Cislunar Positioning,Navigation,and Timing: International Relations and Policy Implications

Abstract

Across the globe, government, military, and commercial interests are rapidly expanding into the Cislunar service volume (CSV). As missions into Cislunar space become increasingly common over the next decade, the need for reliable positioning, navigation, and timing (PNT) infrastructure in the CSV is frequently discussed on the international stage, for which some states have started to propose policy frameworks and technical solutions. NASA's proposed Artemis series of missions may be the most publicly well-developed plan for Cislunar exploration and development, including plans for a Lunar Gateway space station as well as a permanent research station on the lunar surface. PNT systems allow a spacecraft to accurately determine its location and orientation, calculate trajectory corrections, and maintain precise clock time. On Earth's surface and within the geostationary orbit, PNT is commonly achieved via Global Navigation Satellite Systems such as GPS or GLONASS. Currently, there is no standardized PNT system for spacecraft operating in Cislunar space; the United States and other nations have expressed interest in developing this technology to facilitate future space missions. Possible architectures for a Cislunar PNT system include an expanded or augmented system of Earth-orbiting navigation satellites, a network of lunar-orbiting satellites, Earth- or Moon-based surface beacons, and combinations of the above. Technical efforts to develop these systems are ongoing. However, the international political landscape that underpins these activities remains fragmented. In this study, we examine key stakeholders in the creation of a Cislunar PNT system. We also identify and analyze the critical international debates and policy issues that will be relevant in the development of this infrastructure, such as military versus civil involvement, signal characteristics and requirements, possible restrictions on who can access the signals, varying levels of international cooperation, and discussions around system interoperability to offer insights that can help guide ongoing policy development in this area.

INTRODUCTION

Between the edge of Earth's geostationary orbit (GEO) and where the Moon's gravitational influence ends is a region called Cislunar space, encompassing a volume roughly 1,700 times greater than that within the GEO.¹ This region includes the lunar surface, lunar orbits, Earth-Moon transfer orbits, highly elliptical Earth orbits, and the Earth-Moon Lagrange points. As spaceflight for government, military, and commercial purposes becomes increasingly common over the next decade, a dramatic increase in missions to Cislunar space is expected; over 40 such planned and funded missions are currently under development, with scheduled launch dates ranging from 2022 to 2029. These lunar missions hail from a diverse body of spacefaring nations, including the United States, Russia, South Korea, the United Arab Emirates, India, Turkey, the United Kingdom, and China, among others. At least 5 of these missions plan to carry crews of astronauts into Cislunar space.²

Positioning, navigation, and timing (PNT) refers to a spacecraft's ability to accurately determine its location and orientation, apply corrections to its course, orientation, and speed to maintain its desired trajectory, and acquire and maintain precise time. On Earth's surface and within GEO, PNT is commonly achieved via Global Navigation Satellite Systems (GNSS) such as GPS, provided by the United States, or GLONASS, provided by Russia. Beyond GEO, lunar and interplanetary missions have typically relied on the use of radio or optical links to other spacecraft or ground antennas (such as NASA's Deep Space Network) to determine range and trajectory, combined with optical navigation for finer adjustments as the spacecraft approaches a celestial body.³

A dedicated Cislunar PNT system would offer finer timing precision, enhanced positional accuracy, greater spacecraft autonomy, and expanded capacity over existing navigation methods. This could be achieved by an expanded or augmented system of Earth-orbiting navigation satellites, a network of lunar-orbiting satellites, placement of navigation satellites in transfer orbits, Earth- or Moon-based surface beacons, or any combination of the above.⁴

Creating a Cislunar PNT system raises several interesting international relations and policy questions. For instance, if multiple countries develop their own Cislunar PNT systems, will they use the same standards and coordinate systems so as to be interoperable for all users? How would interoperability, or lack thereof, affect a country's national security and robustness of space access? Is there an urgency related to some advantage of being the first nation to create such a system? What will be the roles of civil, military, and commercial entities in the creation and operation of the system? Will PNT signals be restricted or provided openly as a public utility?

This article examines the ongoing debates that surround these issues in the Key Issues and Debates section, following a review of major global stakeholders in the Major Stakeholders section, and a look at the current policy landscape and international cooperation efforts in the Current International Legal and Policy Landscape section and the International Coordination section, respectively.

MAJOR STAKEHOLDERS

Several entities have discussed plans for the development of Cislunar PNT systems. However, most of these discussions are in the early stages of planning and proposal, with many of the technical and policy details remaining to be resolved. The activities and statements made by key stakeholders with an interest in developing Cislunar PNT systems are summarized below.

The United States

The United States has been perhaps the most vocal proponent for the creation of a Cislunar PNT system. The National Cislunar Science & Technology Strategy, published by the Office of Science and Technology Policy in November 2022, explicitly includes the implementation of Cislunar communications and PNT capabilities as 1 of 4 key objectives to support U.S. leadership in Cislunar space and on the Moon, which the Office says will help enable a cooperative and sustainable ecosystem in Cislunar space.⁵ Early in 2022, NASA proposed an interoperability standard for Cislunar communications and navigation, known as LunaNet.⁶ The U.S. Air Force (USAF) has also awarded several contracts to private companies such as Masten Space Systems, Rhea Space Activity (RSA), and Xplore for the development of Cislunar PNT technologies.^7–9 Below is an overview of the U.S. activities on Cislunar PNT in the military, civil, and private sectors.

Military

U.S. military officials have spoken publicly about the need for the United States to extend its military space activities to Cislunar space. Speaking on the creation of the U.S. Space Force (USSF) in 2019, Maj. Gen. John Shaw, previously the Air Force Space Command Deputy, said that “its ultimate destiny is going to be providing security and projecting power through increasingly vast distances: geosynchronous to Cislunar and beyond.”¹⁰ During a virtual meeting of the Space Information Sharing and Analysis Center in 2021, Troy Shafford, senior intelligence officer for the Space and Counter-Space program at the Defense Intelligence Agency, spoke about requirements for USAF and USSF operations beyond the geosynchronous orbit (xGEO), saying, “there are three tiers of requirements before 2030. The main one is going to be xGEO space domain awareness. After that, communications and navigation.”¹¹

Furthermore, the USAF has provided funding to several private aerospace companies through the Air Force Research Laboratory (AFRL) Small Business Innovation Research (SBIR) program. The 2019 SBIR called for Cislunar space technologies, including PNT concepts, in its presolicitation notice; this action was seen by industry as a “turning point” and a sign that the Air Force and Space Force are committed to advancing U.S. capabilities in Cislunar space.¹²

One such company to receive funding was California-based Masten Space Systems, which was acquired by Astrobotic in 2022 and now operates as its Propulsion and Test Department.¹³ Masten was awarded a Phase II SBIR contract in 2021 through AFRL's AFWERX program to develop and demonstrate a lunar positioning and navigation system. The Masten design featured a network of PNT beacons to be deployed in shock-proof enclosures from a spacecraft in lunar orbit to descend and penetrate the lunar surface. Once deployed, these surface beacons would form a mesh network allowing consistent wireless connectivity to lunar spacecraft.

In a press release, Masten said its network would “enhance Cislunar security and awareness by enabling navigation and location tracking for spacecraft, assets, objects, and future astronauts on the lunar surface or in lunar orbit. As the lunar ecosystem grows, the network will also help advance lunar science and resource utilization by improving landing accuracy and hazard avoidance near critical lunar sites.”⁷ At the time of acquisition, Astrobotic stated it would continue to advance the Masten space technology portfolio.

AFRL also awarded a Phase II SBIR contract worth $697,000 to astrodynamics startup RSA for a Cislunar domain surveillance solution, with the USSF as the target end user.¹⁴ RSA intends to provide this capability as a constellation of self-guided cubeSats enabled by another RSA project, the Jervis Autonomy Module (JAM)¹⁴ satellite subcomponent that utilizes a propriety navigation algorithm. JAM was successfully demonstrated on NASA's Deep Impact mission in 2005,⁸ and determines a spacecraft's position based on stars, asteroids, other Cislunar spacecraft, and features on the lunar surface.¹⁴ This allows the spacecraft to navigate autonomously, eliminating the need for 2-way radio ranging with ground stations on Earth. Not only does this reduce the cost of navigation in both dollars and personnel, it also gives Cislunar spacecraft stealth capability by allowing them to operate in radio silence.¹⁴

RSA stated in a press release that “JAM will significantly decrease the cost, number of operators, navigators, and frequency of communications required to maneuver in Cislunar space and, over time, will provide a fundamentally different way to control all manner of spacecraft.”⁸

Finally, aerospace company Xplore also received a Phase II SBIR contract for a Cislunar PNT solution.⁹ Their proposed PNT solution will be enabled by a network of “Xcraft” spacecraft. Xcraft is a versatile, multimission spacecraft designed to handle a wide range of low-Earth orbit, Cislunar, or interplanetary missions.⁹ In a statement, Xplore Founder and Chief Operating Officer Lisa Rich said, “Xplore is developing a first-of-its-kind Cislunar architecture that ultimately allows for communication, data distribution and transportation beyond Earth orbit. With dozens of international, government and commercial Lunar missions planned over the next decade, Xplore's commercial PNT service will support this increased cadence of activity at the Moon.”⁹

Civil (NASA)

NASA plans to return humans to the moon in 2024 under its Artemis program.¹⁵ Along with international partners, NASA is also planning to build a permanent science station in Cislunar orbit called the “Lunar Gateway.” The Gateway is a centerpiece of the Artemis program that will enable transport of human crews to and from the moon, docking of the Orion spacecraft and lunar lander, deployment of science payloads, and CubeSats, communications, and refueling. The Artemis plan also calls for an Artemis base camp at the lunar south pole to support in situ resource utilization and longer human expeditions on the lunar surface.

The plan also states that once the core Artemis elements are operational, NASA should work in partnership with commercial providers and international space agencies on the creation of Cislunar infrastructures, including the LunaNet communications and PNT network. NASA envisions that LunaNet will provide communications, data, and PNT services to robotic landers, rovers, and astronauts on the Moon. LunaNet aims to be scalable and interoperable, so that as NASA and other international agencies progress with exploration of the Moon and solar system, the network may “grow in a manner analogous to the development of the internet on Earth.”¹⁵ NASA has said that LunaNet will enable “more precise surface operations and science than ever before.”¹⁵ LunaNet will also provide more services than simply PNT; the current proposal includes capabilities for lunar search-and-rescue activities as well as space weather alerts for personnel and equipment on the lunar surface.⁶

Private

To address the needs of military and civil Cislunar operations, private corporations will likely play a role in developing PNT solutions. NASA has expressed its intention to partner with commercial providers in the development and deployment of its LunaNet architecture. In its Artemis plan document, NASA states, “This flexible solution to enable exploration and science activities can be provided by a combination of NASA, commercial partners, international partners, and others.”¹⁵ Also, as noted above, the U.S. military has awarded several contracts to private companies for the development of Cislunar PNT solutions, including Masten Space Systems, RSA, and Xplore.

Europe

Similar to NASA, the European Space Agency (ESA) has also embraced the development of a Cislunar PNT system. In May of 2021, ESA released a short video and press release¹⁶ unveiling its Moonlight Initiative, a call to European space companies to put a constellation of communication and navigation satellites around the Moon. ESA states that such a system would facilitate future lunar missions by making individual missions more cost efficient, reducing design complexity, and freeing up payload space for more scientific instruments or other cargo. In a presentation at the United Nations International Committee on GNSS (ICG) in November of 2018, ESA also expressed plans to extend the space service volume of its Galileo network of satellites by working to develop and test a high-sensitivity space-borne receiver to enable the use of existing Earth GNSS services for Earth-Moon missions.¹⁷

In a November 2022 update to the ICG, ESA described a plan for developing lunar navigation services in 3 phases. First, in 2023, the high-sensitivity receivers will be deployed to leverage existing GNSS signals within the Cislunar service volume (CSV). Second, beginning in 2027, the ESA plans to establish a dedicated PNT constellation, Moonlight, to service the lunar south pole. Third, after completion of Moonlight in 2035, ESA will continue to enhance PNT services within the CSV, including additional PNT satellites and lunar surface beacons.¹⁸

Russia and China

Although there is little information regarding a Cislunar PNT system from either Russia or China, both nations do have plans to operate in Cislunar space in the near future, including a joint research station on the lunar surface.¹⁹ In the Artemis plan document released in 2020, NASA stated that Russia had expressed interest in cooperating on the Gateway project.¹⁵ However, later comments by Russia suggest that significant cooperation in this area is unlikely.²⁰

Other nations

Many other nations, including France, Germany, Italy, Luxembourg, Ukraine, the United Kingdom, Australia, India, Japan, South Korea, Thailand, Canada, Mexico, Israel, Turkey, and the United Arab Emirates, have expressed an interest in operating in Cislunar space, either individually or with international or commercial partners.² However, these countries have not directly addressed the issue of Cislunar PNT from a national perspective.

CURRENT INTERNATIONAL LEGAL AND POLICY LANDSCAPE

As Cislunar PNT is still a hypothetical technology and not currently in operation, there exists little policy or regulation that addresses Cislunar PNT specifically. However, some existing treaties and policy may be relevant as Cislunar PNT is developed.

The premiere international treaty presiding over activities in space is the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and Other Celestial Bodies,²¹ more commonly referred to as the Outer Space Treaty (OST). Article IX of the OST states that countries' operations in space must be “guided by the principle of cooperation and mutual assistance.” It also states that countries must undertake appropriate international consultations before proceeding with any activity which would cause “potentially harmful interference” with other countries' activities in outer space. These clauses suggest that countries developing Cislunar infrastructure should aim for interoperability and should make appropriate efforts to avoid causing interference with the systems of other nations, to include frequency spectra and orbital conjunctions.

Article IV of the OST explicitly states that the Moon and other celestial bodies may be used only for peaceful purposes.²¹ This may raise concerns regarding the establishment of PNT nodes on or around the Moon given the potential military applications of Cislunar PNT. Cislunar PNT systems that are funded, built, or operated either fully or, in part, by state militaries could raise questions about adherence to Article IV.

Explicitly grounded in the OST but developed independently outside the United Nations, the Artemis Accords have been signed by 28 nations and one territory as of September 2023.²² Although led by the United States, the Artemis program intends to embrace international partnerships, and the Accords lay out “principles for cooperation in the civil exploration and use of the moon … for peaceful purposes.”²³ Section Five of the Artemis Accords states, “The Signatories commit to use reasonable efforts to utilize current interoperability standards for space-based infrastructure, to establish such standards when current standards do not exist or are inadequate, and to follow such standards.”²⁴ Thus, the Accords expands on the OST, making explicit the responsibility to ensure space systems are interoperable. As part of the Artemis program, the Artemis Accords presumably applies directly to NASA's LunaNet project.

Certain orbital trajectories or areas of the lunar surface may be more desirable for establishing a PNT system, and access to these orbits or locations may be finite in capacity. Some nations are concerned that those who develop infrastructure early may monopolize access to these orbits. The current international regime does not provide clear guidance here. Article VIII of the OST declares that nations who launch objects into space retain ownership and jurisdiction of those objects in perpetuity. This would indicate that an entity that establishes PNT nodes (as either orbiting spacecraft or surface stations) would retain ownership of those spacecraft or stations.

However, Article II states that no country can claim ownership or sovereignty over any areas of outer space, the Moon, or other celestial bodies. Therefore, the physical spacecraft or surface station would be owned by its respective operator, but the orbit it is placed in, or the area of the Moon on which it is constructed, could not technically be claimed by the operator. This presents an ambiguous situation that may require a new regulation if areas of the lunar surface or key orbital positions become contested, whether for PNT or other uses.

Article IX of the OST establishes a “due regard” principle with respect to harmful interference by one state into “the corresponding interests of all other States Parties to the Treaty.”²¹ Under international law covering State Responsibility, a breach of Article IX's due regard principle would amount to a wrongful act, allowing those states affected to sue for recompense or other countermeasures.²⁵ With regard to evaluating and enforcing Article IX, in a 2018 statement to the United Nations, Russia alleges that “harmful interference” cannot be measured objectively, and specifically cites self-defense as a right they are entitled to enforce, even in outer space.²⁶

A concept of safety zones introduced by the Artemis Accords, Section 11, offers a way to address the ownership ambiguity and interference issues resulting from Articles II, VIII, and IX of the OST. By establishing a zone around orbiting spacecraft where others may not operate without causing unacceptable levels of interference, Cislunar satellite operators can gain somewhat exclusive access to a region without claiming ownership of it in violation of the OST. The Accords have not yet addressed limits on the size and function of safety zones other than “reasonable,” and countries that have not signed the Accords (e.g., Russia, China, and India) are in no way bound to respect such zones—the Accords were developed independently from the UN General Assembly—and the concept of safety zones as a whole has not been officially endorsed by any nations not yet included in the Artemis Accords.²⁴

INTERNATIONAL COORDINATION

The United Nations International Committee on GNSS (UN ICG) is an international body under the umbrella of the United Nations that meets annually to discuss terrestrial GNSS and encourage cooperation of global GNSS providers. At the ICG-12 meeting in Kyoto, Japan, held in December 2017, the ICG recommended that service providers, space agencies, and research institutions consider the full potential of GNSS services, including the ability of these systems to support “exploration activities in cis-Lunar space and beyond.”²⁷ In pursuit of this goal, the United States is leading a Cislunar GNSS subgroup with participation by Japan, Europe, China, and Russia. This subgroup allows members to share plans for Cislunar PNT development, as described in the section on Europe, above. The group has also collaborated on a document to help guide interoperability of existing GNSS signals in Cislunar space.²⁸

Another UN body, the International Telecommunications Union (ITU), resolved at the UN General Assembly in July 2022 to accept a recommendation by the Space Frequency Coordination Group (SFCG) that Cislunar communication and PNT satellites be restricted from using certain frequencies that cause harmful interference with radio astronomy observations in the shielded zone of the moon.²⁹ The SFCG is a “voluntary, informal group of frequency managers from all the main civilian Space Agencies of the world,” and meets yearly to agree on policies and regulatory issues affecting space systems and frequency bands.³⁰

Outside the UN system, the International Space Exploration Coordination Group (ISECG), another body created to enable international coordination on space issues, produces regular updates to its Global Exploration Roadmap, first developed in 2011. In October 2022, ISECG released a Lunar Surface Exploration Scenario that identifies PNT capabilities around the lunar surface as a requirement for successful, long-term exploration in Cislunar space. The report suggests that further study should be carried out by the International Operations Advisory Group (IOAG), which formed a Lunar Communications and Navigation Working Group (LCNWG) in 2022.³¹ In 2018, the IOAG also proposed frequency, modulation, ranging, and coding recommendations for the Lunar Gateway that will, among other purposes, help facilitate interoperable PNT signals for member space agencies.*

KEY ISSUES AND DEBATES

While several stakeholders have expressed interest in creating Cislunar PNT systems, details surrounding the exact specifications and implementations of these systems remain under debate. As with the creation of GNSS on Earth, the creation of Cislunar PNT systems raises a variety of international policy questions, including military versus civil involvement, signal structure and access, international cooperation, and interoperability. In the following section, we give an overview of some of these major debates and provide analysis that may help to shape policy in these areas.

Network requirements

While there has been growing support for the development of a Cislunar PNT system, there are still fundamental uncertainties about the core requirements of such a system. What are the essential missions for this system, and how soon will the system be needed?

System need

Currently, there are no dedicated Cislunar PNT systems. Missions to Cislunar space and beyond must use a combination of Earth-based radio stations, onboard accelerometers, and optical navigation to successfully complete their missions.³² In the 2018 Global Exploration Roadmap, which outlines plans for over 20 upcoming lunar missions, the ISECG identified in-space navigation and timing as a critical technology needed for future exploration missions.³³ A Cislunar PNT system would aid Cislunar missions by improving autonomous navigation capabilities, reducing tracking and operations costs, and providing backup or redundant navigation signals to improve safety, especially critical for human spaceflight missions.³⁴

Using existing GNSS signals to aid navigation on Earth-orbiting missions is proven to have several benefits, including improved real-time navigation performance (from kilometer-class to meter-class accuracy), faster trajectory maneuver recovery (from 5 to 10 h to min), reduced need for expensive onboard clocks, and increased satellite autonomy, lowering mission costs.³⁴ Similar benefits may apply to missions that use a Cislunar PNT system to navigate in around the Moon rather than relying on other state-of-the-art navigation methods.

As mentioned above, NASA believes a Cislunar PNT network will enable precision surface operations, and the U.S. military has identified PNT as one of its key requirements for expansion beyond the GEO. This shows that there is institutional demand for such a system.

Development of a Cislunar PNT system could improve the safety and efficiency of commercial activity in Cislunar space as well as a government-sponsored spaceflight. The availability of this basic infrastructure could lead to new innovations and accelerate humanity's development of Cislunar space and the lunar surface. The open availability of PNT signals on Earth is deeply integrated into fundamental modern infrastructure and is used for a massive number of critical services beyond simple navigation and human rescue operations. Precision timing from GNSS clocks is the foundation of secure transactions for data networks and financial systems. It also enables collection of precise data about our planet for the study of earthquakes, volcanoes, and tectonic plates. GNSS precision location determination is also used to design and carry out construction and farming operations.³⁵

As humans establish a greater presence on the Moon and in Cislunar space, access to a PNT network will undoubtedly enable all the same benefits to infrastructure, collection of scientific data, and efficient, secure system operations.

System time line

NASA's Artemis program plans to return humans to the moon by 2024. In the Artemis plan document, published in September 2020, the agency proposed building LunaNet after the core Artemis elements are already operational.¹⁵ The plan states, “After Artemis III, NASA and its partners will embark on missions on and around the Moon that also will help prepare us for the types of mission durations and operations that we will experience on human missions to Mars. … With the core Artemis elements in operation—SLS, Orion, HLS, the Gateway, and potentially the LTV—NASA is engaging international and commercial partners to pursue additional surface capabilities. … One such example is an extensible and scalable lunar communications and navigation architecture, known as LunaNet.”

This language seems to indicate that NASA's priority is completing the first lunar landings to return humans to the moon (the Artemis III mission), after which they will work on building up the lunar infrastructure and improving surface operations capabilities. However, it could be argued that Cislunar PNT infrastructure should be in place early to help facilitate the development and operation of lunar missions, including Artemis.

From a national perspective, there may be an advantage to being the first nation to create a Cislunar PNT system. As with terrestrial systems, the nation acting first in this area would have the most freedom to make use of limited resources, such as Lagrange points, “ideal” constellation orbits, and particularly useful frequencies. Space Force Chief of Space Operations Gen. Jay Raymond recently described Cislunar space as “key terrain” critical to the defense of U.S. forces, saying “But as nations move out, and as the economy grows between here and the lunar surface, and as you look at key terrain for the defense of our nation, I think it's an area that will be significant as we move forward.”³⁶

In a document outlining the strategic importance of Cislunar space and the Moon, the Center for Strategic and International Studies (CSIS) highlighted the importance of Lagrange points, particularly the L2, L4, and L5 points, for PNT purposes because spacecraft stationed at those points would have visibility behind the Moon, a part of Cislunar space that is not visible from Earth.³⁷ Lunar relay constellations broadcast downlink signals for PNT systems. According to the Lunar Communications Architecture Study released in 2022 by IOAG,³¹ the “ideal” lunar relay constellation is one that can provide global coverage for the Moon in a stable orbit with high average contact duration; this is a challenging design due to the Moon's tidal locking and proximity to Earth.

The “ideal” constellation would consist of a minimum of 3 orbiters in 12-h orbits: one circular orbit around the equator and one frozen elliptical orbit around each pole, scaling up to 5 orbiters by adding a second to each polar orbit. The Moon cannot support as many orbiting satellites as the Earth; a trade analysis to determine the Moon's orbital capacity has not yet been published, but congestion within the decade is unlikely given that fewer than 5 agencies are currently discussing placement of PNT systems. A more likely advantage of placing the first PNT system around the moon may be realized by embracing interoperability standards, providing an open signal, and making first use of the strongest frequency ranges available.

Due to this potential first-mover advantage, the readiness and speed of deployment of different technologies may be a deciding factor when selecting a PNT architecture. In addition, some architectures may be usable in some capacity while the system is under construction, while others may require the complete architecture to be built before users can access the system. NASA's LunaNet interoperability document states, “LunaNet will include Earth ground stations and orbiting spacecraft and will provide services to human exploration, lunar science, and space technology missions. LunaNet will start with a simple architecture of a few nodes to meet the needs of the early missions and evolve to meet the growing needs of a sustained lunar presence.

All relay network services are not expected to be met by a single spacecraft, or node. The expectation is that the needs of users will be met through a combination of interoperable systems supplied by commercial and government providers.”⁶ Meanwhile, as mentioned earlier, the USAF has provided funding to several private companies for different approaches to PNT architecture, including lunar surface beacons, spacecraft constellations, and improved navigation algorithms.

Military Versus Civil Development and Use

The needs of these various types of users also raise the issue of military–civil involvement in the development and operation of a Cislunar PNT system. A Cislunar PNT system could be developed and operated by the military, by a civil agency, as a civil–military partnership, or even as 2 separate systems. Most terrestrial global PNT systems are military-owned and operated, but provide an open signal for use by civil and commercial entities around the world. This is the case for GPS, Galileo, GLONASS, and BeiDou. Europe's Galileo system is operated by a civil entity, although it also has a protected signal for safety and security uses.

While this question has not been directly addressed by officials with respect to Cislunar PNT, the recent activities of NASA and the U.S. military seem to suggest that the 2 entities are moving toward the development of separate, independent systems. NASA has made progress toward their envisioned PNT architecture with finalization of the LunaNet interoperability specification, which has also received input from international partners. The military appears to be working toward an independent capability to meet its specific needs, as evidenced by the funding of PNT technologies from private partners. However, as these PNT activities by NASA and the military are still in the early draft and development phases, there exists plenty of room for changes in architecture designs and potential collaboration between projects.

In November 2022, the Applied Physics Laboratory (APL) at Johns Hopkins University released a Cislunar Security National Technical Vision, which emphasizes that currently planned U.S. and international Cislunar programs will require extensive Cislunar PNT capabilities.³⁸ This report paid special attention to the issue of Department of Defense (DoD) versus NASA PNT requirements, and examined the advantages and disadvantages of cooperation between these agencies. The APL working group identified that the current version of LunaNet is built around a NASA-first philosophy, with human exploration, lunar science, and space technology missions being the intended uses of the system. LunaNet has provisions for security and encryption protocols, but these may not be sufficient for the DoD, which will likely have stricter requirements for security and system resilience to adversarial challenges such as jamming and spoofing.

APL recognized that a Cislunar PNT system could be built in either a top-down or bottom-up approach. The former would involve a concerted effort to establish requirements and specifications first, and building a unified PNT system that is acceptable to all users. The latter possibility would allow the system to be built out organically over time, with individual actors establishing PNT nodes for their own use, cases and issues around interoperability or signal specifications being worked out as they arise. The bottom-up approach would involve lower cost initially, but could result in service gaps, a less optimized system, and greater long-term cost as more spacecraft and PNT nodes would need to be fielded to address the needs of all users. Ultimately, this will likely be decided as a matter of cost: if the United States prioritizes lower upfront cost, the bottom-up approach is appropriate.

However, if more funding is allocated to the project, a top-down approach is feasible, and a universal system analogous to GPS for Cislunar space could be created. The working group at APL recommended that the DoD allocate funding to participate in trade studies with NASA to determine a PNT architecture that serves the needs of both organizations. This may involve a new system or a modification of LunaNet to suit the DoD's requirements.

PNT Signal Provision and Access

As noted above, regardless of the entity that owns and operates the system, all of the operational GNSS provide an open signal as a global utility. The global availability of these systems contributes to safety. It is also touted as an example of how the provision of basic infrastructure can lead to significant economic benefits. A 2019 study conducted by the RTI Institute estimated that GPS has generated roughly $1.4 trillion in economic benefits for the United States alone since it was made available for civilian and commercial use in the 1980s. The study further found that loss of GPS service would average a $1 billion per-day impact to the U.S. economy.³⁹

Some point to GPS as an important source of soft power for the United States. “Soft power” refers to the ability to get other parties' interests aligned with your own without use of coercion. The U.S. policy of provision of GPS civil services and civil signal design information, free of charge, and the use of that signal by individuals around the world is an example of soft power.⁴⁰ Provision of a free PNT signal in the Cislunar domain may provide similar benefits.

Shortly before the United States decided to provide open access to the GPS signal, Korean Airlines Flight 007 inadvertently flew into Soviet airspace and was shot down. There was a recognition that the GPS signal could be used to improve safety and avoid such accidents globally.⁴¹ The same logic could apply to the Cislunar domain—ensuring accurate navigation will help to avoid inadvertent collisions as activity in this space ramps up.

It is also possible that an open PNT system will help to spur commercial activity and innovation, as was seen with terrestrial PNT systems. However, policymakers would also need to consider that with an open signal, these benefits accrue to actors from all nations. This could lower the barrier to entry to competitors in Cislunar activity (either private companies or foreign governments), reducing the U.S. advantage in this area. Spurring additional foreign activity in Cislunar space could also be seen as a security risk.

Commercial Involvement in Development

Given the rapid growth of the private space sector, it is possible that a commercial entity may have an interest in developing a Cislunar PNT system. To be financially feasible, such a system would likely need to require that space operators pay for access to the signal. This type of user-fee model would be very different from the model used for GPS and other Earth-focused systems, which provide the signal free as a global utility. It is possible that a user fee would be more feasible in Cislunar space where the number of actors is relatively low (compared with GPS users on Earth). However, commercial control over such a signal could raise safety and security issues, and it would risk the loss of benefits discussed in PNT Signal Provision and Access.

Additional consideration of the states' responsibility for due regard and nonownership under the OST may also encumber commercial implementation of a paid model, as each commercial entity will be required to fulfill the requirements set out by its state so as to not violate international law, and if other state parties who have not paid are excluded from access to a system, it may create international disagreements about interference and fair use of outer space resources.

International Cooperation

The United States (NASA) and Europe (ESA) have both expressed interest in developing Cislunar PNT systems. China and Russia also have plans to operate in Cislunar space, but have not released any formal plans specific to PNT. If there is high enough interest from multiple stakeholders, the question will arise whether to create one Cislunar PNT system as an international partnership, many independent systems, or a combination of these options.

Systems developed through international partnerships may help to save on cost for participants in the program, and such partnerships can help to strengthen relationships among participants. However, given that the system will likely have military involvement and national security capabilities, close cooperation among potential adversaries is unlikely.

Among allies, there is a higher possibility of cooperation. Currently, ESA and NASA are working to develop their own individual PNT networks (ESA through its Moonlight initiative and NASA through the LunaNet network). Given the early stage of these proposals, there are many options for cooperation, including integrating these 2 concepts into one unified system, developing separate but interoperable systems, or moving to an entirely new cooperatively developed infrastructure. In the LunaNet interoperability specification document, NASA reiterates its desire to work with international partners in the development and deployment of PNT nodes and orbiters.³⁵ Furthermore, the document was written and reviewed with input from ESA, indicating their interest in shaping interoperability requirements and eventually being a provider within the LunaNet system.

Interoperability

Although one integrated system involving all major space actors is unlikely given the military applications of such a system, there is still room for international cooperation. The 4 global positioning, navigation, and timing satellite systems—U.S. GPS, European Galileo, Chinese BeiDou, and Russian GLONASS—were developed as stand-alone systems capable of operating independently. This was driven, in part, due to the national security importance of these systems, as well as the desire to avoid reliance on other nations. For instance, the primary drivers for the creation of Europe's Galileo network included concerns over “dependence on systems over which Europe has no control,” a desire for redundancy and guaranteed service, and a need for better performance in higher latitudes and urban environments, which made European reliance on GPS an inadequate solution.⁴²

However, despite being developed as independent capabilities, all of these systems are interoperable. This improves the quality and resilience of the systems. Since users can access and use signals from all satellites, they are more likely to get an accurate position at any given time and location. Also, in the event that one of the systems is damaged—inadvertently or due to conflict—this interoperability allows users to continue functioning with minimal interruption.

It is likely that a similar analysis of Cislunar PNT will come to the same conclusion. This means that Cislunar PNT systems should be designed with interoperability capabilities in mind. Note, however, that this does not preclude benefits to being a first mover—the first entity to develop a PNT system may still have advantages in choosing frequencies and orbits before others. In addition, as mentioned above, existing international policy suggests that space infrastructures should be made interoperable. The Artemis Accords in particular calls this out explicitly. Furthermore, the NSTC National Cislunar Science & Technology Strategy document more concretely laid out the U.S. intention of fielding interoperable Cislunar PNT systems. The document identified the implementation of Cislunar communications and PNT capabilities with “scalable and interoperable approaches” as one of the key objectives for U.S. efforts in Cislunar space, and suggests that any future U.S. policy will be guided by this principle.⁵

One of the most notable projects in the Cislunar PNT interoperability space is NASA's LunaNet Interoperability Specification Document, which is intended to provide “the basis for a comprehensive set of specifications” to create an interoperable communications and PNT network for Cislunar space. The final LunaNet document released September 2022 includes network requirements, capabilities, and interoperability standards (including frequency allocations) for the network and was collaboratively developed with input from the ESA. By outlining the technical requirements for interoperability, NASA hopes that LunaNet can be built as a combination of interoperable systems from NASA, international partners, and commercial providers. As per the document, “Interoperability across this network-of-networks can be achieved through negotiation of mutually-agreed-upon standards that will be reflected in this document and in the specifications defined by other participants in the cooperative lunar network.”⁶

Interoperability for terrestrial GNSS is facilitated within the International Committee on GNSS (ICG), which helps to coordinate GNSS efforts between operators, including nations and private entities. The ICG has already recommended investigating the use of terrestrial GNSS for Cislunar operations,⁴³ so perhaps the ICG would be a natural place for Cislunar PNT operators to collaborate as well. If Cislunar PNT becomes pervasive enough with many stakeholders and operators, a new body may need to be formed to address Cislunar PNT efforts specifically.

Frequency Band Coordination

A consideration for Cislunar PNT will be the coordination of frequencies for the PNT signal. It may be desirable to protect certain frequency bands for other uses, such as radio astronomy.⁴⁴ Operators of Cislunar PNT spacecraft will also need to coordinate with each other when designing the specifications of their PNT signal structures to limit interference issues. As mentioned above, limiting harmful interference and operating in “due regard” to other nations is a principle of the OST.

For Earth-orbiting satellites, radio frequency allocation is currently handled by the ITU. When a new satellite or a constellation of satellites is proposed for launch, the satellite's operator submits a filing to the licensing authority of the responsible government institution in their country, which then submits the necessary frequency requests to the ITU in accordance with the Radio Regulations (the international treaty governing the use of radio frequencies). The ITU ensures that the satellite proposal conforms to the regulations and maintains a Master International Frequency Register that contains all radio frequency assignments in use in space.⁴³

This system could potentially be extended to include spacecraft operating in Cislunar space, but part of the 2022 SFCG recommendations to the United Nations is a set of limited frequency bands proposed for use in the lunar region by communications, PNT, and SAR along with a request for all assignments of lunar local link frequencies to be handled through the SFCG instead of the ITU.

CONCLUSIONS

Government, military, and commercial space actors are seeking to expand their operations in Cislunar space, with many Cislunar missions already funded and scheduled to launch within the decade. To support this uptick in Cislunar activity, there is an interest in developing a positioning, navigation, and timing (PNT) infrastructure for Cislunar space. Such a network (or networks) could improve navigation precision, autonomy, and cost efficiency for Cislunar spacecraft. It may also provide other less tangible benefits such as soft power for network operators, strengthened ties among international partners, and bolstered innovation and economic growth.

While developments toward Cislunar PNT are still in the early stages, several nations have discussed their interest in undertaking Cislunar PNT projects. The United States is perhaps the most vocal of these, and has seen activity in the military, civil, and private sectors. One of the most notable U.S. efforts in this area is NASA's proposed LunaNet network, and NASA has already released draft documentation for the interoperability specifications of this network. ESA has also been actively working on Cislunar PNT with its Moonlight initiative and efforts to extend the GNSS space service volume. Many other nations, including Russia and China, have discussed plans to operate in Cislunar space but have not addressed the issue of PNT specifically.

Current international policy in this area is limited. The primary treaty affecting all activities in space is the OST of 1967, which may have some bearing on interoperability, noninterference, and ownership rules for a PNT system. The Artemis Accords, signed by 8 countries in 2020, provides some clarity in these areas, but still leaves many details open-ended.

As Cislunar PNT continues to be developed and a network or networks are eventually deployed, several major issues and debates will need to be addressed. We identified some of these key debates, to include network requirements, military vs. civil involvement or ownership, signal access, commercial involvement, international cooperation, interoperability, and frequency band coordination. For some of these issues, the development of GNSS on Earth and related policy decisions provides a compelling analogue. We provide discussion and analysis of these areas that may help to shape the development of Cislunar PNT systems as the policy landscape continues to change and grow in the coming years.

Footnotes

AUTHORs' CONTRIBUTIONS

A.D.: Conceptualization (supporting); writing—original draft (lead); and writing—review and editing (equal). M.B.: Conceptualization (lead); funding acquisition (lead); project administration (lead); supervision (lead); writing—original draft (supporting); and writing—review and editing (equal).R.P.: Writing—original draft (supporting) and writing—review and editing (equal).

AUTHOR DISCLOSURE STATEMENT

No competing financial interests exist.

FUNDING INFORMATION

No funding was received for this article.

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