Satellite orbit has been determined autonomously within 10 km (3σ) by observing an X-ray pulsar with the Hard X-ray Modulation Telescope (Insight-HXMT) satellite, according to a study by Chinese scientists just published in the Astrophysical Journal Supplement¹.

On June 15, 2017, China launched its first X-ray astronomy satellite, Insight-HXMT. It consists of three X-ray slat-collimated telescopes – the High Energy X-ray Telescope, the Medium Energy X-ray Telescope, and the Low Energy X-ray Telescope – as well as a Space Environment Monitor. Many black holes, neutron stars and gamma-ray bursts have been observed in Insight-HXMT’s two-plus years of operation. In addition, in-orbit demonstrations of X-ray pulsar navigation technology have been carried out, as described in the recently published paper.

Mankind never stops exploring the universe. Voyager 1 and Voyager 2, launched in 1977, are still cruising in deep space. Nowadays, more and more space probes have been flying towards the Sun and its major planets, asteroids, comets and other objects in the solar system. With these spacecraft far away from Earth, it’s become more and more difficult for global navigation satellite systems (GNSS) to provide reliable navigation services. At present, radio technologies (e.g., the U.S. deep-space network) are applied in deep space, however, with many limitations. Meanwhile, pulsar navigation, an autonomous navigation technology, has been receiving more and more attention since it is less dependent on the support of ground equipment and meets the continuous navigation requirements for deep-space exploration.

“X-ray pulsar navigation is a new type of autonomous navigation,” said Shijie Zheng, principal investigator in charge of the pulsar navigation demonstration, “It uses periodic pulse signals from pulsars – which are distant celestial objects – to provide navigation and timing services for spacecraft.”

Pulsars, a kind of rapidly rotating neutron star, are compact stars produced in supernova explosions. They are sometimes called “celestial GPS satellites” or “cosmic lighthouses” because of their long-term timing stability, which is comparable to atomic clocks on Earth. By detecting the periodic pulse signals of pulsars, a spacecraft can autonomously determine its orbital parameters, i.e., conduct pulsar navigation. After successfully testing X-ray pulsar navigation on the International Space Station (ISS), the National Aeronautics and Space Administration (NASA) stated that X-ray pulsar navigation technology should be applied to the Gateway mission (a lunar-orbiting space station) and Mars exploration mission².

Pulsar navigation is based on a fundamental principle: The time interval (or pulse period) of two adjacent pulses emitted by a pulsar is constant. If a spacecraft moves towards a pulsar, the received pulse interval will be shortened, and vice versa. Thus, the observed pulse profile will change as the spacecraft moves in space. The relative arrival times of pulses also describe the relative position of the spacecraft with respect to the pulsar. Therefore, by analyzing the characteristics of the pulsar signals received by the spacecraft from different directions, the three-dimensional position and velocity (or orbital motion) of the spacecraft can be determined.

Far from Earth (e.g., hundreds or thousands of light-years or more), the pulse signals of pulsars cannot be influenced by mankind, and the positional accuracy does not vary with different orbits in space; therefore, pulsar navigation is an attractive navigation technology in deep space. In 2004, ESA released a technical report on its pulsar navigation feasibility study, noting that it is suitable for large spacecraft³. In January 2018, NASA announced that a test it had conducted using the Neutron star Interior Composition ExploreR (NICER) “proves pulsars can function as a celestial GPS"⁴. By measuring tiny changes in the arrival time of pulses, NICER was able to pinpoint its location to within five kilometers (RSS or 1σ)⁵.

In China, many theoretical and experimental studies on pulsar navigation have been carried out. “In September 2016, the TG-2 space station was launched,” said Shijie Zheng. “With POLAR (Gamma-ray Burst Polarimeter) onboard TG-2, we successfully carried out the first pulsar navigation test in China⁶. In November, the X-ray pulsar navigation-I (XPNAV-1) was launched.”

Shuangnan Zhang, the principal investigator of the Insight-HXMT mission, discussed the in-orbit pulsar navigation demonstration: “From August 31 to September 5, 2017, Insight-HXMT observed the famous Crab pulsar for about five days to test the feasibility of pulsar navigation. The new X-ray pulsar navigation algorithm SEPO (Significance Enhancement of Pulse-profile with Orbit-dynamics) was proposed by our team in 2016, and has been verified by the POLAR experiment7.” This time, the researchers further improved the algorithm and applied it to observational data from the three telescopes onboard the Insight-HXMT satellite. The study showed that the orbit could be successfully determined using data from any of the three telescopes, respectively. By combining all the data from three telescopes, the position of the Insight-HXMT satellite was pinpointed to within 10 km (3σ), which is comparable to that of NICER/SEXTANT on ISS. To test the feasibility and reliability of SEPO, the team carried out theoretical analysis and simulation verification using various types of pulsars. Their results show that the method works for different pulsars.

The referee from the Astrophysical Journal Supplement noted that “the flight demonstrations from the Insight-HXMT satellite are important contributions to the development of X-ray navigation. . . . In particular, the simulation section added to the end of the paper establishes mathematically that the approach is valid. I appreciate the additional hard work that went into the paper and I believe it is a nice contribution to the XNAV community.”

Insight-HXMT is China's first X-ray astronomical satellite. It is supported by the China National Space Administration (CNSA) and the Chinese Academy of Sciences Space Science Project (Phase I). It was launched on June 15, 2017 with a design life of four years. The satellite platform and all payloads continue to operate normally. The current research was supported by the National Key R&D Program of China and the National Natural Science Foundation of China.

1. In-orbit demonstration of X-ray pulsar navigation with the Insight-HXMT satellite，ApJS, 244,1, 2019 
2. https://www.nasa.gov/feature/goddard/2019/nasa-navigation-tech-shows-timing-really-is-everything
3. Feasibility Study for a Spacecraft Navigation System relying on Pulsar Timing Information 
4. NASA test proves pulsars can function as a celestial GPS, Nature News, 47, 9505, 2018
5. https://files.aas.org/aas231/aas_231_press_2018-01-11_10_15_its_amazing_what_you_can_do_with_space_telescopes.mp4
6. Test of Pulsar Navigation with POLAR on TG-2 Space Station，SSPMA,  47, 09, 2017.