posted 04-03-2024 09:31 AM               

Over the next decade, the United States will work with allies and partners to return humans to the Moon and develop capabilities to enable an enduring presence. During this same period, the U.S. government (USG) anticipates that many other actors, including governments and private industry, will also send spacecraft to operate in Cislunar space — namely to the lunar surface, in lunar orbits, and at the Earth- Moon Lagrange points.

A unified time standard will be foundational to these efforts. The National Cislunar S&T Strategy calls for the United States to establish a sustainable "Cislunar ecosystem" with scalable and interoperable position, navigation, and timing (PNT) infrastructure. U.S. leadership in defining a suitable standard — one that achieves the accuracy and resilience required for operating in the challenging lunar environment — will benefit all spacefaring nations.

Knowledge of time in distant operating regimes is fundamental to the scientific discovery, economic development, and international collaboration that form the basis of U.S. leadership in space. There will be an increasing need for the USG to collaborate with other entities interested in operating in Cislunar space on a broad range of capabilities, standards, and infrastructure. Time standardization will be a necessary foundation to enable interoperability across the USG and international partners, promote safe and sustainable operations, and simplify Space Situational Awareness (SSA) for safety of flight and operations. Key technical background information is summarized below:

1. UTC is the primary time standard used by Earth-based systems today. The UTC standard is based on the theoretical ideal, Terrestrial Time, which is an analytical definition for time at mean sea level, taking into account the Earth's center of mass. International Atomic Time (TAI) is the primary realization of this ideal time, 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.

2. Relativity poses challenges to extending operations into Cislunar space and beyond. Due to general and special relativity, the length of a second defined on Earth will appear distorted to an observer under different gravitational conditions, or to an observer moving at a high relative velocity. For example, to an observer on the Moon, an Earth-based clock will appear to lose on average 58.7 microseconds per Earth-day with additional periodic variations. This holds important implications for developing standards and capabilities for operating on or around the Moon.

   Additionally, 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 relativistic effects, events that appear simultaneous at the Earth (e.g., the start of a broadcast signal) are not simultaneous to an observer at the Moon.

   Safety of navigation in Cislunar space also relies on a consistent definition of time among users.

   This includes SSA and continuity of time knowledge during transit operations. PNT systems provide distance measurements by multiplying the time of flight of the signal by the speed of light. Failing to account for the discrepancy between a transmitter clock on the Earth and how it is perceived by a receiver on the Moon will result in a ranging error. Precision applications such as spacecraft docking or landing will require greater accuracy than current methods allow.

   Beyond these operational challenges, the direct use of UTC at the Moon (i.e., without correction) as the local time scale would have cascading effects for applications that require precise metrology. International System of Units (SI) core unit definitions, including the meter and kilogram, rely on the SI definition of time. Due to relativistic effects, a non-SI unit would introduce uncertainty in core unit definitions. These types of errors will have undesired impacts, such as reducing the accuracy of mapping and inertial navigation products. Defining a local time scale can provide a stable reference point for these base units and conversions that must be independently realized on the surface of the Moon.

posted 04-03-2024 10:33 AM               

Secondly, the Webb telescope is operating adequately at the Lagrange point on UTC alone, despite having relativistic time effects to signals sent when setting up the mirror arrays — none of which compromised its operation.

Thirdly, by the very nature of space-time a coordinated lunar time standard at the lunar surface would work less adequately than the UTC and TAI on Earth because the lunar mascons are so much more extreme than geo-mass concentrations.

posted 04-03-2024 02:08 PM               

The use of a common lunar reference time across LNSP infrastructure elements is required to enable synchronization of services and time in the lunar domain. This common lunar reference time is described in detail in Lunar Time System Standard (TBD).