completely true close to the poles, due to low sun angle that makes this proportion vary. TransTerrA’s chosen landing site in the Amundsen crater, located close to the south pole, is illuminated up to approximately 25% of one lunation (≈29.5 Earth days) [Kring et al., 2012]. Therefore a rover powered with SA is operational approximately 7.4 consecutive days each lunar phase cycle (amount of time from one new moon to the next).

4.3.3 Rover specifications

The information required for each rover to carry on the study are taken from various sources.

Allouis et al. [2012] provides all data necessary for the evaluation of the Sampling Fetching Rover.

Opportunity’s data comes from Badescu [2009], [Subbarao, 2011] and [Wikipedia-2]. A description of Sherpa’s performance can be found in Roehr et al. [2013].

Several clarifications are necessary to explain how some of the values presenting in Table 4.3 are obtained:

• SherpaFM mass. Four payload-items, weighing 5 kg each, can be docked to Sherpa during normal operation. Additionally, another element of the logistic chain weighing 30 kg, such as a smaller rover or a base camp, can be transported. This brings the total mass of the

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payload that can be docked to Sherpa using the EMI to 50 kg. It is also estimated that 40 further kilos could be placed in or on the rover main body for a total payload mass of 90 kg.

• Batteries.

– Battery Type. The same type is used or planned to be used for all rovers: Lithium- Ion (Li-Ion). This type is used as it presents main characteristics that are much more advantageous over other battery types [Wertz et al., 2011]:

∗ Specific Energy Density (SED). The amount of energy stored within the battery per unit mass. Typical value for Li-Ion battery is 125^{W ·hr}_kg . MER Li-Ion batteries have a specific energy density of 90^{W ·hr}_kg .

∗ High Energy Efficiency. Transmission efficiency between the battery and the load.

Typical value for Li-ion battery is 98%

∗ Low Self Discharge: 0.3_day^%

– Battery Capacity.The total amount of energy stored, expressed in Ampere · hr or W att · hr. The battery capacity Cb is related to the SED and the total mass of the batteries Mb

through Equation 4.1:

Cb = SED · Mb (4.1)

– Battery Charging Time. Equation 4.2 is used to estimate the battery charging time T_b[hr].

T_b= DoD · C_b

P_sa (4.2)

where

∗ DoD is the battery Depth-of-Discharge i.e. the percentage of total battery capacity removed during a discharge period. Typically 20-40% for Li-Ion batteries. 30% is used in the calculations.

∗ C_bis the battery capacity [W · hr].

∗ P_sais the SA output power, or charging power [W ].

• Solar Array.

– SherpaFM.Comparing the SA output power and the power required during locomotion for Opportunity and SFR, it can be seen the SA output power must be approximately 30% higher than the locomotion power, in order to deal with peak power requirements. With a required average power of 150 W, the SA output power can then be roughly estimated to be 190 W. This approximation enables to estimate SherpaFM SA size and mass.

– Size and Mass Estimation. Maximum output power Psais used to obtain an estimate of the SA size and mass, thanks to Equations 4.3 and 4.4 given by Wertz et al. [2011].

Sa= Psa

η · S_c (4.3)

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M_a= 1

S_p · P_sa (4.4)

where

∗ S_ais the size of the array [m²].

∗ P_sais the SA output power [W ].

∗ η is the efficiency at the panel level. Taking into account the operating conditions and degradation at end-of-life, an efficiency of 15% can be assumed.

∗ S_c is the solar constant [_m^W2]. The rovers used for the comparison are related to Mars exploration, therefore when calculating SherpaFM SA size, the solar constant at Mars must be used (588.6_m^W2).

∗ M_ais the mass of the array [kg].

∗ S_pis the typical specific performanceh

W kg

i

. 25^W_kg was a typical value for SA at the time the MER rovers were designed. For SFR and SherpaFM, equipped with the latest technologies, specific performance approaches 100^W_kg.

Table 4.3:Rover Specifications

Opportunity SFR SherpaFM General

Size (H/W/L) [m] 1.5/2.3/1.6 1/1/1.2 0.7/2.5/2.5

Mass (Total/Payload(/Docked)) [kg] 180/6 73/6 160/40(/50) Batteries

Battery Type Li-Ion Li-Ion Li-Ion

Battery SEDh

W ·hr kg

i

90 125 125

Battery Mass [kg] 14.3 3.85 2.84

Battery Capacity [W · hr] 1287 481.25 355

Battery Charging Time [hr] 2.76 1.2 0.60

Solar Array

SA size [m²] 1.3 1.1 2.15

SA Max. Output Power Psa[W ] 140 120 190

SA Mass [kg] 5.6 1.2 1.9

Locomotion Mode Operation

Required Power [W ] 100 95 150

Average velocity_m

hr

36 80 120

Thanks to these specifications, the performance of the different rovers within TransTerrA’s context, i.e. with the aim of exploring the Amundsen crater, can be evaluated. The time required to travel the distances and perform the different tasks planned during the two legs of the scenario are to be determined.

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4.3.4 Leg 1: normal operation Description

The first leg of the exploration scenario takes place in the illuminated area of the Amundsen crater.

Two points of interests are visited by the rover, b1 and b2, where regolith samples are taken. The deployment of a base camp is also planned at b1. A shuttle rendezvous occurs at b2 to exchange battery and sampling payload elements. The exploration rover and the shuttle then both return to the lander. As shown in Table 4.1, the total distances traveled during this first leg are respectively 22.39 and 10 kilometers for the exploration rover and the shuttle.

Evaluation

Within this part of the scenario, the rover continuously operates thanks to the power provided by the SA. Knowledge of the rovers average velocity enables to obtain an estimate of the time needed to travel the distance involved. Section 4.3.2 indicates that approximately 7.4 days of each lunar month are illuminated in the Amundsen crater, therefore it is also possible to determine how many months are required for the completion of Leg 1 of the scenario. In other words, it allows to determine the number of lunar nights the exploration rovers would have to survive. Table 4.4 summarizes the results. Time required to perform the tasks planned at b1 and b2 is not included.

Table 4.4:Evaluation Leg 1

Opportunity SFR SherpaFM Distance to travel [km] 22.39 22.39 22.39 Average velocity_m

hr

36 80 120

Time Required [hr]-[day] 622-26 280-12 187-8

Lunar nights 3 1 1

4.3.5 Leg 2: PSR exploration Description

The second leg of the scenario sees the exploration rover entering PSR to head towards the next two points of interest: b3 and b4 to take geological samples. It then leaves PSR to encounter the shuttle and deploy a second base camp. Finally, it enters and leaves PSR twice more in order to sample at the two last points of interest: b6 and b7.

During Leg 2 of the scenario, the exploration rover alternates between illuminated and non- illuminated regions. The rover operates on SA power as long as it is encountered in sunlit areas.

The output power from the SA starts steadily decreasing as the rover gets closer to PSR. The battery must progressively take over to compensate for this reduction of power. Eventually, the SA does not produce anymore power and the batteries must provide the required operational power. When the rover heads again towards sunlit regions, the reverse scenario occurs.

The time a rover is operational inside PSR depends on power consumption and battery capac- ities for one battery discharge cycle. A high power harvesting mode leads to a faster discharge of the batteries and thus a shorter time allowed inside PSR. To extend this time and enable the rover

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to reach its objectives inside PSR, TransTerrA’s mission scenario relies on the power resupplying capabilities given by the logistic chain. A key point of the assessment is therefore to determine how many times the exploration rover needs to be resupplied with new fresh battery modules each time it travels inside PSR. This depends directly from the distance which has to be traveled in this region.

For a first evaluation, Leg 2 is divided in six subsections, depending on whether or not the exploration rover is encountered inside PSR. The transition period described earlier when the rover enters or leaves PSR is not taken into account. Figure 4.3 illustrates the chosen landing site for the mission scenario (B), the associated points of interest as well as the regions of permanent shadows. Table 4.5 describes the six subsections with a rough distance estimation, calculated thanks to Table 4.1 and Figures 4.2 and 4.3.

Table 4.5:Leg 2 Subsections

Subsection 1 2 3 4 5 6

Description L to PSR PSR to PSR (b3,b4) PSR to PSR PSR to PSR (b6) PSR to PSR PSR to b7

PSR (IN/OUT) OUT IN OUT IN OUT IN

Distance [km] 4 6.61 6.87 1 4.38 2.5

Evaluation outside PSR

Outside PSR, the evaluation is similar to Leg 1. The rovers can operate using power generated by the SA and the time necessary to travel subsection 1, 3 and 5 can be easily estimated from the average velocity. Table 4.6 presents the results.

Table 4.6:Evaluation Leg 2, outside PSR

Opportunity SFR SherpaFM Cumulated Distance to travel [km] 15.25 15.25 15.25 Average velocity_m

hr

36 80 120

Time Required [hr]-[day] 427-17 191-8 128-6

Lunar nights 2 1 0

Evaluation inside PSR

Inside PSR, the main focus of the evaluation is to determine how many times the rovers must be resupplied with fresh batteries to traverse the different shadowed sections planned during Leg 2. To reach this purpose, the time the rover can remain operational during one battery discharge cycle needs to be calculated, using Equation 4.5.

Cb= Pe· T_e

(DoD) · N · η ⇒ T_e= Cb· (DoD) · N · η Pe

(4.5) where

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Figure 4.3: Landing site used as input for mission scenario (B) and associated points of interest [Sonsalla et al., 2013].

• C_bis the battery capacity [W · hr]

• P_eis the operational power required [W ]

• T_eis the maximum operation time [hr]

• DoD is the battery Depth-of-Discharge.

• N is the number of battery.

• η is the energy efficiency.

Several assumptions need to be done:

• Since Opportunity and SFR are being evaluated in the frame of the logistic chain planned by TransTerrA, their battery can be replaced by fresh ones brought by the shuttle when required.

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• SherpaFM is equipped with four 4-kg Li-Ion battery payload items to enhance the time it can operate inside PSR before it is resupplied. Along with its built-in battery it makes the total battery capacity up to 2355 W · hr.

• In a realistic scenario, only the four battery payload-items docked to SherpaFM can be replaced when required. Therefore after the first battery discharge cycle inside PSR, the battery capacity is reduced to 2000 W · hr. The built-in battery can be recharged only when the rover reenters an illuminated area.

Table 4.7 presents the results of the investigation.

Table 4.7: Rover performance per battery discharge cycle and corresponding number of resupplies required from the shuttle for the exploration rovers to traverse each PSR subsection of Leg 2 (as indicated in Table 4.5)

Opportunity SFR SherpaFM

Exploration Rover Performance

Battery Capacity [W · hr] 1287 481.25 2355 2000

Average Power Pe[W ] 100 95 150 150

Operating Time Te[hr] 3.78 1.49 4.6 3.92

Average velocity_m

hr

36 80 120 120

Distance Traveled [m] 136 119 553 470.4

Shuttle Supplies Requirements

Leg 2 Subsection 2 4 6 2 4 6 2 4 6

Distance [km] 6.6 1 2.5 6.6 1 2.5 6.6 1 2.5

No. Supplies Required 48 7 18 55 8 20 13 1 5

The number of resupplies needs to be as low as possible as each new supply lengthens the duration of the total scenario. SFR would need 55 resupplies to travel the total distance planned by subsection 2 of Leg 2. Even though its lower velocity accounts for this high number, the main reason is its low battery capacity which only allows it to remain operational 1.5 hours when in locomotion mode.

Feasibility Considerations

Theoretically, using the logistic chain planned by TransTerrA for power supply purposes is possible. However, in practice, it can rapidly be demonstrated that the capabilities required by the shuttle to perform this task are too demanding.

A simple scenario for battery resupply sees the shuttle packed with full battery payload-items in the nearest illuminated area from the exploration rover, where a base camp is deployed in order to provide energy harvesting capabilities. The shuttle must first travel to the exploration rover where the empty battery pack are exchanged with the new ones before heading back to the sunlit area while the rover starts being operational again. Leg 2 Subsection 2 corresponds to the longest path and the deepest exploration performed inside PSR in the frame of TransTerrA’s mission scenario. Thanks to Figure 4.2 and 4.3 point of interest b4 can be estimated as the furthest

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point reached inside PSR about 2.5 kilometers from the nearest illuminated area. At this step of the scenario, the shuttle has to travel approximately 5 km, 2.5 km carrying fresh battery module and 2.5 km with empty ones, to supply the exploration rover.

Assuming the shuttle average velocity to be twice as high as SherpaFM, i.e. 240_hr^m, it means that the shuttle should remain operational on its own batteries during approximately 21 hours to travel the required distance of 5 km. Further assuming 100 W power average requirement for the shuttle in locomotion mode, the battery required to meet these needs should have a capacity of 7150 W · hr (Equation 4.5). This number, applied to a Li-Ion battery whose SED is typically 125^{W ·hr}_kg , corresponds to a battery mass of about 57 kg. Eventually assuming that the shuttle is equipped with SA capable of delivering a power of 100 W , the time necessary to charge such a battery would approximately be 21 hours (Equation 4.2).

Sonsalla et al. [2014] presents a concept study for the FASTER (Forward Acquisition of Soil and Terrain Data for Exploration Rover) scout rover which is to be used as the baseline for the design of TransTerrA’s shuttle. It specifies a total maximum rover mass of 20 kg, a 200 W · hr LiPo battery and SA capable of delivering a maximum power of 60 W . It can easily be seen that FASTER’s capabilities dramatically differ from what would be required to supply energy to the exploration rover while deep inside PSR. Furthermore, considering the 21 hours return trip plus the 21 hours charging time for the shuttle battery, the inherent duration of the complete scenario to explore each PSR subsection becomes unrealistic.

4.4 Conclusion

4.4.1 Operation in illuminated area

The studies carried out in Sections 4.3.4 and 4.3.5 evaluate the capabilities of current and future exploration rovers in the illuminated areas of the scenario. Average locomotion power requirements can be provided with the use of SA of reasonable size, enabling the rovers to continuously travel during the 7.4 illuminated days available during one lunar month in the Amundsen crater.

The time to explore these areas depends on the average rover velocity. Due to its lower velocity (36_hr^m), Opportunity would need 26 days of illumination to travel the distance planned by Leg 1 and 17 days of illumination to travel the sunlit subsections of Leg 2. SherpaFM exhibits the highest average velocity (120_hr^m) and consequently it would require respectively 8 and 6 days of illumination to travel the same distances. To equip the exploration rover with SA seems the best solution to travel in illuminated area since the rover can be continuously powered.

4.4.2 Operation inside PSR

In the current version of TransTerrA’s mission scenario planed for terrestrial demonstrations, the energy supply process is ensured by one shuttle whose task is to resupply the rover with fresh battery modules when required. The shuttle comes back and forth between the rover and a base camp where energy harvesting for battery modules is possible. The last part of Section 4.3.5 aimed to determine the feasibility of this approach. Using the deepest point reached inside PSR planned by the scenario, which occurs approximately 2.5 km from the closest illuminated area, simple calculations led to the realization that this energy resupply scheme is impracticable. As a matter of fact, assuming that a base camp has been deployed in the closest illuminated area, a shuttle with average locomotion power requirements would require the mass of its typical Li-Ion battery to be

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in the order of 57 kg to travel the required 5 km round trip. The impracticability is accentuated with the fact that this scenario is to be repeated several times and therefore the time to travel the distance plus the time to recharge the shuttle battery eventually imply a unrealistic duration to cross the PSR subsection.

Therefore, the outcome of this evaluation shows that using the part of the logistic chain formed by the shuttle and the exploration rover for energy supply purposes is not practicable at this time.

A mission scenario involving less deep exploration inside PSR or different means to supply energy to the exploration rover while inside PSR must be considered.

Simultaneous Data + Power Laser Link Investigation with Applications for Lunar

MASTER'S THESIS