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China's XPNAV 1 To Use X-Ray Pulsars For Navigation

China just launched XPNAV 1, the world's first x-ray navigation system. The X-ray Pulsar Navigation 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,


(XPNAV-1 X-ray Pulsar Navigation satellite)

The navigation system relies on x-ray pulsars found in systems with two stars. Essentially a dense neutron star's strong magnetic field pulls in gas from the other star, and when the gas impacts the neutron star, it generates a strong X-ray hotspot. If the neutron star's spin axis and magnetic axis are not aligned, as the neutron star rotates, pulses will be generated as the X-ray hotspots move in and out of the observer's view. This turns out to be a useful tool for navigation.

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.

Science fiction writers of the 1950's were fascinated by this idea. In his 1952 story Troubled Star, George O. Smith described space beacons:

"And what is a beacon?"

"It is a phenomenon caused by the Doppler effect when traveling at galactic speeds. In this case, when coming through this rift at fifteen hundred light years per hour, a three-day variable star will appear to the observer as a rapidly blinking light..."

"We use the three-day variable to denote the galactic travel lanes. Very effective.
(Read more about Smith's space beacon)

In his 1956 story The Repairman, Harry Harrison described a hyperspace beacon:

Every beacon has a code signal as part of its radiation and represents a measurable point in hyperspace. Triangulation and quadrature works for navigation - only it follows its own rules.

For a hyperspace jump, you need at least four beacons for an accurate fix.
(Read more about Harrison's hyperspace beacon)

In 1980, Star Trek Maps was published; it consisted of a set of four maps and an Introduction to Navigation booklet. In the accompanying pamphlet, they described the standard navigation system using "sub-space beacons", and then described the emergency system that used pulsars as a GPS system. It included the real equations as well.


(Star Trek Maps: galactic coordinates and basic vector calculus)

XPNAV 1 is the world's first x-ray navigation system to go in orbit. NASA's Station Explorer for X-ray Timing and Navigation Technology (SEXTANT), will not be installed on the ISS until 2017.

Via PopSci.

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