TEMPO2 Uses Pulsars For Celestial GPS

In his classic 1952 story Troubled Star, sf writer George O. Smith writes about the process by which selected stars might be turned into 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. We use the longer variable types for other things - dangerous places like cloud-drifts, or a dead sun that might be as deadly to a spacecraft as a shoal is to a seagoing vessel. It's all very logical."

In his wry 1959 story The Repairman, sf author Harry Harrison wrote about the intricacies of using hyperspace beacons for galactic travel and navigation:

"The first ships to enter hyperspace had no place to go - and no way to tell if they had even moved. The beacons solved that problem and opened up the entire universe. They are built on planets and generate tremendous amounts of power. This power is turned into radiation that is punched through into hyperspace. 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. For long jumps, navigators use up to seven or eight. So every beacon is important and every one has to keep operating. That is where I and the other troubleshooters come in."

Once rapidly rotating neutron stars - pulsars - were discovered in 1967, scientists eagerly considered using them for celestial navigation, owing to their extraordinary regularity.

Scientists Matteo Ruggiero and colleagues at the Politecnico di Torino in Italy have demonstrated that it is possible to calculate the trajectory of a point on the Earth's surface through spacetime relative to four pulsars.


(TEMPO2 tracks Earth's movement through the stars)

[T]here is a software package called TEMPO2 that simulates the signals that pulsars would produce anywhere on the Earth's surface. Ruggiero and co use this to simulate the signals that the telescope at Parkes would receive over three days, were it able to monitor four pulsars simultaneously.

This situation is relatively straightforward mathematically since the pulsars can be thought of as stationary and their frequencies constant.

Ruggiero then compare the result to a different calculation of Earth's movement made using ephemerides, the position of various astronomical objects in the sky.

Both trajectories are plotted in the figure above and, at this scale, show pretty good agreement. In fact, Ruggiero and co say that the limiting factor is the accuracy of the clock used to measure the pulsar signals. "These preliminary results show the feasibility of the use of pulsating sources for positioning purposes, in a fully relativistic framework," they say.

See also this earlier related story XNAV Steer Your Way By X-Ray Pulsar .

Via MIT's Technology Review blog.

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