December 28, 2016

China launched spacecraft to use and develop millisecond pulsar navigation

In November, China launched the world's first spacecraft with a deep space x-ray navigation system. The X-ray Pulsar Navigation 1 (XPNAV 1) satellite, which the country launched on Nov. 10 aboard a solid-fueled Long March 11 rocket from the Jiuquan Space Launch Center in the Gobi Desert, is the world's first x-ray navigation system to go in orbit, beating out NASA's Station Explorer for X-ray Timing and Navigation Technology (SEXTANT), which is scheduled to be installed on the ISS next year.

Millisecond pulsars generate x-ray pulses at such short intervals, that by measuring the time differential from multiple known pulsars (like a GPS using pulsars instead of satellites), a spacecraft can determine its location in the solar system within 5 kilometers (3.1 miles), which is pretty good for deep space. The trick is to find pulsars that provide pulses at a consistent pace; x-ray pulsars often speed up or slow down the frequency of their bursts.

If all goes as planned, the XPNAV 1 will both gather data to build the pulsar x-ray database and then be able to use that data to independently verify its location. The 529-pound satellite has two detectors to measure x-rays generated by pulsars. Over the next five to ten years, XPNAV 1 will build a database of x-rays from 26 pulsars, measuring their frequencies against other electromagnetic activity in space. It will also measure the accuracy and consistency of pulsar x-rays against background space noise, without having to worry about atmospheric interference. It will verify the usability of the data by testing the data to see if the data can predict the satellite's location, independent of other navigation aids.

The advantages of x-ray navigation include greater accuracy and reliability; spacecraft wouldn't need to rely on radio signals that take longer to travel into deep space and lose signal fidelity. X-ray navigation is also cheaper, because the spacecraft would no longer need large, expensive ground-based radio antennae for navigation signals. Additionally, the spacecraft would be more autonomous, saving bandwidth for the transmission of scientific data back to earth.

Simulations made by the team provided crucial measurements regarding the future development of the XNAV method. The scientists concluded that at the distance of Neptune (about 30 astronomical units from the Earth), a 3-D location of a spacecraft with an accuracy of 18.6 miles (30 kilometers) can be calculated by locking onto three pulsars. Moreover, they estimated that even an accuracy of 1.25 miles (2 kilometers) can be achieved when locking onto a particular pulsar, called PSR B1937+21, for 10 hours. Due to the fact that PSR B1937+21 is a millisecond pulsar, completing almost 642 rotations per second, and thanks to its very stable rotation, it is capable of keeping time as well as atomic clocks.

The Neutron star Interior Composition Explorer (NICER) is an International Space Station (ISS) payload devoted to the study of neutron stars through soft X-ray timing. Neutron stars are unique environments in which all four fundamental forces of nature are simultaneously important. They squeeze more than 1.4 solar masses into a city-size volume, giving rise to the highest stable densities known anywhere. The nature of matter under these conditions is a decades-old unsolved problem, one most directly addressed with measurements of the masses and, especially, radii of neutron stars to high precision (i.e., better than 10 percent uncertainty). With few such constraints forthcoming from observations, theory has advanced a host of models to describe the physics governing neutron star interiors; these models can be tested with astrophysical observations.

From NICER's ISS platform, a star-tracker-based pointing system allows the XTI to point to and track celestial targets over nearly a full hemisphere. The pointing system design accommodates the ISS vibration and contamination environments, and enables (together with NICER's GPS-based absolute timing) high-precision pulsar light-curve measurements through ultra-deep exposures spanning the 18-month mission lifetime. Anticipated launch of NICER is in early 2017.

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