How is GPS used in spaceflight?

This could have been a headline written in the fall of 1846 if George and Jacob Donner had access to the Global Positioning System, a highly accurate navigational technology relying on signals from an array of satellites orbiting about 12,500 miles (20,200 kilometers) above Earth’s surface [source:]. Unfortunately for the Donner brothers and their ill-fated band of pioneers, GPS would require another 100 years of R&D, leaving them to find their way to California using compasses, maps and bad advice. In the end, their long journey turned into a tortuous nightmare. They became snowbound in the Sierra Nevada Mountains, where many in their party died before rescuers could reach them in the spring.

Spacefaring explorers may face similar tragedies ifthey can’t find a reliable method to orient themselves as they travel to distant planets and, perhaps, faraway stars. GPS seems like the logical candidate for such endeavors, but the system only works if your travel is limited to Earthly destinations. That’s because the 24 satellites that make up the GPS «constellation» transmit their signals toward Earth. If you’re located below the satellites and have a receiver capable of detecting the signals, you can reliably determine your location. Cruising along the planet’s surface? You’re good to go. Flying in low-Earth orbit (LEO)? You’re covered. Venture above LEO, however, and your handy GPS receiver will quickly find itself above the satellite constellation and, as a result, no longer be able to record a signal. Put another way: GPS satellites only transmit down, not up.

This doesn’t mean missions to destinations beyond Earth have to fly blind. Current navigational techniques use a network of earthbound tracking stations that look up and out into space. When a rocket leaves our planet for Mars, Jupiter or beyond, ground crews beam radio waves from the tracking stations out to the vessel. Those waves bounce off the craft and return to Earth, where instruments measure the time it took the waves to make the journey and the shift in frequency caused by the Doppler effect. Using this information, ground crews can calculate the position of the rocket in space.

Now imagine you want to travel to the outer reaches of the solar system. When your spacecraft reaches Pluto, you’ll be 3,673,500,000 miles (5.9 billion kilometers) away from Earth. A radio signal sent by a tracking station would take 5.5 hours to reach you and then another 5.5 hours to travel back (assuming the waves were traveling at the speed of light), making it more difficult to pinpoint your exact location. Travel even farther, and the accuracy of earthbound tracking systems falls off even more. Clearly, a better solution would be to place a navigational instrument on the spacecraft so it could calculate its position independently. That’s where pulsar navigation, an innovation of NASA’s Goddard Space Flight Center, comes in.

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.

Although the GPS we use on Earth isn’t helpful for interplanetary travel, its principles apply to other navigational systems. In fact, using pulsars to orient yourself in the solar system resembles earthbound GPS in many ways:

First, just as a GPS receiver triangulates a position using data from four or more satellites, you need more than one pulsar to determine an object’s precise location in space. Luckily, astronomers have discovered more than 2,000 pulsars over the years [source: Deng]. The best candidates for navigation, however, are stable pulsars that blink on and off in the millisecond range and that emit strong X-ray signals. Even with those limitations, a number of possibilities remain. Some pulsars under consideration include J0437−4715, J1824−2452A, J1939+2134 and J2124−3358 [source: Deng].
Next, you need something to detect the signals emitted by the pulsars. This would be equivalent to the GPS receiver, but it would need to be sensitive to X-ray radiation. A number of observatories have X-ray telescopes, though they are far too big to strap to a spacecraft. The next generation of detectors, known as XNAV receivers, will be much smaller and easily carried into space.
Finally, you need algorithms to make all of the appropriate calculations. Teams of scientists have worked out the math over several years, using a complex set of equations to account for variables such as pulsar spin irregularities and the effects of external phenomena — gravitational waves or plasma — on the propagation of the waves. Although the math is challenging, the basic idea is the same as earthbound GPS: The XNAV receiver would detect signals from four or more pulsars. Each signal would carry a precise time stamp, allowing a computer to calculate changes as a spacecraft moved farther from some pulsars and closer to others.
The last hurdle, of course, is testing the theory to see if it holds up. That will be one of the key objectives of NASA’s NICER/SEXTANT mission. NICER/SEXTANT stands for Neutron-star Interior Composition Explorer/Station Explorer for X-ray Timing and Navigation Technology, which describes an instrument consisting of 56 X-ray telescopes bundled together in a mini-refrigerator-sized array [source: NASA]. Slated to fly on the International Space Station in 2017, the instrument will do two things: study neutron stars to learn more about them and serve as a proof of concept for pulsar navigation.

If the NICER/SEXTANT mission is successful, we’ll be one step closer to autonomous interplanetary navigation. And perhaps we’ll have the technology in place to avoid a Donner-like disaster in outer space. Being lost at the edge of the solar system, billions of miles from Earth, seems a tad more frightening than wandering off the beaten path on your way to California.

Author’s Note: How is GPS used in spaceflight?
Remember «Lost in Space,» the campy sci-fi TV show that aired in the late 1960s? I watched it in reruns during the ’70s and loved every minute of it. It seemed kind of cool to be lost in space back then. Now, with some perspective, it seems utterly terrifying. If pulsar navigation becomes a reality, at least that aspect of spaceflight — finding your way — will become less intimidating.

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