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   Since the beginning of early navigation, sailors have relied on the sky to determine their direction, using visual markers like the North Star and the Celestial Poles to locate their whereabouts. Today, we don’t struggle with this issue as much, since we have the Global Positioning System of satellites (GPS) that we use daily in our cars and smartphones. However, this technology relies on humans to make alterations every single day, and the further you get from the satellites, the more difficult it is to pinpoint your location. While GPS is accurate enough for us to navigate on Earth, it isn’t accurate enough to bring a spaceship to Saturn. Based on recent research from NASA’s Goddard Space Flight Center, the University of Texas, and the Polytechnic University of Turin, a new solution has been introduced: a stellar GPS network, or the pulsar positioning system. The premise of the pulsar positioning system is that similar to how GPS utilizes the consistency of satellite signals to determine your area, spaceships would be able to receive radio blips from dead stars that emit radiation at constant intervals, also known as pulsars. Instead of having to rely on faraway radio telescope communications from Earth, spaceships could venture further out without having to worry about inaccurate coordinate readings.    Using the GPS system, the receiver, such as a car or phone, receives radio signals from satellites in Earth’s orbit. These satellites are adjusted with atomic clocks in order to radiate signals at the same time. These satellites are all at separate distances from the receiver, and therefore each transmission reaches the device at different times. From these differences in duration, the GPS system deduces its location. The highest quality consumer devices can estimate your location within about a meter in the best situations, but tall buildings and interference can easily disable the system by 20 meters or more.     Because these GPS satellites orbit the earth so rapidly (they complete two orbits a day), Einstein’s special theory of relativity necessitates that the clocks tick slower than those of Earth. For instance, after two minutes, the GPS satellites are already divergent from Earth’s clocks. The only way to send the correct time to the satellites is by determining the actual time from clocks on Earth and relaying the information to each satellite, which is a perpetual obligation for the Department of Defense.     Conversely, even though a pulsar’s uniform signals are used to keep time just like the GPS system, the math in the pulsar positioning system already accounts for relativity. Therefore, the constant revisal associated with GPS is avoided. Pulsars have a highly advanced ability to keep time similar to atomic clocks and don’t shift very often between intervals relative to Earth. Even when they do, the distance they travel can be predicted.    To prove that the pulsar positioning system could navigate on its own, several researchers experimented with their radio signals. Angelo Tartaglia, a physicist at the Polytechnic University of Turin in Italy, conducted a study on software that imitates pulsar broadcasts. Tartaglia and his team tracked the trajectory of the observatory to the accuracy of several nanoseconds. Additionally, researchers with the Station Explorer for X-ray Timing and Navigation Technology experiment (SEXTANT) reported on a pulsar analysis at a news conference on January 11 during a meeting of the American Astronomical Society. SEXTANT used an array of 52 X-ray telescopes to measure the signals from five pulsars. By analyzing those signals, the researchers were able to locate SEXTANT’s position to within 10 kilometers as it orbited Earth on the space station, says astronomer Keith Gendreau. As pulsars are weak sources and typically require large radio telescopes to track, researchers will have to look for pulsars that emit X-rays, which provide a much brighter signal. X-ray antennas are also much tinier and more lightweight, says physicist Richard Matzner at the University of Texas at Austin. 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, Virginia. Pulsars would be able to track all three dimensions and recognize if the craft was deviating from its course. In a scenario in which we are far from Earth, pulsar navigation systems could improve upon the position estimates currently formed using orbiting satellites, and serve as a backup GPS system if those satellites were to malfunction.         

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