The Artemis generation of lunar explorers will establish a sustained human presence on the Moon, prospecting for resources, making revolutionary discoveries, and proving technologies key to future deep space exploration.

To support these ambitions, NASA navigation engineers from the Space Communications and Navigation (SCaN) program are developing a navigation architecture that will provide accurate and robust Position, Navigation, and Timing (PNT) services for the Artemis missions. Global Navigation Satellite System (GNSS) signals will be one component of that architecture. GNSS use in high-Earth orbit and in lunar space will improve timing, enable precise and responsive maneuvers, reduce costs, and even allow for autonomous, onboard orbit and trajectory determination.

Global Navigation Satellite System
GNSS refers to PNT satellite constellations operated by the U.S., the European Union, Russia, China, India, and Japan. GPS, the PNT constellation created by the U.S. Air Force, is probably the example most Americans are familiar with.

On Earth, GNSS signals enable navigation and provide precise timing in critical applications like banking, financial transactions, power grids, cellular networks, telecommunications, and more. In space, spacecraft can use these signals to determine their location, velocity, and time, which is critical to mission operations.

"We're expanding the ways we use GNSS signals in space," said SCaN Deputy Director for Policy and Strategic Communications J.J. Miller, who coordinates PNT activities across the agency. "This will empower NASA as the agency plans human exploration of the Moon as part of the Artemis program."

Spacecraft near Earth have long relied on GNSS signals for PNT data. Spacecraft in low-Earth orbit below about 1,800 miles (3,000 km) in altitude can calculate their location using GNSS signals just as users on the ground might use their phones to navigate.

This provides enormous benefits to these missions, allowing many satellites the autonomy to react and respond to unforeseen events in real time, ensuring the safety of the mission. GNSS receivers can also negate the need for an expensive onboard clock and simplifies ground operations, both of which can save missions money. Additionally, GNSS accuracy can help missions take precise measurements from space.

Expanding the Space Service Volume
Beyond 1,800 miles in altitude, navigation with GNSS becomes more challenging. This expanse of space is called the Space Service Volume, which extends from 1,800 miles up to about 22,000 miles (36,000 km), or geosynchronous orbit. At altitudes beyond the GNSS constellations themselves users must begin to rely on signals received from the opposite side of the Earth.

From the opposite side of the globe, Earth blocks much of the GNSS signals, so spacecraft in the Space Service Volume must instead "listen" for signals that extend out over the Earth. These signals extend out at an angle from GNSS antennas.

Formally, GNSS reception in the Space Service Volume relies on signals received within about 26 degrees from the antennas' strongest signal. However, NASA has had marked success using weaker GNSS side lobe signals - which extend out at an even greater angle from the antennas - for navigation in and beyond the Space Service Volume.

Since the 1990s, NASA engineers have worked to understand the capabilities of these side lobes. In preparation for launch of the first Geostationary Operational Environmental Satellite-R weather satellite in 2016, NASA endeavored to better document side lobes' strength and nature to determine if the satellite could meet its PNT requirements.

"Through early on-orbit measurement and documentation of the GNSS side lobe capabilities, future missions could rest assured that their PNT needs would be met," said Frank Bauer, who began the GNSS PNT effort at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Our understanding of these signal patterns revealed a host of potential new GNSS applications."

Navigation experts at Goddard reverse-engineered the characteristics of the antennas on GPS satellites by observing the signals from space. By studying the signals satellites received from GPS side lobes, engineers pieced together their structure and strength. Using this data, they developed detailed models of the radiation patterns of GPS satellites in an effort called the GPS Antenna Characterization Experiment.

While documenting these characteristics, NASA explored the feasibility of using side lobe signals for navigation well outside what had been considered the Space Service Volume and in lunar space. In recent years, the Magnetospheric Multiscale Mission (MMS) has even successfully determined its position using GPS signals at distances nearly halfway to the Moon.

GNSS at the Moon
To build on the success of MMS, NASA navigation engineers have been simulating GNSS signal availability near the Moon. Their research indicates that these GNSS signals can play a critical role in NASA's ambitious lunar exploration initiatives, providing unprecedented accuracy and precision.

"Our simulations show that GPS can be extended to lunar distances by simply augmenting existing high-altitude GPS navigation systems with higher-gain antennas on user spacecraft," said NASA navigation engineer Ben Ashman. "GPS and GNSS could play an important role in the upcoming Artemis missions from launch through lunar surface operations."

While MMS relied solely on GPS, NASA is working toward an interoperable approach that would allow lunar missions to take advantage of multiple constellations at once. Spacecraft near Earth receive enough signals from a single PNT constellation to calculate their location. However, at lunar distances GNSS signals are less numerous. Simulations show that using signals from multiple constellations would improve missions' ability to calculate their location consistently.

To prove and test this capability at the Moon, NASA is planning the Lunar GNSS Receiver Experiment (LuGRE), developed in partnership with the Italian Space Agency. LuGRE will fly on one of NASA's Commercial Lunar Payload Services missions. These missions rely on U.S. companies to deliver lunar payloads that advance science and exploration technologies.

NASA plans to land LuGRE on the Moon's Mare Crisium basin in 2023. There, LuGRE is expected to obtain the first GNSS fix on the lunar surface. LuGRE will receive signals from both GPS and Galileo, the GNSS operated by the European Union. The data gathered will be used to develop operational lunar GNSS systems for future missions to the Moon.

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