Lunar exploration has important strategic significance, the current major international space power and organizations will take the Moon exploration as a starting point for deep space exploration. Many have already launched a series of lunar activities.
In planning for future human exploration activities in a lunar environment, the wireless communications infrastructure used will be of critical importance. Using standardized communication protocols such as 802.11 can reduce costs, enhance interoperability, and provide robust networks that build on proven technology and security. While long-range communications are of great importance, short-range machine-to-machine (M2M) communications in a lunar environment are of increased importance as the loading of local communications. As the name indicates, it is the communication between machines without any human intervention. It can be said that different devices interacting with each other and exchanging data over a wired or wireless channel. It is also called as Internet of Things in many scenarios and almost synonymous. M2M allows virtually any sensor to communicate, which opens up the possibility of systems monitoring themselves and automatically responding to changes in the environment, with a much-reduced need for human involvement. Also, voice conversation and real-time video exchanges among astronauts for situational awareness, are expected to grow exponentially with the number of network elements.
The future lunar network will be incrementally realized through a campaign consisting of a series of logistical and explorative missions designed to support the infrastructure required for long-duration stays and, eventually, for a permanent lunar outpost. A large number of network elements, such as habitat modules, power generation units, mobile robotic vehicles, and astronauts, must be networked together through shared access to the network infrastructure.
Machine-to-Machine (M2M) communications
Machine-to-Machine (M2M) communications refer to the technology that enables the communication between different devices and allows them to perform a variety of actions without or with only limited human intervention. M2M and IoT are almost synonymous but it’s important to know that IoT is itself a subset of M2M technology. Both terms relate to the communication of connected devices, but M2M systems are often isolated stand-alone networked equipment. IoT systems take M2M to the next level, bringing together disparate systems into one large, connected ecosystem.
Because of the extremely large number of devices involved as well as the different communication patterns, the implementation of M2M poses many new challenges related to data transmission, protocol design, system integration, power, and spectrum efficiency which make it a very hot research topic. M2M can be viewed as a myriad of different components (sensors, actuators, middleware, software, and applications). In many lunar environment-related applications, the wireless sensor nodes (WSNs), which lie at the bottom of this myriad, are deployed for many different purposes.
Wireless Sensor Nodes at the Lunar Surface
The frequency band, type of modulation, emission power, and communication protocol strongly impact the performances of the WSN. Frequencies ranging from 1MHz to 10GHz would allow a good tradeoff between the antenna size and the communication distance that could be achieved in the presence of noise from external sources, i.e. galactic, Solar, and atmospheric.
Wireless sensor network (WSN) has been widely used in various detection and data transmission applications on the Earth. The extended application of wireless sensors in space is one of the hot research fields, especially in lunar environment applications, such as water detection on the Moon. Because wireless sensor networks are composed of a large number of sensor nodes, which are deployed in different locations, they can monitor large geographic areas remotely and can overcome the limitations of field measurements of the lander and the rovers on the lunar surface.
A lunar wireless sensor network will be composed of wireless detection nodes deployed on the lunar surface in a specific way. These wireless detection nodes constitute a network topology based on wireless transmission links. These nodes are interconnected with each other, and they themselves are connected with the mobile base station of the lunar rover, or with the mobile base station of the lunar rover through some relay nodes. By the mobile base station of the lunar rover, the sensor nodes are connected with the central station of the lunar base. Considering the different geographic environment on the Moon, the deployment of wireless sensor networks on the Moon can consider two situations: one is the plain area on the Moon, the plain area on the Moon can use a lunar rover to throw sensors. For sensors that need to be buried in the lunar soil, they can be buried under the Moon through the lunar rover drilling. For the plain area of the Moon, normally there is no shelter and the dynamic environmental impact of the lunar surface is not very different from the conventional deployment of sensors. The system architecture of wireless sensor networks mainly considers the impact of lunar dust on sensor nodes, which will cause the communication capability of sensor nodes to decline or fail. The other is lunar crater area, which can also be deployed by the way of lunar rover dropping sensors for lunar crater area. Due to the influence of the surrounding environment, the parameters such as the communication range and the communication rate of sensor nodes are limited. Compared with the lunar plain area, the system architecture design of wireless sensor networks needs to consider more factors, including the setting of communication relay nodes, the coverage of wireless nodes, and so on.
Considering the actual application scenario of sensor deployment on the lunar surface, the working frequency of the sensor is to be 2.4 GHz, and the working conditions on the lunar surface are different from those on the Earth. It is known that there is no atmosphere on the Moon and there is a very high vacuum of 10−12 Torr. The communication model of the terrestrial communication network is not directly applicable to the lunar wireless sensor network, so it needs to be modified according to the conditions of the lunar surface. The existing propagation models to the Earth include irregular terrain model (ITM also known as Longley-Rice model), two-ray model, multipath model, and multipath signal distribution. Since there is no atmosphere on the Moon, the applicability of the model for the lunar needs to be examined. Physical phenomena and path losses on the lunar surface, including free space loss, reflection, reflection scattering, diffraction, are studied.
Lunar Surface Communication System
The base camp on the Moon consists of multiple communication terminals, which requires a comprehensive lunar surface communication system. As the first step, landers, rovers, and robotic probes will be brought to the lunar surface. And in the long-term, astronauts will work in the research station as mobile users of a lunar surface communication system. Based on a short-term plan, the prototype of a lunar surface communication system of a lunar research station could be several centralized wireless networks, which is similar to the terrestrial cellular networks.
Lunar Wireless Networks
The UHF (Ultra-High Frequency) band is used cooperatively with Proximity-1 Link Protocol. S-band supports IEEE 802.11 and LTE wireless network protocols. And, Ka-band can apply cooperatively with IEEE 802.16 to support ultra-high-speed wireless networks. All of IEEE 802.11, IEEE 802.16, and LTE employ OFDM in the physical layer, which has good multipath resistance. Compared with 802.11, LTE, which requires the main node to work as a base station, provides a higher data rate along with higher implementation costs. The main nodes in LTE wireless network consume higher power and need more complex constructions. So far, the engineering selection of lunar wireless network protocol is still an open problem waiting for further research. However, in October 2020, Nokia was named a NASA partner for its Tipping Point technologies for the Moon program, receiving a $14 million contract to deploy the first LTE/4G communications system on the Moon.
The LoRaWAN is a Low Power, Wide Area networking protocol specifically designed to wirelessly connect battery-operated ‘things’ to the internet in regional, national, or global networks, and supports features like bi-directional communication, end-to-end security, mobility, and localization services. This technology is also highly suitable for Lunar missions and supports the type of communication required on the lunar surface. Australian Lunar Exploration Mission is planning to send nanosatellites and exploration sensors to the Moon, this will be another technology that can be used for Lunar M2M communications.
While, no matter which wireless network protocol is adopted eventually, it has to be modified for the adaptability of lunar application and lunar environment. Commercial-Off-The-Shelf (COTS) network chips would also require to be changed keeping in mind the space environment and the requirements of lunar wireless communication networks.
Design of Multi-Node Wireless Networking System on Lunar Surface
Lunar wireless communication network for the lunar re-entry plan, mainly uses UHF, S, C, and Ka-bands. The UHF band is responsible for the reliable communication of voice and data between EVA astronauts, lunar modules and stations, and can also be used for emergency communication on the lunar surface.
The S-band has also used some communications satellites, especially those used by NASA to communicate with the Space Shuttle and the International Space Station. The S-band (TT&C communication) completes the high reliable transmission of voice, health parameters, instructions, remote control, and telemetry data at medium and low speeds. The S-band (WLAN) provides wireless access to portable devices, physiological parameters acquisition, equipment location, and status information acquisition for EVA astronauts, robots, lunar rovers, etc. So NASA plans to choose WLAN as part of its wireless communication protocol.
NASA is developing LunaNet, a unique approach to lunar communications and navigation. The LunaNet architecture would enable the precision navigation required for crewed missions to the Moon and place our astronauts closer to scientifically significant lunar sites. LunaNet navigation capabilities will also include a lunar search and rescue network, or LunaSAR to enhance mission safety. Existing Search and Rescue (SAR) systems suffer from operational and performance limitations that limit their effectiveness.
A project SmartSat Cooperative Research Center (CRC) will initially focus on the current internationally used Cospas-Sarsat SAR system, a system constrained by a number of existing specifications and requirements, and then extend to additional applications. The first phase of this work will develop enhanced system requirements and resilient architectural designs, waveforms, and protocols for selected concepts of operation and use cases, that will improve the efficacy and operation of the current system. Specifically, SmartSat CRC will propose new designs for the 406 MHz signal sent by beacons through the Cospas-Sarsat network. NASA’s Search and Rescue team is working to extend beacon services to the lunar surface with the LunaNet communications and navigation architecture.
Through the research and analysis of the above wireless communication protocols, the following enlightenments are given for the future multi-node networking system on the lunar surface:
- The international mainstream space organizations have basically reached a consensus on lunar networking communication, and plan to use the lunar LAN networking scheme based on WLAN technology.
- At present, the application of international CCSDS* recommendations is mainly in the data link layer and network layer and has not yet formed a complete deep space information protocol system, especially in the lunar wireless networking protocol. It is necessary to study the lunar wireless network architecture and protocol architecture on the basis of existing network protocols.
(*The Consultative Committee for Space Data Systems (CCSDS) is an international organization formed by space agencies in 1982.)
Design of Wireless Networking According to the above research and analysis, combined with the needs of the lunar networking task, a scheme combining UHF-based low-speed MESH network with 802.11n-based high-speed WLAN local area network is designed. UHF low-speed MESH network is upgraded on the basis of UHF point-to-point low-speed communication system, which has been successful used on lunar. It can increase network communication capability, provide stable and reliable low-speed communication links for lunar networking. WLAN local area network refers to NASA’s standard requirements and develops a high-speed LAN system suitable for lunar mid-long distance networking communication.
The multi-node wireless networking system on lunar takes the central controller node as the center, the UHF is used as the signaling channel and the minimum communication, and the S-band is used as the data channel.
The UHF band has the characteristics of good channel stability and strong anti-multipath ability, which can realize high reliable and low rate networking communication between detectors on lunar. When the initial link is built, the access nodes of each detector and the central controller node transmit reliable signaling data frames through the UHF band to establish routing links. In the process of detector movement, the central controller node always acts as the central backbone node. The other detector nodes can adaptively change the network topology and access the central controller node or other detector nodes.
When a detector access node and a central controller node successfully establish a link through UHF, they can send high-speed traffic data to the central controller node through the WLAN band. Each node can select the communication frequency and rate adaptively according to the current channel quality. The following are two special application modes:
- Collaborative work mode is the mode of data communication between detector nodes when they carry out scientific exploration tasks. The content of transmission includes the data information that needs the cooperation between detector nodes and the telemetry and key business data that needs to be broadcast to the other nodes and backed up in multiple detector nodes. This mode mainly uses the UHF band to communicate.
- The high-speed networking mode is the working mode of the detector node, which sends scientific detection data and log data to the central controller node. This mode uses S-band as data transmission channel. The central controller acts as the central node (base station). The detector node acts as the access node (terminal). After data transmission, the network resources are released in time.
Communications in a Cave Environment
Lava tubes are thought to exist on Mars and on the Moon and are of special interest for science objectives and human habitability. The communications environment in such a cave is important for exploration. It is essential to investigate the electromagnetic properties of the cave environment and how to plan a communications network, with a focus on identifying communications hazards and their workarounds. The waveform selection and optimizations for a power-constrained environment are also significant.
Use of 802.11 WiFi
The 802.11 OFDM waveform is specifically designed to deal with a frequency-selective fading environment. In literature, it is shown that the biggest time delay spread appeared to be 250 ns (70 meter path difference) without any evidence of a wider spread. This is significantly smaller than the 802.11 OFDM symbol period of ~3 μs. For this reason, Wi-Fi performance in the cave being modeled is estimated based on signal to noise rather than having to consider the inclusion of residual interference (C/I) effects of highly delayed signal energy. In a terrestrial setting, regulation limits the transmitted power to below 100mW. Antenna and cave entrance losses were budgeted at 10 dB. These losses represent any combinations of unmodeled exploration vehicle boundary conditions, poor antenna orientation or deployment, poorer coupling into the cave than simulated, antenna loading, and suboptimal voltage standing wave ratio (VSWR). The freestanding cross-dipole antenna (turnstile antenna) is assumed to be optimally oriented and points toward the cave entrance.
The waveguide nature of the cave and the relative close proximity of 100 meters imply that, even with an obstruction, Wi-Fi coverage (RSSI > -81dBm) is available. The maximum 54 Mbps rate is experienced in the majority of the cave, and locations with weaker signals are covered with reduced Wi-Fi rates. Research shows a -44 dBm mean RSSI throughout the cave with 2.5 percent distributed below -66 dBm. This suggests a spatial fading margin of roughly 22 dB relative to the mean signal level in the cave. This margin can be sub-allocated to:
- Fading margin within any cross-section of the cave due to concentration of signal power in strong reflections,
- Signal energy flux loss due to the obstacle,
- Reduced signal level in shadowed regions,
- Attenuation by lossy cave walls, and
- Fading as a result of backscatter.
Each of the quantities can be assessed based on the specific cave encountered.
Lava tubes do exhibit rough surfaces, and a simulation with surface variations up to 10 cm suggested an attenuation closer to 0.2dB/m. If 15 dB are booked for losses due to rough surfaces, the distribution suggests that communications are still possible in 97.5% of the cave.
With the exception of side passages, if the signal is lost, the spatial distributions shown from the simulations suggest that mobility would allow regaining communications.
If exploration is to include side passages, transmit power can be increased since the regulatory requirement would not apply. However, if the option is available, a mission capable of placing a relay is a more reliable approach.
Finally, it is noted that signal processing power, not transmit power, may be a limiting factor in a power-constrained environment, and especially if older technology is deployed to counter the space environment. A much simpler waveform, where WiFi coding and OFDM processing can be traded off for higher transmit power, is possible. A simplified 802.11 waveform may be deployed along with the full waveform, and the simplified waveform could be used when mobility allows the exploration vehicle to move to locations of greater signal concentration.
IEEE 802.11ah is a wireless networking protocol published in 2017 called Wi-Fi. It uses 900 MHz, license-exempt bands, to provide extended range Wi-Fi networks, compared to conventional Wi-Fi networks operating in the 2.4 GHz and 5 GHz bands.
A benefit of 802.11ah is the extended range, making it useful for rural communications and offloading cell phone tower traffic. The other purpose of the protocol is to allow low rate 802.11 wireless stations to be used in the sub-gigahertz spectrum. The protocol is one of the IEEE 802.11 technologies which is the most different from the LAN model, especially concerning medium contention. A prominent aspect of 802.11ah is the behavior of stations that are grouped to minimize contention on the air media, use a relay to extend their reach, use little power, are still able to send data at high speed under some negotiated conditions, and use sectored antennas. It uses the 802.11a/g specification that is downsampled to provide 26 channels, each of them able to provide 100 kbit/s throughput. It can cover a one-kilometer radius. It aims at providing connectivity to thousands of devices under an access point. The protocol supports machine-to-machine (M2M) markets.
Use of 802.11p
It is evident that autonomous vehicles can boost productivity in operations carried in lunar caves. These challenging environments generally need to have V2V (vehicle to vehicle), V2P (pedestrian), and V2I (infrastructure) enabled communication. IEEE 802.11p is an approved amendment to the IEEE 802.11 standard to add wireless access in vehicular environments (WAVE), a vehicular communication system. It defines enhancements to 802.11 (the basis of products marketed as Wi-Fi) required to support Intelligent Transportation Systems (ITS) applications. The lunar cave environment resembles the mining operations carried on Earth. This 802.11p has been implemented for mining operations and its noted that it has an outstanding communication range and it can determine its location precisely, even deep underground.
IEEE 802.11p standard typically uses channels of 10 MHz bandwidth in the 5.9 GHz band (5.850–5.925 GHz). This is half the bandwidth, or double the transmission time for a specific data symbol, as used in 802.11a. This allows the receiver to better cope with the characteristics of the radio channel in vehicular communications environments, e.g. the signal echoes reflected from the surroundings.
Effective Planning of Wireless Sensor Network Deployments for In Situ Lunar Surveys
Wireless sensor networks (WSNs) are a promising tool for in situ planetary exploration missions.
A critical issue in these missions is deployment, which must be performed by scattering groups of nodes – clusters – over relevant areas in a three-dimensional (3D) terrain via airborne sensor launches. Deployment planning is, therefore, subject to random network connectivity and coverage, and must be tackled by selecting suitable cluster launch parameters (e.g., position, height, etc.), to achieve a good observational performance, while meeting constraints such as lifetime and mass limitation. So far, the issue of modeling and computational challenges inherent to these environments is not addressed much. Research filling this gap was found by developing a full analytical model, which enables to search for optimal deployment configurations.
Industrial, Scientific, and Medical (ISM) Ultra-High-Frequency band of 2.4 GHz with a transmission power between 0 dBm and 20 dBm. Communication ranges of up to a few hundred meters are ideally possible with a direct line of sight. However, irregular terrain elevation causes them to be considerably lower. Connectivity is determined by the signal-to-noise-plus interference ratio, SNIR, which must be above a minimum SNIRmin (the minimum SNIR value) threshold (radio-hardware parameter). It’s assumed that each cluster operates in a separate frequency channel, so the interference power on a receiver depends exclusively on nodes of the same cluster.
Lunar satellite coverage can prove to be an important part of the solution of this problem, whether it is connecting all nodes via satellite or a subset of satellite-connected nodes. ESA through its Project Moonlight intends to put a constellation of satellites around the Moon to make each individual mission become more cost-efficient.
NASA’s LunaNet intends to provide communication service for lunar missions using satellites. Envisioned much like the internet on Earth, it would enable communications among robotic landers, rovers, scientific devices, and astronauts, sensors and also allow them to transmit data back to Earth through Moon-orbiting relays, such as satellites, small satellites, or a Moon-orbiting space station.
Cluster-based Wireless Sensor Network Deployment for Lunar Exploration
Most existing deployment schemes focus on deploying nodes on the Earth. However, WSNs deployment on lunar is facing many challenges which are not realized on the Earth, such as mass restriction, the anti-impact ability of nodes, non-replaceable sink, etc. In other words, we need a new WSNs deployment scheme that aims at the requirements and considerations of the lunar WSNs deployment. Researchers proposed a positive n-edge cross deployment method based on the existing deployment framework and give the anti-impact conditions.
Let’s first define some terms used and then give an overview of the deployment scheme for the lunar wireless sensor network. Terms definition:
- Prober: The spacecraft which scatters out several cluster managers.
- Cluster manager/master node: The sink node of each cluster that is responsible for the deployment, aggregation, and forwarding of data.
- Cluster deployment point: The point where the cluster manager starts deploying nodes.
- Prober separation point: The point at which the prober starts to scatter out several cluster managers.
In order to ensure the robustness of the deployment and support applications including synchronization, localization, and data aggregation, the distance between two neighboring master nodes should not exceed their maximum communication radius.
Because the lunar surface environment is very different from the Earth’s environment, we need to discuss the impact of the lunar environment upon the sensor node. We need to resolve key technical problems of WSN. That is very important for navigation and localization of lunar vehicles and long-term detection.
The Impact of Lunar Surface Environment Upon Sensor Node and Corresponding Solutions
Because the lunar surface environment is very different from the Earth’s environment, we have to consider more factors when we design sensor node which is applicable to lunar surface environment.
Buffer and Anti-buried Module
Most of the surface of the Moon is covered by a layer of dust called lunar soil. Lunar soil has a loose structure, mainly includes gravel, powder, breccias, and the impact melted glass. The separation characteristic of lunar soil is generally poor. Its granularity is similar to sand. Most of the particles are smaller than 1 mm in diameter, mainly from 30µm to 1mm. This is to say that nearly half of particles are beyond the human eye’s ability to distinguish. The thickness of lunar soil is different in different places, thin places only a few centimeters, but some thick places up to 5～6 m. In principle, the design of the sensor node requires that the volume and quality should be as small as possible. According to the situation of the surface of the Moon, the area of the sensor node should be big enough to avoid being buried. According to the comprehensive test of the physical and mechanical properties of lunar soil, the result showed that most of the surface of the Moon can withstand 18～27 Kpa. When we design the sensor node, we should consider the reference data above.
Because sensor nodes are thrown out of the lunar rover, we need to add cushioning material to the sensor node to prevent damage by the impact. Its characteristics must include small density, thin thickness, smooth surface, high flexibility, high toughness, good insulation, and so on.
Temperature Control Module
The average temperature on the surface of the Moon (at the equator and mid-latitudes) varies from -298 degrees Fahrenheit (-183 degrees Celsius), at night, to 224 degrees Fahrenheit (106 degrees Celsius) during the day. With no significant atmosphere to block some of the Sun’s rays or to help trap heat at night, its temperature varies greatly between day and night. A temperature control module must be needed to protect the sensor node from bad temperatures. In order to guarantee the normal operation of sensor nodes, researchers mainly used a combination of active and passive thermal control programs. A vacuum multilayer insulation (MLI) structure is used to passively control the temperature variation. Electric heaters and heat pipes are used to increase the temperature actively if needed. To protect antennae from a bad environment, we can refer to the microwave antennae thermal control program of communication technology satellite (CTS) designed by the USA and Canada. Radome coated with thermal control layer can prevent it from sunlight penetrating, but allow electromagnetic waveband of communication to go through the layer. Thermal control coatings used a combination of black and white paint, in order to meet the requirement of the specific absorption rate and radiation rate. Radome was made by polyimide film material into the shape of film, and the aim of film-shaped is to reduce the absorption of communication signals.
The Moon suffers from three kinds of radiation, solar wind, solar flare, and cosmic rays. Apart from electromagnetic radiation, UV radiation can not be ignored. The solar flare creates a safety issue for astronauts. Detection needs to be reported to astronauts quickly so they can take shelter. In the space environment, solar ultraviolet radiation has a strong impact on metals, ceramic, glass material, and polymer, so that the sensor node can’t work well or life is very short. Therefore we must consider radiation protection to improve the reliability of the sensor node. To prevent electromagnetic radiation, we can use an aluminum electromagnetic Bi-screen to make the internal device free of the impact of electromagnetic fields. Aluminum density is relatively small, so this can reduce the quality of sensor nodes. To prevent ultraviolet radiation, we often add absorption coating at a low rate.
There is almost no air on the surface of the Moon, and air pressure is 1.3×10-14 Pa or 3.10^-15 bar (1 bar on the Earth). Sensor nodes bear an atmospheric pressure on the Earth, but the surface of the Moon is an almost close vacuum. This environmental change makes the structure would bear the additional 0.1 Mpa pressure. In order to resolve this problem, if the devices need to be sealed, we have to make sure that the seal structure should be able to bear the additional 0.1 Mpa pressure. If the devices do not need a sealed environment, we may dig a hole in its seal structure so as not to damage the package.
Radio Frequency (RF) Environment for Lunar Application
It is expected that the communication would be better on the Moon as compared to that on the Earth, due to the absence of atmosphere on the Moon. The lunar side is unique in the inner solar system. It is free from human-made radio frequency interference (RFI) over much of its surface.
For communication on the Earth, there can be factors like steady and moving reflectors and scatterers, atmospheric absorption, etc, and can lead to multipath components at the receiver. Due to multipath components, signal strength can vary at the receiver due to moving scatterers. However, in the case of lunar application, there are no moving objects on the surface and therefore there can be multipath signals at the receiver due to surface topography, but received signal strength is not expected to vary randomly due to steady terrain, but it can show periodic variations due to signals traveling in different time durations. Also, there is no atmosphere on the Moon and therefore there are no atmospheric losses, which are present on the Earth. The main possibility of signal getting affected is that of the surface reflections due to uneven terrain structure. This can cause multipath propagation and signal reaching at the receiver by direct path will be modified due to multipath components. The expected number of multipath components is few as the mission lander should land on a comparatively plane or smooth surface, where a lunar vehicle can move easily. The wavelength of the signal is 12.5 cm at 2.4 GHz wireless operation and objects should be of larger size to cause the reflection.
On the lunar surface, it is likely to have such obstacles in between transmitter and receiver, which may be considered only for direct path but the deployment is supposed to be in almost plane terrain for smooth movement of the rover and hence the possibility of occurrence of such loss is rare and may be neglected.
On the lunar dayside, photoelectrons are quasi-constantly emitted from the Moon’s surface and this electron flux acts to typically charge the dayside lunar surface a few volts positive. In arriving at an equilibrium surface potential, the surface will charge to balance the two primary currents: the outgoing photoelectron flux, Jp, against the incoming solar wind electron thermal flux, Je. In nominal solar wind conditions, Jp > Je and the surface charge positive, trapping most of the photoelectrons. The photoelectrons are related to surface charging, as well as the charging of exploration vehicles on the lunar surface. They also play a role in the mobility of lunar dust particles, which consequently varies based on the solar wind conditions. Therefore, the best possible understanding of the photoelectron sheath is important for future exploration of the lunar surface.
The lunar/remote planetary surface network would include nodes with limited, albeit renewable, energy source. The same holds true for the satellites in orbit. We envision that the planetary network would have a large number of sensor nodes with constrained energy. The design and implementation of the communication network and protocols should take into account this energy limitation – the protocols should not be such that they deplete the node energy before it can be renewed. The energy of each sensor node must be renewable, based on solar sources.
Constraints of the Network Architecture
The sensor nodes in the lunar network have finite energy, even if the energy is renewable. The ad hoc routing path between a sensor node and its base station might not be available if an intermediate sensor node’s energy is depleted. Also, the path might go through a sensor node with a critical function (for example, it being the only sensor in a location observing a particular phenomenon). The lifetime of a critical node should be maximized; hence it should be avoided as a routing node if possible. Therefore ad hoc routing protocols in the sensor network should have multiple routes, and the routing parameters should include the “importance” of a sensor node in the network and the amount of energy available to each sensor node. The mobile base station might not reach certain clusters due to various circumstances, and therefore data from the nodes in these clusters cannot be collected in time. Therefore the sensor nodes should have sufficient storage to cache previously collected data for certain time periods beyond the normal collection time, if necessary. Also, contingency measures to collect data from the sensor nodes should be there, if the mobile base station fails. Similarly, to avoid data loss in case of interruption in connectivity between the gateways on the lunar network and the satellite constellation, or between the constellation and the terrestrial gateways, both the lunar gateways and the satellites should have the provision for data caching. A critical requirement for the space mission is to ensure that communication between the command center on Earth and the planetary surface network is always available, in extreme cases where this link breaks, the lunar services must continue its work irrespective of this link breakage, and also that the planetary surface network performs its functions correctly. Therefore, the network should be designed with the following requirements in mind:
• the network should be robust,
• additions/modifications to the functionalities of the network components on the remote planetary surface should be possible after deployment, and
• the command and data traffic is secure.
Security is an important issue, security agencies do not want other countries monitoring everything that happens on the moon. Also, it needs to handle malicious actors. High degree of trust and integrity is required when it comes to the data traffic security feature.
Lunar Communication Terminals Architecture
This section describes the technological elements of lunar surface segment of the overall architecture. The overall lunar architecture includes other elements such as the habitat, LRS, and pressurized rovers. The Lunar Communication Terminals, whether small as for a rover or large as for a main gateway, are comprised of various common items that differ only in transmit power and data capacity. The Lunar communication architecture contains a surface radio, lunar space gateway radios as needed for the time frame being implemented, a router, data storage, tracking hardware, and avionics. The small components will contain ultra-stable oscillators while the large will carry an atomic clock to establish local time and to facilitate time transfer between the lunar surface and Earth. Also, NASA plans to have a proper satellite positioning system for the Moon and China also is working closely. This satellite system will be capable of providing a combination of three standard service types: networking; positioning, navigation, and timing (PNT). The large Lunar Communication Terminals, if not integrated with the habitat, will use a fiber optic link laid with the power cable to establish a high 100+ Mbps link to the habitat. The size of the antennas on the various communication terminal types will differ depending on the trades between transmit power, mass, antenna gain and complexity, and cost. The small communication terminals will use dish antennas as small as 0.3 m in diameter for the gateway applications and a dipole antenna for the surface links. The large terminals with a 1-meter diameter dish antenna for the gateway and 15 dBi sector antennas for the surface-to-surface links.
• Delay Tolerant Networking (DTN): Need a large number of space systems to automatically intercommunicate using techniques (based on IPv6) that broadly parallel those used in the terrestrial Internet, but in a space communication large delay environment. Need disruption tolerant networking techniques to successfully bridge islands of connectivity (i.e., lunar network and terrestrial network). NASA is working on LunaNet that is a communication and navigation system for the Moon, it will leverage Delay Tolerant Networking (DTN) Bundle Protocol (BP) as the principal internetworking protocol. Although similar in foundation to the traditional Internet Protocol (IP), DTN BP will allow end-to-end networking, even under circumstances that IP will not, such as disconnections and/or delays anywhere along the complete end-to-end path.
• Demand Assigned Multiple Access (DAMA) and Quality of Service (QoS): Need QoS protocols to support traffic prioritization and DAMA schemes to enable auto-discovery of the network load for self-managed, autonomously reconfiguring networks.
• High-Performance Router: Need to support multiple 100 Mbps network interfaces to terminate high-speed RF links (backbone and proximity). Need to support processing/routing at 350-500 Mbps for proximity and backbone links.
• Multi-beam High-gain Antenna: Need a lightweight two-beam S-/Ka-band (2 GHz/40 GHz) High Gain Antenna (HGA) to enable communications with both a habitat and rover separated by 250 km distances (Non-LOS) via a relay orbiter.
• Surface Wireless Network: Need to support >15 simultaneous users with aggregate bandwidth of 80 Mbps at ranges of 6-10 km and data rates from 16 kbps to 20 Mbps. Convert conventional IP stacks to Space Network (SN) and Command, Control, Communications, and Information (C3I architecture) stacks. Support time synchronization service to all surface elements.
• Surface Network Radios: Need IP-based radios to link humans, robots, habitat, power stations, ISRUs, rovers, and science packages together for line-of-sight applications. Support surface mesh networking using 802.16 protocols. User network radios need to have MAC layer protocol support for both SN signaling at high rate Ka-band and 802.16 protocols on the lunar surface to the LRS. Radios need to support one and possibly two-way radiometric tracking.
• High Data Rate Modem: Need to provide throughput of 100 Mbps interfacing to SN signaling side using Quadrature Phase Shift Keying (QPSK) and down-conversion to Intermediate Frequency (IF).
Integrated Wireless Technology
• RFID Development: Need space-qualified interrogator that reads RFID tags for inventory management and also reads passive wireless sensors. The fundamental physics of interrogation are very similar, so SDR for interrogator should reduce cost and permit re-use. Need interoperability of spectrum with international and commercial partners, as well as inventory management commonality between multiple Constellation Program elements.
• Miniaturized EVA Radios: Need a miniaturized lunar EVA suit radio that integrates an 802.16e WLAN radio and S-band voice/navigation radio. The EVA suit radio must fit within a difficult-to-achieve two-pound weight limit for the radios, avionics, radiation protection, and cooling. Need to provide two-way navigation to enable relay of crew position back through the voice channel. Need a new antenna system, combining the S-band and 802.16e dipole antennas which are comparable in wavelength.
Countries Actively Participating in Lunar Missions
Over the next few years, we’ll see the launch of more than a dozen different lunar missions. Some will just orbit or loop around the Moon, some will land, some will deploy rovers and other robots for deeper exploration. Many are tasked with prospecting for water ice and other resources. And all of these missions are just a prelude to a permanent human presence on the Moon and establishing a pit stop for traveling to Mars.
- NASA: Artemis 1, November 2021
- China: Chang’e 5 and 6, Late 2020 and 2023
- India: Chandrayaan-3, 2021
- Russia: Luna 25, 26, and 27; July 2021, 2024, and 2025
- Australia: Australian Space Agency to jointly work on NASA Artemis program for 2024
- Astrobotic and Intuitive Machines: CLPS 1 and 2, July 2021
- Japanese company ispace and Draper Lab: Hakuto-R Mission 1 and 2, October 2021 and 2023
- PTS: ALINA, 2021 German company Planetary Transportation Systems (PTS)
- Japan: SLIM, January 2022
- South Korea: KPLO, July 2022
- NASA: VIPER, 2022
- NASA: Artemis 2, Late 2022
- SpaceX: #dearMoon project, 2023
Lunar Stationary Orbital Communications Solutions
The term stationary orbit refers to an orbit around a planet or moon where the orbiting satellite or spacecraft remains orbiting over the same spot on the surface. From the ground, the satellite would appear to be standing still, hovering above the surface in the same spot, day after day.
In practice, this is accomplished by matching the rotation of the surface below, by reaching a particular altitude where the orbital speed almost matches the rotation below, in an equatorial orbit. As the speed decreases slowly, then an additional boost would be needed to increase the speed back to a matching speed, or a retro-rocket could be fired to slow the speed when too fast.
It takes the Moon about 27.3 days to rotate once on its axis. So, a lunar equivalent of a “geostationary” orbit would have a period of 27.3 days.
The gravitational constant here is (6.67408 × 10-11 m3 kg-1 s-2) and the mass of the Moon is (7.34767 × 1022 kg). The orbital period (27.3 days equals 2360620 seconds).
Putting these values in the formula we get a radius of 88,441 km. That would be the “geostationary” orbit for the Moon. There’s just one small complication. 88,441 km is outside of the Moon’s Hill sphere. A Hill sphere, or Roche sphere, is the region around a body in which the body is the dominant gravitational influence. The Moon’s Hill sphere has a radius of 66,100 km.
That means that a satellite that attempted to orbit the Moon at a radius of 88,441 km would find itself pulled away from the Moon’s orbit by the Earth.
In this article, I have discussed in detail the topic of lunar communication systems, particularly M2M communication. I have discussed the communication requirements for such a mission, highlighted some important issues related to the design of the communication network that are unique to the space environment, and analyzed a few important constraints of the mission. I have also discussed communication in different scenarios like in the lunar caves. The discussion also contains the list of upcoming missions that will use such technologies.
The article also contains information about the influence of the environment on radio communication, how reflection, diffraction, and electronic flux affect the environment. Along with the suitable frequency bands, which feels to be the one for WLAN. I included the wireless sensor networks architecture on the Moon and hardware requirements, plus the power issues. I also discussed some network constraints in this work and the possibility of the geostationary orbit of the Moon. This article provides good information on the subject from different angles.