Deep Space GPS from Pulsars

By Mark Trodden | March 31, 2012 7:12 am

This semester I’m teaching General Relativity, and as part of discussing gravitational waves, this week I briefly discussed pulsars. It was quite timely therefore when I learned of a new proposal that pulsars may ultimately provide a perfect navigation system for spacecraft far from Earth.

Here on Earth, the Global Positioning System (GPS) gives us a highly accurate way of determining position, and many of us now use hand-held devices every day to help with directions. These work because GPS satellites provide a set of clocks, the relative timings of the signals from which can be translated into positions. This is, by the way, another place where both special and general relativity are crucial to how the system works. Out in deep space, of course, our clocks are unfortunately useless for this purpose, and the best we currently can do is by comparing the timing of signals as they are measured back on Earth by different detectors. But the accuracy of this method is limited, since the Earth is a finite size, and our terrestrial detectors can therefore only be separated by a relatively small amount. The further away a spacecraft is, the worse this method is.

What Werner Becker of the Max-Planck Institute for Extraterrestrial Physics in Garching has realized (and announced yesterday at the UK-Germany National Astronomy Meeting in Manchester), is that the universe comes equipped with its own set of exquisite clocks – pulsars – the timing of which can, in principle, be used to guide spacecraft in a similar way to how GPS is used here on Earth. Of course, it isn’t quite as simple as all that.

A significant obstacle to making this work today is that detecting signals from the pulsars requires X-ray detectors that are compact enough to be easily carried on spacecraft. However, it turns out the relevant technology is also needed by the next generation of X-ray telescopes, and should be ready in twenty years or so. Perhaps one day our spacecraft will map their routes through the cosmos thanks to yet another spinoff from basic research.

  • Mohsin Rasheed

    What are your personal views about Einstein’s theory (General + Special).
    By the way, good article.

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  • Bob F.

    Great idea. But does anyone know what is the life expectency of a pulsar? And does it end its life by slowing down (which could be figured into the calculations) or does it explode? I understand we’re dealing with time spans that are many times greater than human civilization and the age of the Earth, but nobody wants to be in the middle of deep space and hear “recalculating!”

  • Bjoern

    I thought that most pulsars emit mainly radio waves, only some emit X-rays? So why are X-ray detectors crucial here?

    @Bob: Pulsars usually slow down at known (measured) rates, they usually don’t explode (only if they are in binary systems with another stars from which they can draw material, but that is also known for most nearby pulsars, since those stars can be seen!) And their life expectancy is *very* long. (I don’t have exact numbers at hand right now, but “billions of years” is quite a short time for them)

  • Chris

    So maybe we were wrong about the origin of pulsars. Perhaps the aliens have set up a giant GPS (Galactic Positioning System)!

    Although didn’t we already know this decades ago? On the Pioneer and Voyager plaques we put the location of the Earth using the pulsar timing and distances.

  • Keith Arnaud

    I e-mailed the BBC journalist about this to point out it is not a new idea. There is a group at NASA Goddard (including my office mate) who have a space station experiment (NICER/Sextant) which will test this idea if it gets pass the final selection hurdle. The experiment is a 2-for-1 because it is also designed to do pulsar science as well. If all goes well it will be installed in 2016.

  • EB

    Also, the precise timing provided by pulsars is already being used by scientists as a means to detect gravitational waves. Similar to LIGO and VIRGO, but at much lower frequency, these “Pulsar Timing Arrays” will detect large ripples in space-time as the gravity wave shifts individual pulsars closer and farther away from us.

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  • Mark Trodden

    EB: indeed – that’s one of the things I mention in my GR class.

  • Tom Renbarger

    “So maybe we were wrong about the origin of pulsars. Perhaps the aliens have set up a giant GPS (Galactic Positioning System)!”

  • Lab Lemming

    Do pulsars transmit in all directions? I thought they were rotating collimated emissions.

  • Chris

    @11 Lab
    They are collimated, but over the vast distances even a point source will diverge. Let’s assume a pulsar is 1000 light years away and it’s emitting 1 square degree from its surface. Well 1 square degree at 1000 light years is about 304 square light years, compared to the 12.6 million square light years of a sphere 1000 light years in radius. At least in our local neighborhood the pulsars should easily be seen and tracked.

  • Lab Lemming

    304 square light years is a square ~17 light years on a side, or a circle with a radius ~10 light years. There aren’t that many stars in such a small area. In fact, I don’t think there are even any known planets closer than 10.5 light years from here.

  • Rhys

    @Lab Lemming:
    My immediate thoughts were similar. But the article I read suggested worthwhile use for such a system at the scale of the solar system (for example, the uncertainty in the positions of the Voyager spacecraft is apparently huge). Since it seems we won’t be sending anything over light-year distances any time soon, our concerns are irrelevant for now.

  • Chris

    @Lab Lemming
    I’m sure that over the many many years it would take to traverse these distances, they would have plenty of time to find new pulsars and use those as new position markers. Just as if you or the satellites move, you switch to a new satellite to keep your GPS working. However if they are moving at relativistic velocities, they may not have as much time as they’d like to calibrate new pulsars.

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  • Aeronin

    @Lab Lemming:
    My understanding of Chris’ relevant calculation/ contention was that it should be fairly easy to detect a 1 square-degree collimated source, as any within a 1000 ly radius would project at least a 304.62 ly^2 area (or 17.45 ly-square, as you point out). This is ~4x the area projected by the Moon (0.518 deg-square). I’m not quite sure what your point was about there not even being any planets within a 10.5 ly (or, by implication, pulsars within a 17.45 ly) radius sphere, as 1000 ly was the distance in question?

    On the other hand (and perhaps this was your underlying point), I do wonder about the probability of sufficient Earth-wise orientations of such bi-polar pulsar collimations. Imagine a ‘shell’ of space of arbitrary radius centered on the Earth; any such spherical shell contains ~41,253 1-deg-square regions. Let’s denote the number of pulsars existing at an **observable** distance somewhere within this sphere as N. Therefore, the probability of one of these pulsars being found in any given 1-deg-square region is at most N/41,253.

    Now, if the orientations of these N pulsars themselves are randomly distributed, the odds of **observing** one of these reduce to about 2xN/41,253^2, or 1.18E-9xN (odds of at least one of the pulsars in any given 1-deg-sector having one of its two polar ‘beams’ oriented in the reciprocal 1-deg-square region containing, and therefore visible from, Earth). So, to have even a 1% chance of **observing** a pulsar in a given 1-deg-square sector, there would have to exist at least 8,509,034 pulsars within such a sphere.

    I’m sure that at some radial distance from Earth, even given the low percentage of pulsars/stars, this number would be satisfied, as there are 100’s of billions of stars in the Milky Way galaxy; but would said distance exceed the observable limits for pulsars? I’m not so sanguine about that.

    Additional thought: This would represent a minimum probability, as many pulsars have quite a large, measurable precession of their polar emmission spectra that would effectively enlarge the area of observability to an annulus swept by this 1-deg square. Just thinking out loud …

    Now, how about quasars instead? Any takers? More ‘stable’ from a relativistic speed-time perspective, but not as dependable over time?

  • Simon

    It is a cool technique – only thing is that this idea has been around for about 8 years, and as Keith says, is already is close reaching the implementation stage at GSFC. So it is a bit misleading to say that Werner Becker has just realized …. and announced yesterday .
    Some publication history:
    Implementation details:


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Cosmic Variance

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About Mark Trodden

Mark Trodden holds the Fay R. and Eugene L. Langberg Endowed Chair in Physics and is co-director of the Center for Particle Cosmology at the University of Pennsylvania. He is a theoretical physicist working on particle physics and gravity— in particular on the roles they play in the evolution and structure of the universe. When asked for a short phrase to describe his research area, he says he is a particle cosmologist.


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