# How is GPS used in spaceflight?

Galactic GPS
This artist's rendition shows the NICER/SEXTANT payload. The 56-telescope payload will fly on the International Space Station.
Image courtesy NASA

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:

1. 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].
2. 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.
3. 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.