2.1

Future lunar settlements

The incoming new Moon exploration era will be driven by both Agencies and private sector.

In particular, the following organizations are currently taking care of the Moon exploration future plans:

The future Lunar settlement, according to [1], is composed by the Lunar Orbiting Platform-Gateway (LOP-G), plus an unprecedented number of lunar missions foreseen for the next decade. This includes institutional missions, but also commercial ones – this being a new trend: in total more than 40 missions, approximately 80 space vehicles planned by or involving 10 space agencies, plus several endeavors by private sectors, e.g., Moon Express, Astrobotic, and Blue Origin).

Such missions will foresee:

A brief description of the most relevant ones is reported here below.

The Lunar Orbiting Platform-Gateway (LOP-G), also simply named Lunar Gateway, is an in-development space station in lunar orbit intended to serve as a solar-powered communication hub, science laboratory, short-term habitation module, and storage for rovers and other robots. It is an international program, including participation of Canada (advanced external robotics), Japan (habitation components and logistics resupply), ESA (providing the International Habitat (I-Hab), and the European System Providing Refueling Infrastructure and Telecommunications (ESPRIT), both of which will dramatically enhance the capabilities of Gateway, contributing to sustainable operations while paving the way for a future human mission to Mars), Russia (airlock).

Figure 1:

Lunar Gateway concept

The Gateway Communication System specification currently includes ([5]) S- and Ka- bands Radio Frequency (RF) communication interfaces which cover links with Earth, visiting vehicles, local users (EVAs) and Moon users, but OCRSG proposes also optical communications for all of these scenarios (also under study at NASA side with Lemnos).

Artemis is the name of NASA’s program to return astronauts to the lunar surface.

The Artemis program foresees a sustained long-term presence on the lunar surface to use the Moon to validate deep space systems and operations before embarking on the much farther voyage to Mars. The Artemis program will also enable commercial opportunities on the lunar surface.

Artemis’ final goal is to build the Artemis Base Camp at the South Pole of the Moon, depicted in Figure 2.

Figure 2:

An artist’s impression of what a lunar base could look like

In addition, NASA is also foreseeing a dedicated communication network around the Moon, named LunaNet.

ESA is working on plans for a European Large Logistic Lander to provide different types of uncrewed missions, from supply runs for Artemis astronauts, to stand-alone robotic science and technology demonstration missions and even (still under discussion) a lunar return mission to bring samples to laboratories on Earth.

This European Large Logistic Lander (shortly EL3, [6]) is aimed at being an intermediate vehicle with size (8/9 tonnes) between the small landers being developed for science payloads (e.g. NASA CLPS, Roscosmos Luna-27, JAXA Smart Lander for Investigating the Moon- SLIM) and the large human lander employed for the Artemis program.

At this stage, EL3 relies on the Gateway for relay communications in S band and Ka band, having also a direct to Earth X-band capability.

ESA’s Moonlight initiative aims at providing commercial communications and navigation services to future lunar missions. The Initiative encompasses two steps:

- Commercial services (CommStar)

Currently, space communications have been primarily the responsibility of government owned, operated communications networks and managers. Commercial companies’ participation as vendors of communications equipment, facilities, or as a network manager of hardware and software infrastructure, will accelerate Agencies’ goals of commercialization.

CommStar Space Communications™ LLC, (“CommStar Space”) seeks to lead that transition to commercial space communications and is planning a communication network to support the future lunar explorations. This network will be a “system of systems”, providing an “end-to-end” data communications service between the Earth and the Moon. This objective requires access to a significant infrastructure, not only in space, but also across the Earth.

This design will start from the deployment of an advanced, proprietary data relay satellite (“CommStar-1”), under development to be located between the Earth and the Moon, which will not act purely as a data relay satellite, but it will also process the data in order to enhance the signal quality and provide robust and reliable communications. CommStar-1 relay infrastructure will be designed as a hybrid system for both radio frequency and optical (laser) communications.

The above-described lunar infrastructure will also necessarily foresee an Earth-based support, i.e.:

they are not in the perimeter of OCRSG study, but mentioned because of paramount importance, also during the initial design phases, because their performance and availability directly impacts the sizing and design of the space segment.

2.2

Communications’ scenarios and regulations

The foreseen lunar communications’ infrastructure from [1] is shown here below in Figure 3.

Figure 3:

Lunar Communications architecture and Interfaces from [1]

This architecture is based on three networks:

Such systems will involve a large set of terminals, located on different crafts or assets and with different needs and performances.

Concerning Moon orbiters and spacecrafts, we can see the Gateway and other orbiters as part of a relay satellite network (DTN, or other networks which can include also Cubesats in low orbit). Those nodes are inter-connected through ISL (inter-satellite links), and at the same time they will communicate also with the lunar surface (Proximity link).

On the lunar surface, on the other hand, there can be fixed (ground stations, both optical and RF, and the habitat modules) or mobile assets (rovers, astronauts in EVA but also portable communication stations), all interconnected. Of course, communication support will have to be tailored differently for the two types of users.

The possibilities for each asset to communicate with the desired other asset directly or relaying through other nodes depends on the subsystem constraints, on the visibility and on the networks’ management.

As an example, a Lunar habitat can communicate with Earth via a DTE (direct to Earth) link or via a Moon Station as a relay, and in space this link can be either direct or through a proximity link.

A relevant effort was put into the extensive analysis of the above wide set of use cases. Indeed, the optical terminal design is heavily impacted by constraints such as the needed signal power (linked to distance), Field of Regard and environment. Thus, a compromise between an all-tailored design (i.e. one terminal per use case), which would have allowed the best optimization but was not possible at this stage of the study due to the lack of input mission analyses, and an all-purpose design, which would have led to an inefficient over-design, has been proposed ending up with a limited set of terminals covering all needs with the maximum exploitation of state-of-the-art technologies (sec.4) as building blocks.

In particular, the studied type of links are:

From the scenarios gathered above, an outlook of a potential future optical communication system covering the needs of the future Lunar exploration activities can be drawn and is shown in Figure 4.

Figure 4:

pictorial view of lunar, cis-lunar and Moon-to-Earth optical links.

3.1

RF vs optical comms

To meet the demands of high-definition video and data-intensive scientific research, the radio bands traditionally allocated for space research are showing their limits. For example, the Orion spacecraft will transmit mission-critical information to Earth via an S-band radio at 2 megabits per second. Barely 1 Mb/s will be allocated for streaming video from the mission. That’s about one-fifth the speed needed to stream a high-definition movie on Earth.

To boost data rates even higher means moving beyond radio and developing optical communications systems that use lasers to beam data across space. Laser communications systems will allow transmission from the Moon of ultrahighdefinition 4K video back to Earth. Moreover, robust optical communications will allow future missions to receive software updates in minutes, not days. The scientific community will have access to an unprecedented flow of data between Earth and the moon.

In the next Table, a (not exhaustive) list of aspects showing the main differences between RF and optical links is presented.

Table 1:

Key differences between RF and Optical links

3.2

RF survey and analyses

During OCRSG study, a technological survey and a link budget analysis have been carried out in order to give a complete overview of the achievable performances with «traditional» RF communications (compatible with the indications reported in the previous section) w.r.t. optical links. The main driver of this state-of-the-art survey was on RF solutions which can compete to provide links on the Moon at 2.1 Gbps or more.

The survey for the spacecraft subsystem has been conducted to find units which should:

Table 2 summarizes the main outcomes.

Table 2:

RF state-of-the-art survey for the spacecraft subsystem

Some considerations have been also collected concerning the Ground Stations: even if they are not in the scope of this study, they have to be properly addressed as input for the link budgets.

The Ground Stations we are looking for must:

Furthermore, a nice-to-have aspect for Ka-Ground Stations would be a favorable location from the climatic point of view. Dry conditions would be preferable in order to be able count on few moisture/rain/fog/snow effects.

ESA ESTRACK network provides three Ground Stations which are “nearly” compliant to the study needs. Cebreros and for Malargue GS in particular implement OQPSK modulation.

Concerning frequencies, there is no complete compliance instead. Indeed, the Ka-band is covered by Cebreros-1, Malargue-1, New Norcia and Redu-2, but with different frequencies with respect to the ones assigned to the Moon missions (22.55-23.15 GHz uplink, 25.5-27.0 GHz downlink).

According to ESOC (informally), the Moon Ka frequencies are in program to be implemented in the ESTRACK network. An extrapolation from the above data leads to the following indicative performances for the future developments for a 15 m antenna: G/T of 46-48 dB/K, EIRP in the range 86-92 dBW assuming a 50 W – 200 W amplifier. These input has been considered as baseline in the link budgets.

The NASA DSN (Block V) receiver does not currently include GMSK, however it is able to cross -support GMSK modulation by use of the O-QPSK receiver implementation, at expenses of additional 0.7 dB to 1.0 dB demodulation mismatch losses. Concerning frequencies, there is again no complete compliance instead. Indeed, the Ka-band is covered, but, for the uplink, with different frequencies with respect to the ones assigned to the Moon missions (22.55-23.15 GHz uplink, 25.5-27.0 GHz downlink). The Ka-band receive capability in this band exists at Goldstone (DSS -24 and -26), Canberra (DSS-34, -36), and Madrid (DSS-54 and -56). Goldstone DSS-25 has also Ka-band uplink (but on the “wrong” frequency) at 300 W and, together with Canberra (DSS-35) and Madrid (DSS-55) it will receive 800 W Ka-band uplink before the end of 2024.

Besides ESTRACK and DSS networks, also the following companies, potentially compliant with the above requirements, have been found: Goonhilly (UK, Australia) and Astro Digital (USA). But again, frequency compliance is still not ensured.

Here below, the results of the link budgets based on the above input are summarized. They show that the requested performance is at the edge of current possibilities, again evidencing the advantage of optical communications in this scenario.

The requested data rate of 2.1 Gbps is:

3.3

Optical survey and analyses

Figure 5 illustrates the simplified block diagram of a FSO (Free Space Optical) communication system for space applications. The modulated optical signal is collimated and transmitted by means of a telescope and, after the propagation through the atmosphere or the vacuum in space, it will be collected by another telescope and focused in to a small spot in a focal plane or directly coupled into an optical fiber, where a photodetector will transform it into an electrical one, which will be decoded to extract the original information.

Figure 5:

Simplified block diagram of the FSO communication system, with a propagation through turbulent channel

The modulated optical signal is collimated and transmitted by means of a telescope and, after the propagation through the atmosphere or the vacuum in space, it will be collected by another telescope and focused in a small spot in a focal plane or directly coupled into an optical fiber, where a photodetector will transform it into an electrical one, which will be decoded to extract the original information.

The main components at the transmitter side are:

The main components at the receiver side are:

Three main factors affecting the link budgets’ result are derived from the propagation of the signal through the medium between the transmitter and the receiver:

Still regarding the propagation of the optical signal, but besides the pure free space losses and atmospheric effects, other two factors are accounted for in the link budget:

All the factors introduced so far concur in the optical link budget analysis as:

Optical link budgets have been computed for the following scenarios:

Table 3:

optical link budget cases and input and considering the following atmospheric conditions:

Table 4:

considered optical atmospheric parameters

The following scenarios have been analysed:

Table 5:

optical simulation cases for each scenario

For each scenario, the link budgets have been computed starting from the assumptions presented in the Table and analyzing the absorption, the scattering, the free space losses, the scintillation index, the arriving spot radius, the turbulence losses and the corrected Zernike modes as a function of the elevation angle.

An example for Daytime DTE downlink is shown here below (Figure 6), to present the adopted methodology. The computation shows the resulting PPB number for each elevation angle in different cases concerning the TX telescope diameter.

Figure 6:

Daytime DTE downlink with D_RX=40 cm. Arriving PPB vs the elevation angle for single aperture transmitter telescope, having dimension D_TX=[5;13;20]cm. The minimum PPB required for OOK, DPSK, PPM modulation schemes are also reported.

The agreement between the PPB results and the dashed thresholds for each modulation scheme gives as a result the feasibility of that combination of variables to close the link. From this feasibility, a baseline design for each case is derived. For example for this plot, a 5 cm transmitter telescope is not sufficient to achieve the minimum required PPB (equal to 5 considering a PPM modulation scheme). The 13 cm transmitter telescope is quite better for a PPM signal, but for a DPSK signal we can’t close the link for elevation angle below 40° with the 3 dB margin considered. The OOK is not an option for this small transmitter aperture. Among the considered ones and in daytime condition, the 20 cm diameter transmitter telescope is the only possible choice to establish a communication link at 2 Gbps from the Moon to the Earth using a 40 cm diameter single aperture receiver telescope. In this case the best modulation scheme is the PPM, so that we can ensure a link margin to increase the robustness and the reliability for each elevation angle.

The optical links budgets’ results for all the combinations shown in Table 5 are reported in Table 6.

It is worth noting some key facts:

Table 6:

synthetic summary of the optical link budget results. In red we show the links that cannot be closed under present assumptions. In orange we indicate those that are partially achievable (e.g. under limited range of elevation angle) or that could be attained only by high-speed PPM (not yet available). The other cells have a light-green background, to indicate that these configurations are achievable under the assumptions we made in this document. In the bottom Tables, the sizing of the low rate proximity is also reported.

According to the above results, the following sizing is baselined (see Table 7, Table 8, Table 9).

Table 7:

DTE terminals baseline design. Column “DTE Earth terminal” is related to the terminals based on Earth (not under OCRSG design, but recalled here for reference on the considered assumptions) i.e. either the Optical Ground Station (OGS) or the GEO Optical Terminal (OT); column “DTE Moon terminal (either on Moon surface or in Moon orbit)” is instead related to the space terminals (objective of OCRSG design).

Table 8:

High rate Proximity terminals baseline design. Column “Orbiter” is related to the terminals in orbit around the Moon; column “Moon Surface” is instead related to the landed or surface terminals.

Table 9:

ISL terminals baseline design. Same design is assumed for all the terminals, even if on different orbits and space crafts.

A technical specification was derived from the above Tables; in the next section the derived design is reported.

4.1

Use cases’ analysis

Within each one of the mentioned type of links, several (11 in total) use cases have been studied, as far as the (limited) knowledge of the involved orbits and mission analysis allowed. The study is too extensive to be completely reported here, however some examples are shown here below in Figure 7 and Figure 8.

Figure 7:

Direct to Earth links (DTE) and, below, an example of FoR study for DTE Link 1 - LCS OT, where it is shown that from the simple trigonometry applied, the LCS OT needs a very small FoR to cover the full Earth disk, due to the large distance between them. This amounts to approximately 2° in total (± 1°).

Figure 8:

Capability 3 - Lunar Cross Links (OISL): Due to lack of knowledge of the lunar orbits exact placement, the safest approach dictates a fully hemispherical LORB OT for the ISL application.

Indeed, the relative motion of the two terminals in an optical link determines the relevant FoR requirements, which directly drive the selection of a suitable pointing mechanism per use case. These requirements are added to the sizing constraints already shown in Table 7, Table 8, Table 9, which feed the optimization process that leads to the final definition of the OTs’ design. After merging as many use cases requirements as possible, the minimum amount of different space terminals (6) is given, in order to meet all the use cases.

This is shown in Table 10. EGS terminals are not included in this list, since their design is not in the scope of this activity.

Table 10:

OT variant list & main requirements

4.2

OT Variants Conceptual Design

In Figure 9, the detail on a single, generic optical terminal is shown.

Figure 9:

A generic Optical Terminal diagram

The 6 variants proposed in Table 10 follow the above architecture, mainly differentiating one from the other for what concerns the OHU (including the telescope and the CPM (Coarse Pointing Mechanism)) and LMU (Laser Modem Unit also covering power boosting, modulation and demodulation and coding).

The conceptual design was mainly focused on evidencing, for each variant, the estimated power consumption and mass, as well as any other relevant property from the preliminary conceptual design derived from the state-of-the-art market research (addressed in sec.5). The OHU power and mass estimations contain the following components: CPM, OH Structure, OH Electronics, Optical Bench, Telescope & MLI. The LMU power and mass estimations contain the following components: LMU structure, HPA, and the rest of the LMU electronics.

Table 11:

OT variant s’ mass and power budget.

A major factor for the LMU design is dissipation of the DCDC converter and HPA thermal losses. This aspect impacts directly the mass of the total LMU structure. A rough calculation shows that in order to successfully dissipate 315 W of thermal losses, additional HW with a mass of 9.5 kg is needed, together with a radiative area of 1.3 m2 at platform level. The current OT conceptual design is not taking into account redundancy in any of the subsystems. Combination with the high reliability requirements (99.9%), yields to an additional total mass of 5.1 kg (again for the LMU).