A ‘cosmic GPS’ for future space missions: a study proves the viability of X-ray pulsar navigation

Currently, most space missions rely on Earth-based guidance and tracking systems, which involves human intervention and limitations related to communication delays. Autonomous navigation systems aim to reduce this dependence, especially in interplanetary or deep-space missions, where technologies like GPS become ineffective. A study led by researchers from the Institute of Space Studies of Catalonia (IEEC — Institut d’Estudis Espacials de Catalunya) at the Institute of Space Sciences (ICE-CSIC) and Politecnico di Milano, published in the journal Acta Astronautica, confirms the viability of X-ray pulsar-based space navigation (XNAV) in deep space.

Pulsars, rapidly rotating neutron stars with an intense magnetic field, emit periodic and stable signals like ‘cosmic beacons,’ making them an ideal natural source for space navigation.

To assess the viability and performance of autonomous spacecraft navigation using X-ray pulsars—that is, without intervening from Earth—the study presents a systematic and realistic approach to pulsar selection, taking into account their brightness, temporal stability, geometric configuration, and visibility limitations. In addition, the team uses observational data from NASA’s NICER mission, an X-ray telescope launched in 2017 to study neutron stars, black holes, and other phenomena from its base aboard the International Space Station to simulate the performance of an autonomous XNAV system.

“Unlike many previous studies, we have used real observational data to assess the system’s performance, which allows us to estimate the capabilities of pulsar-based autonomous navigation much more realistically,” says Sui Chen, a predoctoral researcher at Politecnico di Milano who did a stay at ICE-CSIC developing this work, and now works at the University of Liverpool.

This work is not based solely on analytical noise models, but also uses real data to estimate measurement uncertainty. The study proves that autonomous navigation of space missions without ground support is feasible, particularly for deep space missions beyond our Earth orbits, where traditional positioning systems like GPS are unavailable.

“The challenge is not simply to identify the brightest pulsars, but to determine the optimal combination of sources capable of delivering accurate, precise and stable navigation performance throughout the mission,” says Emilie Parent, former postdoctoral fellow at ICE-CSIC and IEEC, currently working at the Institute for Planetary Sciences and Astrophysics of Grenoble. 

The XNAV navigation system has been tested using an Extended Kalman filter—an algorithm to estimate the spacecraft state by combining a dynamic model with external measurements under uncertainty—under two different scenarios. The first one: in low Earth orbit; and the second one: an Earth-to-Jupiter transfer. In addition, the team used data from the NICER mission to construct realistic pulsar pulse profiles and to estimate measurement uncertainties based on simulations built on real data, extrapolating to various types of future X-ray detectors.

The results show that pulsars that emit large amounts of energy, such as the Crab Pulsar (PSR B0531+21) located in the Crab Nebula, can achieve high accuracy (below 7 km in low Earth orbit), but suffer from poor long-term stability. Meanwhile, millisecond pulsars (with rotation periods between 1 and 10 milliseconds) offer greater long-term reliability at the cost of lower navigation accuracy.

The work presents a systematic study of navigation performance, which assesses the achievable position accuracy across different effective instrument area ranges. This provides a realistic study to begin building XNAV devices to be used on small satellites.

“We wanted to take an important step towards navigation systems capable of operating autonomously in deep space for a long time, where dependence on terrestrial infrastructure is increasingly limited,” says Nanda Rea, IEEC researcher at ICE-CSIC and co-author of the study.

This work, developed entirely at ICE-CSIC, was co-funded by a Proof of Concept (PoC) grant from the European Research Council (ERC) awarded to the DeepSpacePULSE project, led by astrophysicist Nanda Rea. This project aims to study the feasibility of autonomous navigation using X-rays emitted by pulsars, making these satellite positioning devices competitive in both the public and private space markets.

This article contributes to the development of autonomous navigation technologies for space exploration, reducing dependence on ground-based systems. In the long term, it could enable more efficient deep-space missions, both for planetary exploration and interplanetary travel.

In the future, studies in this field will focus on improving pulsar timing models, combining multiple pulsars for greater robustness, and integrating XNAV systems with other navigation systems. Furthermore, a first engineering model of the device, with all its components, will be built at the ICE-CSIC laboratories. “This paves the way for fully autonomous spacecraft navigation for deep-space missions, where no other navigation system can be effective in the long term,” Rea concludes.