Pulsars are highly magnetized, rotating neutron stars that emit electromagnetic radiation at regular intervals. They radiate energy across a broad range of frequencies, but they are most visible in their X-ray beams. Pulsars emit powerful beams in opposite directions as they spin. These beams are observable only when they’re pointed toward Earth, making it seem as if these objects pulse (hence the name). In some cases, this apparent pulsing occurs with the predictability and consistency of an atomic clock.
X-ray pulsar-based navigation and timing (XNAV) is a theoretical navigation technique whereby the periodic X-ray signals emitted from pulsars are used to determine the location of a vehicle, such as a spacecraft in deep space. A vehicle using XNAV would compare received X-ray signals with a database of known pulsar frequencies and locations. Similar to GPS, this comparison would allow the vehicle to triangulate its position accurately (±5 km).
Current spacecraft navigation systems rely on global network of large ground-based radio antennas like NASA’s Deep Space Network (DSN) and ESA’s European Space Tracking (ESTRACK) which require a spacecraft to communicate with ground-based systems for months or years. This guidance from Earth could be disrupted by war or natural disasters such as solar storms, which may threaten military, government and business operations.
XNAV would enable autonomous navigation, minimizing the necessity of communications with Earth. Moreover, the new method is expected to provide faster estimation of spacecraft location as current systems are limited by the time delay at great distances. XNAV is also seen as the cheaper alternative for radio-based systems, as it would require reduced ground infrastructure and because X-ray telescopes can be made smaller and lighter.
In 2014, a feasibility study was carried out by the National Aerospace Laboratory of Amsterdam, for use of pulsars in place of GPS in aircraft navigation. The advantage of pulsar navigation would be more available signals than from satnav constellations, being unjammable, with the broad range of frequencies available, and security of signal sources from destruction by anti satellite weapons.
Many nations have joined the race to develop this new satellite positioning and navigation systems , but none are yet ready for use in the field. There are many challenges, including developing detectors with extremely high sensitivity that can pick up weak signals from tiny stars thousands of light years away.
The Advanced Concepts Team of ESA studied in 2003 the feasibility of x-ray pulsar navigation in collaboration with the Universitat Politecnica de Catalunya in Spain. After the study, the interest in the XNAV technology within the European Space Agency was consolidated leading, in 2012, to two different and more detailed studies performed by GMV AEROSPACE AND DEFENCE (ES) and the National Physical Laboratory (UK).
NASA has already announced plans to use the technology on spaceships taking astronauts to the moon and Mars. Keith Gendreau, an astrophysicist at NASA’s Goddard Space Flight Center and a team of NASA researchers announced in Jan 2018 that they had finally proven that pulsars can function like a cosmic positioning system. With the help of an enhancement known as the Station Explorer for X-ray Timing and Navigation Technology (aka Sextant), Neutron Star Interior Composition Explorer (NICER), a pulsar-measuring instrument was able to determine the station’s position in Earth’s orbit to within roughly three miles—while it was traveling in excess of 17,000 miles per hour. SEXTANT is a NASA-funded project at the Goddard Space Flight Center that tested XNAV on-orbit on board the International Space Station in connection with the NICER project.
On 9 November 2016 the Chinese Academy of Sciences launched an experimental pulsar navigation satellite called XPNAV 1. XPNAV-1 will characterize 26 nearby pulsars for their pulse frequency and intensity to create a navigation database that could be used by future operational missions. The satellite is expected to operate for five to ten years. XPNAV-1 is the first pulsar navigation mission launched into orbit. China’s Insight-HXMT satellite and Nasa’s NICER/SEXTANT instrument, which is mounted on the International Space Station, are both aimed at neutron stars that emit pulses of electromagnetic radiation with a regularity that makes these pulsars more stable than an atomic clock for timekeeping.
In August 2019, Researchers with the Institute of High Energy Physics at the Chinese Academy of Sciences in Beijing said that a Chinese scientific satellite had managed to calculate its own position in space by using the X-ray emitted by a small, distant star for reference. With a margin of error of just 3.3km (2 miles), the accuracy was an improvement of more than 30 per cent on a similar Nasa experiment last year, which had a 5km margin of error.
Zhang Xinyuan, a scientist studying pulsar navigation technology at the Qian Xuesen Laboratory under the China Academy of Space Technology in Beijing, said the Insight-HXMT team had done some “excellent work”. But he said it was difficult to say whether China’s technology had outperformed that of the US because they had used different technical approaches. For instance, the Chinese satellite was looking at only one pulsar with three different types of telescopes while the US instrument monitored five stars using more than 50 small, identical telescopes.
Zhang said the pulsar navigation system could provide emergency backup when ground-based navigation failed, but in the near future it would not replace the existing technology – especially for many civilian applications that need a fast, precise positioning service. GPS and Beidou can now pinpoint a target with accuracy that is within centimetres.
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