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Author for correspondence: Clément Vidal, E-mail: [email protected], http://www.clemvidal.com
Introduction
Navigation is a universal problem whenever one needs to go from point A to point B. Around Earth's orbit, Global Navigation Satellite Systems (GNSSs) such as the American Global Positioning System (GPS) or the Russian GLObal NAvigation Satellite System (GLONASS) have revolutionized the way planes, boats, cars and pedestrians navigate. GPS did not appear overnight but was the culminating outcome of ground-based radio navigation in the 1920s, gradually complemented by satellites. These satellites broadcast timing information that allows users to determine accurately not only their instantaneous position but also their instantaneous velocity. GPS requires at least 24 satellites equipped with precise atomic clocks and algorithms calculating the position of satellites, which must correct for relativistic effects predicted by Einstein's theory. GNSSs constitute a great achievement of modern science and engineering, and will continue to prove revolutionary for all kinds of location-based services in our Internet era. However useful on Earth, GNSSs are of little use for space missions in the solar system and in deep space. We could only dream of the equivalent of a GNSS on a galactic scale. Remarkably, though, it seems already to exist.
Back in 1972, Carl Sagan, with Linda Sagan and Frank Drake, famously composed ‘A message from Earth’, to be placed on Pioneer 10 as a way of communicating our position in the galaxy to any extraterrestrials who happened upon the probe. The spacetime coordinates of Earth are encoded in the message, thanks to a reference to 14 pulsars and the galactic centre (Sagan et al. 1972, see Fig. 1).
Fig. 1.
The Pioneer 10 plaque. On the left, the position of the sun is shown relative to 14 pulsars and the centre of the galaxy.
[Figure omitted. See PDF]
However, the potential of radio pulsars for galactic positioning and navigation was fully explored only two years later by Downs (1974). At that time, Downs showed that spacecraft position could be determined with an error on the order of 1500 km in the solar system, and he pointed out ways to narrow down the error.
Achieving an accuracy of even a 1000 kms may seem largely sufficient on a galactic scale, but it is not enough for solar-system missions. For...