Positioning on the Moon: Developments in Cislunar Navigation

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Several sessions at this year’s ION GNSS+ conference addressed the challenges and potential solutions for spacecraft positioning and navigation in cislunar space, a region spanning the Moon and the Earth-Moon Lagrange points. Technologies equivalent to GNSS will become essential to the multiplying number of public and private lunar missions.

In this article, we will explain the need for cislunar positioning systems, some of the technologies under development, and how NASA and ESA are promoting interoperability.

Traditional methods in space navigation

Although the Apollo Command Modules had inertial guidance systems, the primary sources of navigation information for each mission were the radio signals exchanged by NASA and the spacecraft. NASA’s Deep Space Network (DSN) consists of large radio dishes in California, Australia, and Spain that handle all communications between the space agency and its missions beyond Earth orbit.

Analyzing these signals helps missions track their spacecraft across the Solar System.[1] The DSN’s navigation techniques include:

  • Measuring the Doppler shift of the spacecraft’s downlink.
  • Sending ranging pulses in the uplink signal and analyzing the returned pulse in the downlink.
  • Using two locations to perform very long baseline interferometry techniques, such as three-way Doppler tracking.

NASA’s recent Artemis I mission used this third technique to track the uncrewed Orion module during its 25-day journey to the Moon and back.[2]

Some planetary missions have also used optical navigation techniques by collecting images of star fields, which mission controllers analyze to determine the spacecraft’s orientation and position.

Ground station bottlenecks

Ground stations are the common element of all these deep-space navigation methods. Whether it’s NASA’s DSN or the Estrack network of the European Space Agency, a limited number of radio dishes must exchange signals with distant spacecraft to generate the data mission controllers use to calculate navigation data.

The Artemis I mission placed this bottleneck in stark contrast. The mission itself required over 900 hours from the DSN’s antennas. Moreover, the ten CubeSats accompanying the mission consumed just as much time.[3]

With Artemis I taking precedence, several Mars missions and the James Webb Space Telescope spent nearly a month with limited access to their spacecraft.

Using GNSS in cislunar space

During the post-Apollo lull, lunar missions were few and far between. That has changed as space agencies and businesses worldwide plan dozens of missions to lunar orbit and the Moon’s surface. The DSN and similar networks cannot provide navigation and positioning services to all of them.

A logical alternative is to use signals from Earth’s constellations of GNSS satellites. Missions could leverage this established infrastructure by placing multi-constellation receivers on their spacecraft and alleviate pressure on space agencies’ ground stations.

Several experiments around the turn of the century demonstrated GPS reception from beyond the constellation’s 20,000-kilometer orbit. However, the record for GPS signal detection was set in 2016 by NASA’s Magnetospheric Multiscale mission (MMS).[4] The spacecraft successfully received GPS signals at an altitude of 76,000 kilometers — about a fifth of the distance to the Moon.

Receiving GNSS signals at the Moon will be more challenging due to low signal strength and the clustering of signals around a single point. Considerable work must be done before robotic or human missions to the Moon will rely on GNSS receivers.

Lunar GNSS Receiver Experiment (LuGRE)

NASA and the Italian Space Agency will send their LuGRE mission on a commercial lunar lander in 2024.[5] In transit, LuGRE will monitor the GPS L1 and L5 signals and the Galileo E1 and E5a signals. Once on the lunar surface, LuGRE will receive these signals to generate positioning data.

Lunar Pathfinder

ESA’s Lunar Pathfinder mission is a communications satellite that will provide S-band and UHF links for orbital and surface missions, relaying their data to Earth over an X-band signal.[6] The mission will also host the NAVIMOON receiver, an experiment to use GNSS to calculate position, velocity, and time while orbiting the Moon.

Novel positioning technologies

Using established GNSS technologies promises a simple on-ramp to lunar positioning. They may not be the only solution for cislunar navigation. Researchers continue investigating new technologies for spacecraft near the Moon and deeper into the Solar System.

Spacecraft-to-spacecraft positioning

Central to NASA’s plan to return to the Moon is the Lunar Gateway, a space station in a near-rectilinear halo orbit about the Moon. The space agency launched its Cislunar Autonomous Positioning System Technology and Navigation Experiment (CAPSTONE) in 2022 to characterize operations in this orbit.

The mission also tested a spacecraft-to-spacecraft positioning technique.[7] Last May, CAPSTONE analyzed signals exchanged with NASA’s Lunar Reconnaissance Orbiter to measure the distance and relative velocity between the two spacecraft.

Future spacecraft without positioning receivers could use this technique to determine its location relative to other spacecraft with precise positioning.

Navigation by pulsar

Milli-second pulsars generate X-rays with predictable patterns. Comparing X-ray photons from multiple pulsars of known positions to reference models should let a spacecraft determine its orientation and position in space.

NASA mounted the Neutron Star Interior Composition Explorer (NICER) on the International Space Station to observe X-ray emissions from collapsed supergiant stars. In 2017, the Station Explorer for X-Ray Timing and Navigation (SEXTANT) mission used NICER to collect milli-second pulsar data and successfully calculated the space station’s position within a ten-mile radius.[8]

A smaller version of SEXTANT would give spacecraft positioning capabilities independent of Earth-based systems.

Developing lunar navigation systems

GNSS positioning in the Moon’s neighborhood will be a practical short-term solution, but not for all missions. Surface missions without line-of-sight to Earth — or any mission on the lunar farside — cannot see the GNSS constellations. Barring the development of a small, light, and cheap alternative, the future fleets of lunar spacecraft will rely on dedicated lunar navigation systems. Several such systems are under active development.

Lunar Navigation Satellite System (LNSS)

Japan’s space agency, JAXA, is planning an eight-satellite constellation in orbits to allow high positioning accuracy at the lunar South Pole.[9] JAXA plans to launch a demonstration mission consisting of an LNSS satellite and a lander.

Lunar Communications and Navigation Service (LCNS)

ESA plans to develop a European service for private and public lunar missions.[10] Its two-tiered service will provide one-way positioning similar to how GNSS works on Earth. A two-way service would require active communications between the LCNS satellite and the user.

Lunar Communications Relay and Navigation Systems (LCRNS)

Established in 2022, NASA’s Lunar Communications Relay and Navigation Systems (LCRNS) define a set of requirements for supporting the space agency’s lunar operations.[11] The LCRNS would consist of lunar-orbiting satellites that broadcast PNT signals for crewed and robotic missions on the lunar surface.

LunaNet: A cooperative approach to lunar positioning

Given the many technical and financial challenges of lunar exploration, competing positioning systems would waste resources. NASA and ESA are promoting the LunaNet framework to bring GNSS-style interoperability to these developing satellite networks around the Moon.

This framework defines how compliant system operators would provide shared communications, navigation, and other services to customers in cislunar space. One of these capabilities is the Lunar Augmented Navigation Service (LANS). LANS defines common signal, message structure, lunar reference, and time systems to let multiple providers form a cohesive navigation network.

Each new mission to the Moon will be additive to LunaNet’s capabilities. Service providers will be free to offer proprietary services while contributing to this shared navigation resource. Ultimately, navigating the cislunar environment will involve a mix of traditional ground-based positioning, GNSS reception, lunar satellite networks, and other technologies. The advantage is no single mission must bear the burden of this infrastructure, lowering the cost of lunar exploration for everyone.

 

October 31, 2023