In January 2018, the International Space Station (ISS) tested a new navigation system called XNAV using the NICER instrument. This system used X-ray signals from four pulsars to determine the station's position without relying on GPS. The experiment, part of the SEXTANT project, achieved a precision of about 16 kilometers, sometimes as accurate as 5 kilometers, within less than 8 hours. The goal was to develop a method for future deep space probes to navigate independently, without depending on Earth-based systems like the Deep Space Network. Pulsars are the dense, spinning remnants of massive stars that exploded as supernovas. These objects emit beams of X-rays with such regularity that they can be compared to the most precise atomic clocks on Earth. By measuring the slight differences between the expected and actual arrival times of pulsar signals, scientists can determine the position of a spacecraft in space. This method is particularly useful for missions far from Earth, where GPS signals are unavailable. The idea of using pulsars for navigation dates back to 1974, but it took decades of research and technological development to make it a reality. The SEXTANT project, named after the maritime instrument used by sailors to navigate using the stars, represents a major step in space exploration. The NICER instrument, which includes 52 X-ray telescopes and silicon drift detectors, was installed on the ISS to conduct the test. During the experiment, all GPS data links were cut off, and the system successfully calculated the station's position with a precision better than the initial target. The United States was not the only country exploring this technology. Just a month before the ISS test, China launched its own experimental satellite, XPNAV-1, which aimed to use the Crab pulsar for navigation. This shows that the development of pulsar-based navigation is a global effort, with multiple spacefaring nations working on similar projects. While pulsar navigation is not as precise as Earth-based GPS, it offers a critical advantage: autonomy. For missions beyond the orbit of Mars, GPS signals are too weak to be useful. Instead, spacecraft would rely on the Deep Space Network, a system of ground-based antennas that is limited by the speed of light. With pulsar navigation, a spacecraft can calculate its position in real time without waiting for signals from Earth. This capability is now being used to synchronize clocks for future lunar and cislunar missions, supporting plans for manned journeys to Mars and beyond.