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.