To find your favorite coffee shop in an unknown city, getting directions via satellite works like a charm. But that technology won't get you from Earth to Jupiter.
So theorists have proposed a new type of positioning system based on blinking stars instead of satellites. By receiving radio blips from pulsars, stars that emit radiation like clockwork, a spacecraft above the atmosphere could figure out its place in space.
Unlike the Global Positioning System of satellites used in cars and smart phones, the pulsar positioning system wouldn't need humans to make daily corrections.
"You could be on a spacecraft and you could be able to navigate without having any help from Earth," says Angelo Tartaglia, a physicist at the Polytechnic University of Turin in Italy.
Though the navigation system proposed by Tartaglia and colleagues is just a proof of concept, a GPS-like system under construction in Europe called Galileo could implement the ideas within a decade, he says.
The principle behind the pulsar positioning is not too different from ordinary GPS. The GPS receiver in a car or phone receives radio signals from satellites orbiting the Earth.
The satellites are synchronized with atomic clocks to emit signals simultaneously. Because the satellites are all different distances from the receiver, each message reaches the device at a different time. From those time differences, a GPS device infers the distance to each satellite, and hence can calculate its own position.
The best consumer devices can pinpoint your location to within a meter under ideal conditions, but tall buildings or other interference can throw them off by 10 to 20 meters or more.
Because the satellites move so fast (they orbit the Earth twice every day), Einstein's special theory of relativity must be considered. Relativity requires that clocks on board tick slower than those on Earth.
After two minutes, the satellite's clocks are already out of sync with Earth clocks. Transmitting the correct time to each satellite is a constant chore for the Department of Defense, which determines the real time from an ensemble of clocks on Earth.
A pulsar's regular blips can be used to tell time just like the signals received from GPS satellites. But the math in the new pulsar-based system already accounts for relativity, so those corrections aren't necessary.
Pulsars, the dense leftovers of supernovae that sweep beams of radiation from their poles, serve as really good clocks, in some cases comparable to atomic clocks. Plus, a pulsar doesn't move much relative to the Earth in the time between its pulses, and the distance it does move over several months is predictable.
Instead of tracking real pulsars, the Italian team simulated its proposed navigation system on computers by using software that mimics pulsar signals as if they were received at an observatory in Australia.
The researchers recorded these fake pulses every 10 seconds for three days. Inferring the distance between the pulsars and the observatory, the team tracked the trajectory of the observatory on the Earth's spinning surface to an accuracy of several nanoseconds, or the equivalent of several hundred meters, the team reported in a paper posted at arXiv.org on October 30.
Pulsars are extremely weak sources, however, and detecting them normally requires a large radio telescope -- a heavy payload for spacecraft. So the researchers propose to create their own sources of pulsing radiation by planting bright radio wave emitters on celestial bodies like Mars, the moon or even asteroids.
At least four sources have to be visible at a time to determine a position in the three dimensions of space and one dimension of time. Including just one particularly bright radio pulsar outside the plane of the solar system would be ideal because it would be the tip of a tetrahedron, a configuration that would make calculations more accurate, Tartaglia says.
Or, you could look for pulsars that emit X-rays, a much brighter signal. X-ray antennas are also smaller and lighter, says physicist Richard Matzner at the University of Texas at Austin. Their drawback is oversensitivity to electrons surrounding the Earth.
But an X-ray–based positioning system could pinpoint an object to within 10 meters, an improvement on the 100-meter or so accuracy of the radio pulsar system.
Either system would be accurate enough to track a spacecraft speeding at 19,000 meters per second, the maximum speed the exploratory spacecraft Cassini reached zipping past the Earth in 1999 on its way to Saturn.
It's easy to calculate a satellite's position along the line of sight by measuring Doppler shift -- the change of frequency with an object's speed -- but more difficult to create a three-dimensional picture of a spacecraft trajectory, says Scott Ransom, an astronomer at the National Radio Astronomy Observatory in Charlottesville, Va. A pulsar system could track those three dimensions and detect if the spacecraft was straying from its course.
Pulsar-based systems may not be as precise as GPS, but they could be a backup system for GPS if the ground control for the satellites fails.
"It would be better than nothing," says Matzner. "It's an insurance policy."