NASA investigates upper limits of global navigation systems for Artemis lunar missions


A permanent human presence on the Moon will be created by the Artemis generation of lunar explorers, exploring resources, making groundbreaking discoveries, and testing key technologies for potential space exploration.

NASA’s Space Communications and Navigation (SCaN) software navigation engineers are designing a navigation architecture to support these goals, which will provide the Artemis missions with accurate and robust location, navigation, and timing (PNT) services. One part of this architecture will be global navigation satellite system (GNSS) signals. In low-Earth orbit and lunar space, the use of GNSS can enhance timing, allow for accurate and sensitive maneuvers, reduce costs and even enable autonomous orbit and trajectory determination on board.

Satellite Framework for Global Navigation
GNSS applies to the U.S., European Union, Russia, China, India and Japan running PNT satellite constellations. GPS, the constellation PNT developed by the U.S. Air Force Air Force

The Air Force, possibly, is the example most Americans are familiar with.

For critical applications such as banking, financial transactions, power grids, cellular networks, telecommunications and more, GNSS signals enable navigation and provide accurate timing on Earth.

In space, these signals can be used by spacecraft to determine their location, distance, and time, which are important for mission operations.

“We are expanding the capabilities for using GNSS signals in space,” said J.J. Miller, Deputy Director of Policy and Strategic Relations at SCaN, who coordinates PNT operations around the company. “This will help NASA plan for human exploration of the moon under the Artemis program.”

For PNT data, spacecraft near Earth have long relied on GNSS signals. At altitudes of less than 3,000 kilometers, spacecraft in low-Earth orbit can measure their location using GNSS signals, just as on-the-ground users can use their phones for navigation.

This gives these missions enormous benefits, allowing many satellites the autonomy to react in real time to unexpected events and ensure mission protection. The need for a costly on-board clock is also removed by GNSS receivers and ground operations are streamlined, both saving money for missions.

Furthermore, GNSS accuracy can help missions conduct accurate measurements from space.

The extension of space service sizes.
Navigation with GNSS is becoming increasingly difficult above an altitude of 1.800 miles.

This region of space is called the Volume of Space Operation, and the geosynchronous orbit ranges from 1,800 miles to an altitude of approximately 36,000 km (22,000 miles).

Users must depend on signals obtained from the opposite side of the Earth at altitudes above the GNSS constellations themselves.

The Earth blocks most of the GNSS signals from the opposite side of the Earth, so spacecraft must now “listen” to signals that extend beyond the Earth in the space service volume.

These signals spread from the GNSS antennas at an angle.
Formally, GNSS receipt in the volume of the space service relies on signals received from the antennas within around 26 degrees of the strongest signal. NASA, however, has had tremendous success in using weaker side-lobe GNSS signals – coming from the antennas at an even higher angle – for inside and outside Space Service Volume navigation.

NASA engineers have focused on understanding the capabilities of these side lobes since the 1990s.

NASA tried to better record the intensity and function of the sidelobes in preparation for the launch of the first Geostationary Operational Environmental Satellite-R weather satellite in 2016 to determine if the satellite could fulfill PNT requirements.

“Early on-orbit measurement and documentation of GNSS sidelobe capabilities allowed future missions to be confident that their PNT requirements would be met,”Early on-orbit measurement and documentation of GNSS sidelobe capabilities allowed future missions to be confident that their PNT requirements would be met. “Our understanding of these signal patterns revealed a variety of potential new GNSS applications.”

By studying the signals from space, navigation experts at Goddard reverse-engineered the characteristics of the antennas on GPS satellites.

By observing the signals the satellites receive from the side lobes of the GPS, engineers were able to piece their structure and strength together. A


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