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Reliable Communications on the Moon: How ITS is Enabling Lunar Spectrum Management

Almost 57 years after President Richard Nixon’s long-distance telephone call — via radio — to the Apollo 11 astronauts, Artemis II marked mankind's return to the lunar neighborhood. With the potential for future space travel, economic development, and innovation, the moon represents a frontier for innovation in communications technologies. To ensure the reliability of communications on and around the lunar surface, new radio-wave propagation models are necessary to avoid conflicting communications uses of the electromagnetic spectrum.

The first such model has now been finalized. In September 2025, a new P-Series Recommendation was published by the ITU-R, Recommendation ITU-R P.2170, Methods and models for predicting lunar radio-wave propagation characteristics for the modeling of radio-wave propagation on the lunar surface.

Notably, this new Recommendation is based on the Institute for Telecommunication Sciences’ (ITS) theory and methodologies developed for the Irregular Terrain Model (ITM) for propagation of radio-waves along the surface of the Earth. The ITM was modified to account for the unique characteristics of the lunar surface, such as its lack of atmosphere and roughness (due to the lack of erosion), as well as properties of the lunar regolith and its unique mineral composition — in particular, the presence of titanium and iron. While the technical material for this new Recommendation is based on ITS theory and methodologies, it was born of NTIA ITS’s collaboration with outside U.S. experts and additional collaboration with NASA. The original text, developed within the U.S. Study Group 3, was submitted as an initial draft to ITU-R Study Group 3 for the May 2024 meetings in Denver, Colorado.  ITS engineers continued to refine the model, both in collaboration with colleagues within U.S. Study Group 3 and within the Correspondence Group 3J-26, chaired by ITS computer scientist Erik Hill. Example software is actively being developed and made available via the NTIA GitHub repository https://github.com/ntia/ilm.

Three core modifications to ITM in the areas of atmospheric refraction, tropospheric forward-scatter (or troposcatter), and time variability allow for its application to the lunar environment. The changes all relate to the lunar atmosphere — or lack of one.

Atmospheric refraction, which is a function of water vapor, causes radio waves to bend towards the surface of the Earth under standard atmospheric conditions.  Terrestrial propagation models generally use the concept of an effective Earths radius, in which the radius of the Earth is modified to compensate for the curvature of the radio wave path due to the atmosphere — thus allowing the propagation model to consider ray paths in a linear fashion (the classic “4/3rd’s Earth radius assumption” approach). On the lunar surface where water vapor is absent, atmospheric refraction is also absent, necessitating the removal of effective Earth radius approaches and relying only on the physical radius of the Moon.

For transhorizon paths on the Earth, as the distance between terminals is increased, propagation by diffraction gives way to the more efficient propagation method of troposcatter.  With troposcatter, inhomogeneities in the atmosphere due to its turbulent behavior forward scatter electromagnetic energy in a much more efficient manner than diffraction theory alone would predict. Although the exact mechanisms driving troposcatter are still an active area of research and measurements, such signals can be reliably measured and have been reliably utilized for long-distance communication in the past.  Propagation via troposcatter cannot exist on the lunar surface due to the lack of atmosphere, thus its removal from the lunar propagation model.

Lastly, on Earth, the radio channel is a random process due to the inherent randomness of the atmosphere that the radio waves must propagate through. This randomness is visible to anyone who’s ever tried to take a measurement of radio system and has seen variations of signal strength over time. Measured over long durations, signal variations can be seen to be impacted by diurnal cycles, seasons, and meteorological changes. These variations, referred to as time variability within ITM, result in propagation predictions that themselves are statistical representations of signal strength and accurately represent the random process of the channel.  On the lunar surface, time variability does not exist due to the lack of atmosphere.

Finally, one further modification was performed to ITM to adapt the model to the lunar environment. ITM contains built-in statistics for modeling location variability, which is dependent on the roughness of the Earth surface. These statistics needed adjustment for operations on the lunar surface, where values of surface roughness are much greater due to the lack of erosion. Using a digital terrain model of the lunar surface, ITS performed a statistical analysis of lunar surface roughness. The results of this analysis were incorporated into the lunar model to modify statistics of location variability suitable for lunar propagation.

The model is needed to support WRC-27 agenda item 1.15 “to consider studies on frequency-related matters, including possible new or modified space research service (space-to-space) allocations, for future development of communications on the lunar surface and between lunar orbit and the lunar surface.”  These studies may include evaluating interference and co-existence between manned lunar systems, autonomous drones, and sensitive scientific radio telescopes.

“Modeling radio waves on the lunar surface forces us to re-think some of our default assumptions,” said William Kozma Jr., Head of NTIA’s ITS Propagation Modeling Research Program and Head of the U.S. Delegation to ITU-R Study Group 3. “Atmospheric effects, such as atmospheric attenuation and ray bending, are phenomena we prioritize for terrestrial models due to the dynamic nature of Earth’s atmosphere. The presence of water vapor is absent on the lunar surface, resulting in simplified atmospheric modeling.  Instead, diffraction and surface scattering effects will dominate due to the unique mineral composition of the lunar surface.”

“Although the lunar surface appears visually distinct from the Earth’s surface, it’s still governed by the fundamental laws of physics,” said NTIA Associate Administrator and ITS Director David Goldstein. “A physics-based model such as ITS’s Irregular Terrain Model can be adapted to apply to the lunar surface since the laws of electromagnetics are universal, thus allowing for the creation of a lunar model without the prerequisite of measurement data, as one would need for an empirical approach.”

ITM, also known as the Longley-Rice Model, was designed to be broadly applicable to frequencies from 20 MHz to 20 GHz over distances between 1 and 2000 km, taking into account the impacts of radio propagation on Earth's terrain and climate conditions. ITS is proud to have modified this essential communications tool to work on the moon’s surface to enable reliable, predictable lunar communications — ensuring that as we return to the Moon, our communications systems are as pioneering as the missions they support.