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Galactic GPS Put Through Its Paces

For the first time, astronomers have reconstructed Earth’s trajectory through the cosmos using pulsar signals

Pulsars are rapidly rotating neutrons stars that emit a beam of radio waves. We seem them pulsing when the beam sweeps over us as they rotate. The rotation rate is amazingly regular, with some pulsars rivalling atomic clocks in their precison.

Soon after pulsars were discovered, astronomers and science fiction writers began to speculate about the possibility of using pulsars as celestial navigation beacons allowing interstellar travellers to find their way home, rather like the GPS system we have now.

That’s a simple idea but it belies some technical difficulty. At the speeds that most astronomical objects are travelling, relativity becomes important and that significantly increases the complexity of the calculations. But there’s no reason in principle why pulsars can’t be used in this way.

Today, Matteo Ruggiero and buddies at the Politecnico di Torino in Italy show us exactly how such a system would work by calculating the trajectory of a point on the Earth’s surface through spacetime relative to four pulsars. This movement is essentially the result of the Earth’s rotation and its orbit around the Sun.

The point they choose is the Parkes Observatory in Australia, a radio telescope whose roll in the Apollo landings was made famous by the movie The Dish.

In one sense, Parkes is good choice because it can easily observe the signals from pulsars. In another it is entirely arbitrary because Parkes can only observe one pulsar at a time and at least four are needed to tackle this problem .

Luckily, there is a simple way out of this limitation since there 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.

That’s not to say we’ll be using celestial GPS any time soon. But it does show the potential of the technique for, say, interplanetary navigation, perhaps with the help of artificial ‘pulsars’ in the form of beacons on interplanetary spacecraft.

Ref: arxiv.org/abs/1011.0065 : Pulsars As Celestial Beacons To Detect The Motion Of The Earth

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