GPS uses precise measurements of time to make calculations. Each GPS satellite contains an atomic clock, and its time is synchronized with a receiver's. A receiver can calculate the range to the satellite by multiplying the time it takes the satellite's signal to reach the receiver by the speed of the signal, which is the speed of light. If it takes 0.07 seconds for the signal from a satellite to reach the receiver, then the satellite's range is 13,020 miles (186,000 miles per second × 0.07 seconds).
A rocket could make similar calculations if it could receive time signals emitted by something out in space. As luck would have it, the universe contains more than a few highly accurate timekeeping devices. They're known as pulsars -- rapidly rotating neutron stars that emit regular pulses of electromagnetic radiation. At one point in its life, a pulsar was living large and burning bright. Then it used up its nuclear fuel and died in a massive explosion. The product of that explosion was a rapidly spinning, highly magnetized object whose poles emitted powerful beams of energy. Now, as the dead star spins, the beams sweep around, much like the beacon of a lighthouse. An observer on Earth can't see the star itself, but he can see the pulses of light that come streaming through space.
Some pulsars blink on and off every few seconds; others blink far more rapidly. Either way, they always pulse with a constant frequency, which makes them useful in keeping time. In fact, as timekeeping devices, pulsars rival atomic clocks in terms of their precision. In 1974, a scientist at the Jet Propulsion Laboratory -- G.S. Downs -- first proposed the idea of using pulsars to help spacecraft navigate through the cosmos. The concept remained on paper because scientists still didn't know enough about the enigmatic stars and because the only instruments available to detect pulsars -- radio telescopes -- were enormous.
Over the years, the field advanced. Astronomers continued to discover pulsars and to study their behavior. In 1982, for example, scientists discovered the first millisecond pulsars, which have periods of less than 20 milliseconds. And in 1983, they found that certain millisecond pulsars emitted strong X-ray signals. All of this work made it possible to move pulsar navigation from paper to practice.