US looks to establish Coordinated Lunar Time

US looks to establish Coordinated Lunar Time

Technology News |
By Nick Flaherty

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The US is aiming to establish a standard for timing on the Moon to help with accurate positioning and coordination.

The US National Cislunar S&T Strategy aims to create a standard for timing on the Moon called Coordinated Lunar Time that relates back to Coordinated Universal Time (UTC) on Earth.

The difference in gravity between the Earth and the Moon means there is a difference of 58.7ms a day between the two timing systems as a result of special and general relativity. This timing difference can be an issue for secure data transmission and accurate positioning as well as coordinating multiple systems on the lunar surface.

With standards set in Paris and in French, an internationally agreed Coordinated Lunar Time standard is set to be called LTC.

Accurate positioning has been an issue with the two most recent Moon landings for JAXA and Intuitive Machines, where the landers ended up in the wrong place and toppled over. Precision applications such as spacecraft docking or landing will require greater accuracy than current methods allow and the direct use of UTC at the Moon without correction as the local time scale would have cascading effects for applications that require precise metrology.

The approach to establish time standards consists of the definition, development, and implementation of a distinct reference time at each celestial body and its surrounding space environment. Each new time standard developed will include the following features:

  1. Traceability to Coordinated Universal Time (UTC);
  2. Accuracy sufficient to support precision navigation and science;
  3. Resilience to loss of contact with Earth; and
  4. Scalability to space environments beyond the Earth-Moon system

The initial focus is on the lunar surface and missions operating in Cislunar space as the US space agency NASA plans to return astronauts to the Moon.

NASA will work with the Institute of Standards and Technology (NIST) which is part of the Department of Commerce, as well the US Departments of Defense, State and Transportation to provide a finalized strategy to the Executive Office of the President to implement lunar timing standardization no later than December 31, 2026.

NASA will also include consideration of LTC as part of its annual Moon-to-Mars Architecture Concept Review cycle at the end of this year.

However there are significant challenges. Atomic Time (TAI) is the way UTC is delivered and is produced through a weighted average of hundreds of atomic clocks around the world. UTC differs from TAI by an integer number of “leap seconds” inserted periodically to keep UTC aligned with Earth solar days, despite changes in the rate of Earth’s rotation. A similar system may be needed for LTC, taking into account the relativistic issues.

The navigation accuracy a system can achieve with signals from multiple space- based assets, such as a person navigating on Earth with signals from Global Positioning System satellites, depends on the synchronization of those assets with each other. At the Moon, synchronizing each lunar asset with an Earth-based time standard is difficult due to the relativistic effects, events. The safety of navigation in Cislunar space also relies on a consistent definition of time among users.

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