NASA satellites navigate without GPS using other satellites as moving landmarks

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No GPS available in space? A satellite experiment may have a solution for that.

NASA's Starling mission — made up of a swarm of four cubesats in low Earth orbit (LEO) — is testing out navigation by using other satellites as moving landmarks. The hope is that this tech will allow future missions to navigate if GPS is not available.

The three-year-old tech demo will be extended beyond its previous expiration date this year until at least 2028, NASA announced on Monday (Aug. 17). The aim is to get the most out of the onboard experiment, a collaboration with industry partner EraDrive called FALCON (short for "Fast Autonomous Lost-in-space Catalog-based Optical Navigation").

"As NASA prepares for more missions beyond Earth's orbit, technologies like FALCON can support lunar satellite swarms, distributed science missions and human exploration," NASA officials said in Monday's statement.

While NASA's applications are civilian, alternative GPS solutions are also being investigated by the U.S. Department of Defense (DoD) out of concern that the limited number of GPS satellites could be vulnerable to adversaries. Earlier this year, for example, Space Force entities SpaceWERX and Space Systems Command jointly launched an initiative aiming for new ideas for "positioning, navigation and timing" (PNT) capabilities in space.

The aim of FALCON, however, is to deal with sheer distance. While satellites in Earth orbit have ample access to purpose-built navigational markers such as GPS, that access rapidly thins out as missions go farther afield. And this isn't just an abstract concern; NASA aims to send astronauts to the surface of the moon again as soon as 2028 on the Artemis IV mission, as part of a larger effort to build a moon base .

Industry and the military are also very interested in the moon and the area around it, known as cislunar space — and how best to navigate in the new environment. The moon's orbit also introduces complications to navigation; in 2025, for example, Canada's Outer Space Institute reported that "mass concentrations," or mascons , pull down on orbiting satellites and may affect their pathway, in some cases even inducing crashes into the lunar surface.

But before testing at the moon, NASA wants to learn more closer to home. It partnered with EraDrive, an autonomous spacecraft navigation company that originated with a group at Stanford University, which provided flight software and algorithms for FALCON. The experiment uses data from Starling's cameras, as well as a catalog on the spacecraft that charts thousands of known satellites and space objects around it.

The mission not only is trying out "GPS-independent navigation" but is also showcasing space situational awareness, or SSA — that is, the ability to understand the environment around the spacecraft (including other satellites). Some other spacecraft, like SpaceX's Starlink broadband satellites, use SSA to automatically dodge threats of space debris in LEO.

Starling's team is hoping to bring even more capabilities for navigation and SSA. "The results from FALCON can have far-reaching implications for on-orbit space-traffic monitoring, collision avoidance and alternative navigation," Roger Hunter, program manager for NASA's small spacecraft and distributed systems program, said in the statement.

NASA plans a to build a permanent base on the moon over the next decade or so, via a step-by-step approach. (Image credit: NASA) FALCON notched two major milestones in its first three years. For PNT, the spacecraft used its cameras to observe other satellites and objects — and to match their information with the onboard database, which includes roughly 20,000 objects from publicly available DoD records. "FALCON then used the observed and verified objects as reference points to determine Starling's orbit," NASA stated.

FALCON's in-orbit observations also helped mission managers better understand the pathways of 200 individual objects, beyond the orbital estimates in the DoD catalog. The estimating work was performed over just three days and autonomously, meaning ground operators did not participate.

"The self-orbit determination capability made possible through FALCON is a first for spacecraft using optical cameras, to navigate by their relative position to other objects in space," NASA stated. "Separately, the catalog-update experiments produced better object position predictions onboard Starling than those provided by ground stations."

Starling's four spacecraft operate at an altitude of about 350 miles (565 km), which is about 6 miles (10 km) higher than originally planned. That's because SpaceX advised mission managers that Starling's mission plan put it at risk of coming close to Starlink satellites already operating in an orbital shell at 340 miles (555 km), according to Space News . By comparison, the International Space Station typically orbits slightly lower than Starling or Starlink, at 250 miles (400 km).

All of which is to say that the Starling mission demonstrates operations in crowded conditions. But in the future, NASA is hoping to port the lessons learned to the moon and Mars for science applications — meaning, to improve measurements from spacecraft — as well as for interplanetary traffic management.