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Pulsar GPS

September 11, 2013

One question that philosophers and scientists have asked themselves throughout the ages is how you could communicate information to a remote, non-human culture. One problem before the middle of the twentieth century was communicating the difference between right and left, or up and down. That problem was settled by an experiment which showed that parity was violated in beta decay.

Since it's presumed that the laws of physics are the same no matter where you are in the universe, it makes sense to define quantities with reference to fundamental physical constants. If you could somehow communicate the concepts of helium, neon, and lasing, then you could communicate the dimensions of objects with reference to the 632.8 nm wavelength of the He-Ne laser. You need to be careful in your wording, however, since a He-Ne laser will also emit at other wavelengths, notably 1150 nm.

When it was realized that the Pioneer 10 and Pioneer 11 spacecraft would eventually escape our Solar System, it was decided that there should be information placed on them as to their origin in the unlikely event that they would find their way into extraterrestrial, or perhaps future human, hands (or tentacles). This was more of a symbolic gesture to contemporary humans than anything else.

This task was undertaken by American astronomer Carl Sagan, principally because he was one of the few scientists at the time with an avowed interest in communicating with extraterrestrials. In 1966, Sagan had coauthored the book, "Intelligent Life in the Universe," with Soviet astronomer, I. S. Shklovskii, a copy of which sits in my library.[1]

Carl Sagan in 1980.

Carl Sagan at the founding of The Planetary Society, 1980.

(Planetary Society/NASA/JPL photograph, via Wikimedia Commons.)

Sagan and colleague, Frank Drake, who was the first to listen in 1960 for extraterrestrial radio signals from nearby stars in Project Ozma, designed a plaque to convey essential information on Earth's location in our Milky Way Galaxy, the composition of the Solar System (with Pluto as a planet), and the appearance of male and female humans. The artwork was rendered by Sagan's wife, Linda, an artist.

The Pioneer plaque was conceived as not just a pretty picture, but as a means to convey some quantitative data. Sagan and Drake faced the problem of how to transfer position information to a scientifically knowledgeable, but alien culture. As radio astronomers, their choice of a length scale was obvious. The visible universe is mostly hydrogen, and hydrogen undergoes a fundamental, but extremely rare transition, from the spins of its electron and proton being parallel to anti-parallel.

This transition may be rare, but there's so much hydrogen around that there's an easily detected radio emission, called the hydrogen line, at 1420.406 MHz. This has a corresponding wavelength of 21.1061 cm, which is a good ruler for macroscopic objects such as humans. The emission frequency can be used as a time reference with a period of 0.704024061 nanoseconds.

As a map locating the Earth, Sagan and Drake used a radial pattern showing the relative distances of fourteen pulsars from the Sun (see figure). The pulsars are identified by their periods, in binary code, in units of the hydrogen period. There's a diagram of the hydrogen transition that specifies it as a unit time and a unit length.

Pulsar graphics portion of the Pioneer 10/11 plaque.

Pulsar graphics portion of the Pioneer 10/11 plaque, as designed by Carl Sagan and Frank Drake.

The hydrogen transition reference is depicted, also.

(A vector graphics reproduction of the Pioneer plaque by Oona Räisänen;, via Wikimedia Commons.)

The pulse period of pulsars is at least as stable as an atomic clock, but pulsars do run-down. The pulse period of pulsars is the same as their rotation rate, and they slow down as they lose energy. The period of normal pulsars increases by about 10-15 parts every rotation. The period of millisecond pulsars increases by about 10-20 parts every rotation. The period of the pulsars on the Pioneer plaque will change in time, but this is an advantage, since it will indicate the age of the spacecraft.

The idea of using pulsars for space navigation was first proposed in 1974.[2] A 2011 paper in the journal, Advances in Space Research, by physicists at the Politecnico di Torino (Polytechnic University of Turin) and the Istituto Nazionale di Fisica Nucleare (National Institute for Nuclear Physics), both located in Turin, Italy, studied this idea in greater detail.[3] This research has been followed-up by Australian scientists at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in a paper scheduled to appear in the same journal.[2,4]

The Australian research team demonstrated that pulsars can be useful for interplanetary navigation as well.[2,4] Spacecraft are presently tracked from the Earth, but precision decreases with distance from the Earth.[2] If as few as four millisecond pulsars can be observed for about an hour every seven days, spacecraft position could be ascertained to within 20 kilometers, and velocity to within a tenth of a meter per second.[2,4] One enabling technology for this is the development of compact Xray detectors that can observe the Xray component of some pulsars.[2]

Another interesting result of the study is a determination that the Earth goes around the Sun, and not vice-versa. Says team leader, George Hobbs, "This was nailed a couple of hundred years ago... but if you still need proof, we've got it."[2]


  1. I. S. Shklovskii and Carl Sagan, "Intelligent Life in the Universe," Dell Publishing paperback, January 1, 1968 (via Amazon).
  2. Pulsars make a GPS for the cosmos, CSIRO Press Release, August 16, 2013.
  3. Angelo Tartaglia, Matteo Luca Ruggiero and Emiliano Capolongo, "A null frame for spacetime positioning by means of pulsating sources," Advances in Space Research, vol. 47, no. 4 (February 15, 2011), pp. 645-653. Available, also, from arXiv.
  4. X. P. Deng, G. Hobbs, X. P. You, M. T. Li, M. J. Keith, R. M. Shannon, W. Coles, R. N. Manchester, J. H. Zheng, X. Z. Yu, D. Gao, X. Wu and D. Chen, "Interplanetary spacecraft navigation using pulsars," arXiv Preprint Server, July 20, 2013.

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